i
GUAYULE
wn
(PARTHENIUM ARGENTATUM GRAY)
A RUBBER-PLANT OF THE CHIHUAHUAN DESERT
BY
FRANCIS ERNEST LLOYD
Professor of Plant Physiology, Alabama Polytechnic Institute
WASHINGTON, D. C, ey
-PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON
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A. Guayule field with all plants above 40 cm. removed. Lomas of Sierra Zuluaga.
B, Guayule field of maximum density, near Apizolaya.
GUAYULE
(PARTHENIUM ARGENTATUM GRAY)
A RUBBER-PLANT OF THE CHIHUAHUAN DESERT
BY
FRANCIS ERNEST LLOYD
Professor of Plant Physiology, Alabama Polytechnic Institute
WASHINGTON, D. C.
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON
IQII
J
CARNEGIE INSTITUTION OF WASHINGTON
PUBLICATION NO. 139
Copies of this Book
were first issued
JUL 27 i911
PRESS OF J. B. LIPPINCOTT COMPANY
PHILADELPHIA, PA.
PREFACE.
In 1907 the author of the present paper was engaged by the
Continental-Mexican Rubber Company and the Intercontinental Rub-
ber Company to organize investigations looking toward the successful
cultivation of a Mexican desert rubber plant, the guayule (Parthenium
argentatum Gray). Dr. Theodore Whittelsey and Dr. J. E. Kirkwood
later became identified with this undertaking, the former as chemist,
the latter as assistant botanist. The headquarters for the investiga-
tions were established at the Hacienda de Cedros, Partido de Mazapil,
Zacatecas, Mexico. It was not a matter for congratulation that, at the
close of a year, the directors found it inadvisable, for financial reasons
consequent on the panic of 1907, to continue the department of inves-
tigation. By the courtesy of the company, however, the author carried
on his studies for some three months beyond the termination of his
business relations with it, and this period, falling during the growing
season of 1908, brought to light many important facts. Still further
observations of capital importance, in part on experiments begun in
1907 and 1908, were made by the writer in April 1909, while represent-
ing the United States Rubber Company, a commission which could not
have been prosecuted without the kind concurrence of President C. C.
Thach and a number of the writer’s colleagues at the Alabama Polytech-
nic Institute. As silence was not imposed by the United States Rubber
Company, it has been possible to include these observations.
No less than hearty recognition is due also to Mr. W. H. Stayton,
formerly captain, U. S. Navy, sometime president of the Continental-
Mexican Rubber Company, and now president of the Texas Rubber
Company. It is stating an open secret to say that it was through the
initiative and enthusiasm of this gentleman that the work of the inves-
tigation was undertaken and would have been continued but for cir-
cumstances beyond his control. Mr. Stayton has shown a liberal and
scientific spirit, qualities not of necessity nor at all times associated.
Thanks are due further to Prof. J. C. Arthur and Prof. W. G. Far-
low for reports on various pathological matters; to Dr. M. T. Cook for
contributing manuscript on the galls found on guayule; to Dr. A. D.
Hopkins for a report on the guayule bark-borer; to Dr. L. O. Howard
and Dr. J. G. Sanders for the identification of certain insects; and to Prof.
B. L. Robinson for his courtesy in causing a photograph of the type speci-
men of guayule to be made. Mr. Charles S. Ridgway has rendered sub-
stantial aid in the preparation of certain figures.
The drawing for figure 5 was supplied by Professor Arthur; the nega-
tive of plate 2, fig. B, was made by Dr. W. E. Hinds; Professor Trelease
furnished the illustration (fig. 4) and description of the Cedros sotol, and
kindly made several other determinations; the negatives of plate 3 and
lll
iv Preface.
plate 4, fig. A, were made by Mr. Victor Blanco. Dr. H. van der Linde
obtained for me valuable material of irrigated plants from Caopas.
Dr. Theo. Holm has afforded me the benefit of his criticism of the
portion of this work treating of the anatomy, and has been good enough
to examine inaccessible literature for me. Dr. W. E. Safford did a like
service regarding a few pages in the first chapter.
To Prof. W. L. Bray I am indebted for information about the Texas
guayule fields, later verified by me personally; and to my colleagues, Prof.
C. L. Hare and Prof. J. P. C. Southall, for assistance in making chemical
analyses and for mathematical formule, respectively.
With reference to the chapters which follow, no pretensions are
made with regard to completeness. The exhaustive study of a single
plant from all points of view might well be numbered among the labors
of fable. The reader is asked also to remember that the study of but
a single growing-period was possible. Much of the experimentation,
therefore, was done, as it eventually turned out, during the most un-
favorable season; but in the case of field experiments this was not
entirely a misfortune. That the theoretical bearing of many observa-
tions and more refined methods of making them are less attended to than
the matter warrants has been due to the urgent necessity of practical
success. With these qualifications, the work may be regarded as a
report on a unique opportunity, unhappily shortly terminated, to bring
a hitherto feral desert plant under the subjugation of culture. That suc-
cess may ultimately be attained is not an unreasonable nor an unwar-
ranted expectation, for which statement the interested reader will find
not a little evidence in what follows.
Francis Ernest Lioyop.
ALABAMA POLYTECHNIC INSTITUTE,
January t9gto.
TABLE. OF CONTENTS:
Page
1 DIGTIE Cae sini tin GI aE Ree ator cin eri eran iG clears ot nns Tae iil
[uy SLATE! 02] 2c) a Pe ere inreecen Oneness rscre ERENCE vii
CHapTER I.—HistoricaL ACCOUNT.
Original discovery and description
The vulgar name
© cla) o LQieu siWiateegellel.s) seit LBuC) © .0he) =) elalte ce) ss ees lel mse Les) sam
3
4
ippimaibiverana (ater USES. . oo... eee oe nein e 6 ws cin bie eee oh gees mon tac aas Viera 5
Phsioryror manwlacture: 62.5 s2) kos oe eye vee Sane Se = Sg oer eee prarace Ene 7
Methods of extraction 8
The natural supply of shrub )
Attempts at culture 2
Lan!
Lal
CHAPTER II.—THE ENVIRONMENT.
Geographical distributions: < (i625 fede 1. eon oe tet eee ee ee es bee ees 13
Jinn hie | Ghieinaleiptorn. 925 anooomUbnOe Oo odo cUM oe: Oomocu Pca sur Uigmn Cow or 13
(Clan fae Se oe Re ee ene ee bie Bhs cer Oe ei ee eee eC 14
TEQairte Oh Re ee ORE a Soe rcn cg Rey Dacca re teen aio. arn serar esi aoe 14
AgrAtemiperatanes: cee fen cim.. 0. nse Ans orcs ns ey een We 15
Soibtemiperatuces. 6c ba care cc wh Ge set es oses sce eae Mme vin eres ee Ras 18
Solem O1S TUL Mee ee es epee syste ei ciees cach er susie eiieucite coils otis) sates een vodens 19
Relation of rainfall and temperature to growth...........---+-+-++5-5 20
Relatinse WemmiGtG yes ean ste oie e 2s pS ta sie ysitie oiele set vale eee me Oe 21
Mapornraply ame Silo. char sae 5k ee ee Seles le Hee Se ho 23
Wretiastiis OleTOW EH: acne: eins an eee Fin Foe He Ke as pele weet toe ee Hee 25
TEVOYHTS. IREIENBIONTIG) 4 5. cilees ay ell auc lec on che SNOT: CICIIDNG Gc ce nO ICec Oy rR baci cum aaeinecitica 35
CuapTEeR III.—DeEscRIPTION OF THE GUAYULE.
Era CA ee TN: Ma NU ag I in Pe erat, Rust a ee eres t eae sie 6 Tete eee ete 46
eerie pe cme ERR APR once ae a a se cea, Se egeia = cess elnig bees 48
he matures oe eee Set ee Se ers Settles os oe me ee eee ee ee aes 50
oGt oi Suen Sa ee ein ae oi des 2 eee FR ce © a alleen 50
REEOTIOSE eee MONE Oe a UR QNE RGN store tle Sele Custis nt > A talelter ede S: opus 51
Method of branching =. 8 eae Sate eae hs RSs tees 54
Nit Ss Sena. Ree NSS SR Sd Sek IRN ISR TREN, Pam Gis) Shrek ei sites seo. SERNA 55
ee ee RENN, NRCS DENS Oat, | 5 Ree tin ok emeederts ste). absteaten tals = Biss 56
Surface characters of the stem and method of determining age............-. 57
J ENVEL KE al ani SS PURI eA oS RR oe BY)
ere eG FATES accents edie anise, acl SERA eae eats nies AS 58
aN ine eens ji 2 ee RS re ees o Blo don ceossjgcs Gn Core Dinan uc como Ro ce 58
The inflorescence and the flowering period................ 220s eect e reese 59
Me sraddenmem Or seedy as 2c ence ens eee ene ees ina MLS ole ie SERIE 60
CHAPTER IV.—REPRODUCTION.
Methods of reprodaetione ds jcreiie - fire 2a Satsiene sna sieve 2 EG Hee oe ee 61
Retonos, nermal and induced 20... 2. es Se ee be hiner lene 61
See ee ea ree tr ctitade erases + clang treme aye orese Facet 68
Rate of reproduction and! Of Prowth. =... 2. ee ee eee 75
Rate of growth during germination...............-2-0ee ere e eset ie
Rate of growth in maturer plants beyond the seedling stage..........-.- 79
Rate of growth in terms of stem-length............----++2+s+-steeeee? 79
Rate of growth in earlier years after germimation......-.....----+++++> 79
Rate of growth in medium-sized plants............--+ +++ ess eeeeeteee 81
Rate of growth in irrigated plants. ..).... 625-68 ssyce neste settee 84
iecheh ala tiesee aie ee eile meee iat oS hire, date ee etnias soiling 85
CHAPTER V.—ANATOMY AND HISTOLOGY.
To ee aE et eyes riser deecageyaas pn crust enone sco Fr [eileh ahcgemens coolness go
Pipieaetones tated sIhes 2 2 Te Eleni ees She ieee iel = oF he Set * Sys sa a es ear rere)
Senanbbrgecomueiiies (ice .)4). le Sl Ss Pe 2 vient einen nr se a go
gees ide he ee a ONAL Sen nee ee A Ra aT a 96
Piiessum starred... /\ a dhleida se. ss a es ee eee ee weet ao 96
Rie GOS, J aes es ine se ees Sa ne see HP RE eis ee - 99
Peerceemndangy Stractire. ..- = 6. 5 noe ed ate eee mae Rees ees IOI
vi Table of Contents.
CHAPTER V.—ANATOMY AND Histo_Locy—Continued.
Page
Age and structure in the seedlings, #3.) eee ee ee eine ce as 104
| Dor clo) nd eee eee ITE Re 318 iG diS10. ca A:o.c0G 5.0°0.9-0 O00 uo, Groin an conenan Aer 105
The definitive stem... ..codckh cae ee oe In onion: nie Geen nee 107
Primary Structure). 2) -..c. sa deer rea Ren a wee earner acre carer ec cee eae 107
secondary Structure... tae ee Pieces he ce ote ere ae tae een 109
Origin of the medullary andicortical stereome ss 252... ... 1.02 dss eens 110
Annular structure: micas irs, 5 ee Ee ra ahs ce eds cat ee 114
The effect of abundant water upon anatomical structure.................-. 116
Relative volumes oiveontexandswOOdre =e ae ee nnn eee D7
Effect of various amounts of water of irrigation....................+-- 121
Effect of :droaght-fellowine irrigation. © os.2y oh nc ee Re, oe 122
Effect of irrigation on the physical characters of the wood ............. 122
The peduncles 222 e 2 ot, oS RR ee = ee ee 124
The leatircn ache 3 hc Oe awe ec gees & Oe ne ok a ee eee Tis
COLVIERDODS irs sit gens opitnus Sar spo auernikly Biya aaa: es aan cy eR ae ee 125
Prophylls... ¢ 05"... cS BR OE Re ee eee 126
The definitiveleat (ts eRe oe ain ss ee eee eee 126
CHAPTER VI.—THE RESIN-CANALS IN THE GUAYULE.
The “camal=sysStemis,- 4... qs ds 4 oe ee wie. © cco on i ee 165
Primary eanalsin-the rootwand hiypocotyli.. 05... 250, 0) eee eee 165
Primary ‘cortical canals... 0 [22 2968 eee wae Thick cna EE eee ae 166
Medullary canals 2 rd vcu0ls a Xun fase aide 4 3 sue eels wate ae os ck 5 a 169
The canals in the jleaf..... <0. 2 a Se Oe Oe eee ee Ee? See regi
Primary canals an: branches...3)° 525. .cas. ice, oe ee eo ee eee i 7)
Secondary canals in root, hypocotyl) and stem) J o.8 .. «1.3 ben te eee 172
Canals im the pedunele oc a5. oe ons, sos ye = ee ce ee 172
The canals in retonoses0i ut oe oc cs atc, Patae oy Sree oe ee ee ee eee 173
The contents of the canals: Gheirioripin: bg aee cee ae ee ct ee 174
The, réle:of resin See Ke C8 G ARES EE eo Bd a rs a 174
Resin-content oreuayule! byvanalysis..\.v..+..c. ase 0e. 7 oe Ge ee ee 175
CHAPTER VII.—THE ORIGIN AND OCCURRENCE OF RUBBER.
Methodsy. soi itor oteu Pith atygie: a oe eatin Helse oie yale iota cuenta ea 176
General) distribution: of rubberdinthe plant... i0.... 6.02.4 oe ne eee 177
The appearance of ‘rubber.in richly, loaded! tissues... .. eee eee ee eee 179
Behavior'of peridermal divisions toward rubber. 2.... 25.5.2) .s 2056 eee 179
fhe development jaf tubber muthercelle, anc> acc oe ere 180
Centers of séeretion 4,2. ih, ie Bostinis Tite Jee are ae ee 181
Rate of rubber secretion. relativeto erowtic, <.t ic. 20455 oe oe ee ee 183
Rubber-content, by, chemical methods" 21 150.... 450 o/s sce eee ee 185
Variation in relative amount of rubber in field plants...................... 187
Relation, of rabber-and resing ‘2... 2 cok ee oe ae ee 188
(hei significance Ofimabber:* cies tak wx does, creas aco eral ene eee ae 188
SHUM AI thes Spe tek Sia oe Bees Laden! cette carey, Sen Gna Ch ens aetces ange oe ce a 190
CuapTER VIII.—VeEGETATIVE REPRODUCTION.
Induced ‘root-regeneration: «2% ¢ 425 haat Acie. eae ee ete ee ee 193
Propagation by cuttings:):05 1 05.2 ede 2 De ee ae ee 195
CHAPTER IX.—THE CULTIVATION OF GUAYULE.
Foréstal ‘operations..2: fet) oc «ac oe ood cee cle nee cen alee 199
Present field. Qperabiginige ye a acer yenaieaer ose a 199
Supsested rules of practice. 0. st cpda- Rui. eee el ee ee 200
Harvesting period. 05 900 Rt L.A, fe oe er ee 202
Reseeding barren ‘sreund. 5 oS... oe ree ee ee 202
Cultural“opérationss 01a 2 0 cp.ec ca cow teed tac ic eRe ee ee 203
Seed). oui uc... Rear oa vee fon te ee Rie eer eee 203
The raising of seedilitigs. 8.5 Ss. a2 eh ens 2 ace eee ee eet 203
Trrigavionte ic aah aati nit dtea cee a eeaeo mene ce. eee Tie lebaseteaeeg Doie a ee eee Ie 208
Transplamtimeon ta. ty0 i 5k cn cre wre eheeeehe Cacti oc ks y ae eee eee eee 209
Harvesting enltivated Saayulle.) hoe ose... hts on nein er: crear oe ees 210
Cateh ‘crops. 3) Sal 2505 Ge shop oiieion ao aan eer fettnc Celene eae aan 210
Bibliography 0752). hott de ae or ehe e ae ee ee eee 211
PLATE 1.
tow Bw POW Pw wp
LIST. OF. PEATES.
Facing
page.
. Guayule field with all plants above 40 cm. removed.
Lomas of Sierra Zuluagas2ato% ra ee- ~10eet: lr rontis piece
. Guayule field of maximum density, near Apizolaya
The type specimen of Parthenium argentatum Gray........-.- \
Transverse section of avery old Stet: 2.2... 005. a. oe. e- 4
Upper floor in a guayule factory from which pebble-mills are
Claveh a7 205 ela ee ies sarc ean, lites inlet ler an pwne ni Pin cee IPE a: 8
Lower floor: discharge chutes and ditch from pebble-mills ....
A battery of washing and sheeting machines.............--- J
. Stacks of guayule in bales. Continental-Mexican Rubber Com- |
PetyyS LACHOLY ap aplnjrasige) ys nays a ep ie ys , 8
Experimental ground, with plants two years old from stocks. |
(CO GhROS. SOR o Soe c.s Gao Sis oo Sisie OSS ciO c oleicre eISIeRO. © erencirmege J
. Station 2, Quadrats 5 and 6. Lomas of Sierra Zuluaga.....
Station 3, Quadrat 1, near Cedros. A good stand of mature 24
POPs TAGS Ses pers Beh esl ncheelm cig wane oi spouse eine, cere = ings
lants from Quadrats 5 and 6, Station 2..........-.-.-- ee eeeee 24
A. Quadrats (Station 12) with very dense growth. Apizolaya... \
B. The same, the guayule removed..............----+-+--e0e: 3?
A. Narrow type of guayule............. cece eee eee e cence eens \ 6
B. Spreading type of guayule..... 2... gece ncesssemeet ese 3
A. The root-system of guayule............ 2. cee eee eee e tenes
B. Groups of plants which started as retonos.........--.-++++++- 48
C. A strongly monopodial retono............-.+ eee eee eee
. An exceptionally tall (130 cm.) individual. Caopas.............: 52
. A. A widely spreading (130 cm.) plant of guayule............... \
B. A large plant of the usual habit. Apizolaya................ 5?
. A biotype of guayule. The winter condition on the Le TG gE cactersncyee< 52
. A-C. Seedlings of typical and atypical guayule................--. l 6
D. Seedlings and mature plants of these biotypes............-- s 8
A. An irrigated plant, from a small stock, at the height of flowering \ oh
B. ‘‘Hembra” (on the left) and ‘‘macho”’ (on the right) guayule. J
A. Induced retofios on a tap-root. One season’s growth......... \ 6
B. Induced retofos on a lateral root. One season’s growth...... :
. A-C. New growths after pollarding.................e sees eee \ 68
D. Seedlings in limestone soil; E, in ‘‘garden” soil.............
. A. Minimum, average, and maximum seedlings. (Station 2, a |
TERE Aran tbc c G2 ia Ook 65.6.0, tidi OS. ROR OG O88 Occ ono ieee oo 68
B. Irrigated plant, two years old from a stock. Cedros..........
. Seedlings growing in different soils............. essere eee ences 72
. Seedlings growing in different soils..............0 eee eee eeees 72
_ A. (1) Root-cutting; (2-4) sectorial root-stem cuttings (Exp. 146). \ oe
B. Seedlings grown in different soils... ......-. 265+ - see e eee eenee
. Anatomical and histological details, figs. 1
. Anatomical and histological details, figs. 1
. Anatomical and histological details, figs. 1
. Anatomical and histological details, figs. 1
. Anatomical and histological details, figs. I tO 10............-++4-- 138
. Anatomical and histological details, figs. 1
. Anatomical and histological details, figs. 1
. Anatomical and histological details, figs. 1
. Anatomical and histological details, figs. 1
. A. A branch, one year’s growth under irrigation..............-.. \ 8
B. A branch in the height of flowering, second season.......... 4
. Anatomical and histological details, figs. 1 to 16.............+++-- 128
vill List of Plates.
Facing
page
Pirate 32. Anatomical and histological details, figs: 5 to 7...........-.:.---- 148
33. Anatomical and histological details, figs. 1 to 1o.. I50
34. Anatomical and histological details, figs. 1 to9.........:......-.-. 152
35. Anatomical and histological details, figs. 1 to 15.. 154
36. Anatomical and histological details, figs. r to 8... 156
37. Anatomical and histological details, figs. 1 to 8... 158
38. Anatomical and histological details, figs. 1 to 18...............--- 160
39. Anatomical and histological details, figs. 1 to 8................... 162
40. 1. Rubber in canal-cells, nearby cortex and inner ray-cells...... i]
Older root... _More-rubberaugraiys: free se eae heees ae cs ccs
. Root:eumridiameter- ea eetee Peer eeeenieeee coer eee ook. cs
. Parenchymaynaysinomienor gait. a ea cee ians eee Oe ee eee
» Upper part of hypocotyl, isame.ace;asfGs.te cee ae ee oe @ -
. Longitudinal section through old wood
. Longitudinal section through mature leptome parenchyma, with
a few parenchyma-ray cells
) Leptomes "elongated elemetter a... Ccorecn seatate ae eee mee
Companion cells and sieve-tubes. No rubber in younger leptome
on the left
172
oO TANHWH
41. 1, 2. Cortex, stem of field plant with maximum rubber-content.. }
30° COFLEX OF A BO-yeat-Old StEM occ 2. one coco see ee ot oe
4. Root; rapidly grown seedling, 2 monthsold. Rubberin granules. | 172
5.. Rubber in process of accumulation in an irrigated plant....... |
6. “Primary “restii-catiall 5.7) fisse © sto ne een cee ne tie aa s,s J
42. 1; Apexjef terminal twiciot 190s streld plant ./2 1... os -.5.ee« )
a. Near baSeigitsamie. * hn anes eee i eee eee a ore |
3. Pseudotylosis with rubber inthe cells... 6... .c.c0 «ocr ae es
A. Leptome; held ‘plant. 76.04 eee Come eee
5° Pith ofa-field steny ro: mm- diameterte. 2). aoe noe eee 176
6 An ‘old leaf-trace... 3.24 0Ann ges he ia Tree oe ic a eek ee
7. Outer cortex of-a. field stemAse- aan ee oe eee eee |
8. Outer edge of cortex and inner zone of cork derived from collen-
C01 9p 641: OPP MORMMR SS roe RA Cec c thre ning Aoi olen Gee
43. 1. Base of 1908 growth, August. Cedros, irrigated..............
2. Growth of 1908 in April 1909. Cedros, irrigated..............
3. Cortex, 2-year-old ‘stem: “Caopas) irrigated 2 i. -% 95.08.15. 0-s.- r 192
Ad Pithvotisame plants: 22-2 Na ee as, oe ee Oe iy Oe oe |
5. Epidermis and palisade of an old leaf, field plant............ J
44. A. Irrigated plant, 2 yearsold. Basal branches which have rooted |
arerspread apart! | ate. dy AMER 2 ee SPREE Ok ee oe 192
B. Mariola showing the same behavior, normal in this species... .
45. A. Flat filled with paper tubes, 1 square inch in transverse section. }
5B. lat with 4-sqtiare-tnich tubes. ? o:.5.222 Sel) Eee
C. Exp. 141 (3), 1-inch tubes; very poor growth. Exp. 143, 4-inch | 204
PUES oS Na PMeNe oh ARE. aly hee Aa AOR fh I ee ce
D. Dhesame, seedlings wellsrown). § cc. cnn nwa cam peerepee ces a
40. A. Seedlings from! experiments andicated 5... ...fn.ie¥<. cess oe ) 8
B. Irrigated plant (Caopas) with a retofio a:
GUAYULE (PARTHENIUM ARGENTATUM GRAY):
A RUBBER PLANT OF THE CHIHUAHUAN DESERT,
By
PRANCIS: ERNEST LLOYD,
Professor of Plant Physiology, Alabama Polytechnic Institute.
CHAPTER I.
HISTORICAL ACCOUNT.
Since about the middle of the last century, after the epoch-making
discovery of Charles Goodyear was made, the demand for crude rubber
has been steadily increasing. This demand was for a long period satis-
fied by the products harvested from the tropical forests of the Old
and New Worlds by natives whose methods are resulting in a gradual
depletion of the natural supply. This, in turn, has stimulated research
in three directions : toward obtaining a synthetic rubber, the ambition
of the chemist; toward discovering other rubber-producing plants, for
which search has been made into the farthest reaches of the tropical
forests of the world; and, finally, in the direction of the culture of the
various plants which before had been, in their feral condition, the source
of the much-desired material. Whatever the promise of the chemist
may be, there appears to be no abatement of interest at present in the
culture of those better-known trees which have been found to adapt
themselves to the hand of man, or in the discovery of hitherto unknown
rubber plants. Each new announcement, however vague the authority
may be, that a new rubber plant has been found, is hailed with precipi-
tous interest; and one that is well founded is soon followed by a period
of exploitation scarcely less fevered than on the finding of new gold-
bearing fields. When, a very few years ago, it became more generally
known that the plant commonly known as the guayule, and containing
an economically valuable amount of rubber, grew in abundance in the
desert country of northern Mexico, the vegetation of the adjacent arid
areas underwent minute examination in the hope of finding either this
or other similarly valuable plants, and even at the present moment the
excitement has not died away.
The mere fact, however, that a plant indigenous to the desert
should be found to be of enough value to set in motion large business
operations involving millions of capital, based on the amount of the
Taw material in sight, is sufficient to awaken definite interest. The
economic value of the desert is changed, and possibilities for the devel-
opment of wealth in a supposedly barren country take on new dimen-
sions. This has occurred, in point of fact, as a direct result of the dis-
covery that the plant guayule produces in the neighborhood of 1o per
cent of its weight of ‘‘bone-dry”’ marketable rubber. With the eco-
nomic history, bionomics, structure, and micro-chemistry of this plant the
present essay has to deal.
ORIGINAL DISCOVERY AND DESCRIPTION.
The guayule was first discovered by J. M. Bigelow, M.D., in 1852,
while attached to the Mexican Boundary Survey, ‘‘ near Escondido Creek,
Texas.’’ It was first described by Professor Asa Gray some years later.
His original description was based upon the type specimen, which is now
3
4 Guayule.
in the Gray Herbarium of Harvard University. A reproduction of this
specimen is here given (plate 2, fig. A). The name in the right-hand corner
is in the writing of Professor Gray. The label is Bigelow’s field label. Fol-
lowing is the description published in the “ Botany of the Boundary,”’
p. 86, 1859:
Parthenium argentatum (sp. nov.): fruticosum, pube brevi appressima
sericeo-incanum; foliis spathulato-lanceolatis oblongisve in petiolum longe attenu-
atis parce dentatis seu laciniatis sub-triplinerviis; ramulis floridis elongatis nudis
oligocephalis; involucri squamis obtusissimis; acheniis sericeis; pappo e paleis
2 membranaceis lanceolatis— Near Escondido Creek, Texas, in rocky places,
Sept., 1852; Dr. Bigelow.—A well-marked species, connecting the sections Argy-
rocheta and Parthenicheta; the leaves and branches whitened with a very fine
and close silk-silvery pubescence, which appears to be wholly or nearly persistent.
Leaves one to two inches long, including the tapering base and petiole; 2 to 5
lines wide, mostly acute, scarcely veined, beset on each margin with from one to
three salient teeth, or sharp lobes. Flowering branchlets slender, 4 to 8 inches
long, nearly leafless and peduncle-like, bearing 3 to 7 sub-sessile heads (as large
as those of P. incanum) in a cluster. Exterior scales of the involucre short, orbic-
ular-ovate; the inner orbicular, scarious-membranaceous. Palee of the pappus
lanceolate or oblong-lanceolate, rather narrower and less obtuse than in P. hyster-
ophorus, puberulent, the inner edge more or less adnate to the base of the broadly
obovate and cucullate emarginate ligule. 1 (Fig. 9.)
As will be seen, the crowding of the heads to form a “‘cluster’’ de-
pends upon external conditions. In a later description published by
Gray in the ‘“‘Synoptical Flora,’’* we find the first hint of the peculiarity
which later brought it into economic prominence. This description is
as follows:
P. argentatum Gray. Suffrutescent, a foot high, silvery-canescent with close
tomentum; branches erect, rather leafless above, bearing comparatively large and
few heads (of 2 lines in diameter); leaves lanceolate to spatulate in outline, some
entire or incisely 2-3 toothed, the larger incisely pinnatified into 2 to 7 acute
lateral lobes; pappus a pair of lanceolate chaffy awns (Bot. Mex. Bound., 86;
Southwest border of Texas, Bigelow; Adj. Mex., Parry, Palmer; produces a gum
or resin in Mexico).
THE VULGAR NAME.
The name ®* “guayule’’ is properly applied only to Parthenium argen-
tatum Gray. On account, however, of a superficial resemblance it has to
certain other plants, especially because of similarities in size and in the
gray color (so often seen in the desert) of the foliage, these have been
wrongly called by the same name.* The mariola (P. incanum H. B. K.,
plate 44, fig. B), a closely related species, is one of these; and its very
general association with the guayule proper has led to much error in
estimating acreage of guayule. It is of interest in this connection to note
that the mariola is known to the peon, in some parts at any rate, as
‘‘hembra de guayule,”® apparently because of the very constant associ-
1Gray, in Torrey, Botany of the Boundary, U.S. and Mex. Boundary Sury.,
. 86, 1859.
3 eed ea Flora of North America, vol. 1, pt. 2, p. 245, 1886.
8Investigated by Endlich, 1905.
The name is also applied to Crysactinia mexicana Gray, and more recently
also to Euphorbia misera, material of which was sent to Dr. J. N. Rose, of the
U. S. National Herbarium, from southern California, on the supposition that it
contained rubber.
5’ The female guayule.
LLOYD
|
42
Ja (Manes 7
Fr. Near St; Confiada
Dr fetor ro )
>
Seeks
So 83
i PES Mp
“a*2
“~~ |
A. The type specimen of Parthenium argentatum Gray.
B. Transverse section of a very old stem.
PEATE 2
A _ SYN. FL. N. AMER, >
a)
A . Soe wah
Oh he nnn erpyeuta AANA
=, 9), 5
Historical Account. 5
ation of the two species, and because of the belief that this association
is in some way necessary to the production of seed. Other species of
the genus, some of which are annuals, have also received the name guay-
ule, while a plant of the Sonoran Desert (Sonora and southern Arizona),
Encelia farinosa, is not only mistaken to-day for guayule but is believed
by many to contain rubber. The amount, if present at all, is so insig-
nificant that it would certainly not repay consideration from a com-
mercial point of view.
The guayule is known also as ‘“‘yerba de hule”’ in the region of
Pasaje, Durango, and simply as ‘“‘hule”’ in some parts of Zacatecas and
of Chihuahua. An alternative spelling “ yule’’ (which occurs incorrectly
as ‘“‘Ilule”’ in “guallule’’) is used in some parts of San Luis Potosi. The
name xihuite! occurs in northern Zacatecas and “about Saltillo”’;
copallin and afinador are other less-used designations. But the name
‘“suayule’’ thus spelled is in the ascendant and will in all probability
replace other names. Its derivation, in common with other Mexicanisms,
has speculative interest. Seler ° would refer it to quahu (wood, tree, or
forest) and olli (rubber, Sp. hule), evidently believing it to be of Aztec
origin. This etymology finds support in the aboriginal term ulequahuitl,
said by Sahagun (1529) and Augustin Torquemada (1615) to be applied
to a latex tree (probably Castilloa) producing uli, a dark resin which
becomes very elastic (Jumelie, 1903). By inversion, we have quahu + ule.
The suggestion that the derivation is from the Castilian hay (there is)
and the Aztec olli, from which we therefore have hayoll1, which becomes
hayule and so guayule, can not be seriously entertained.
PRIMITIVE AND LATER USES.
Contact with the country peon of Mexico reveals a great deal of
resourcefulness in the use of many plants. In out of the way places a
game is played with a small, very resilient ball, not purchased in the
market. It proves on examination to be of very pure rubber, obtained
by communal mastication of the bark of the guayule. Altamirano
(1906) tells us that country boys obtain rubber in a similar manner also
from ‘‘tatanini,”’ a name applied, in Querétaro, to Parthenium incanum
and to P. lyratum. This custom dates back with fair certainty to the
middle of the eighteenth century, having been noted by a Jesuit, one
Negrete.®
Mr. W. H. Stayton, formerly captain in the U. S. Navy, when on
duty in the Gulf of California, observed the Yaqui Indians ashore playing
a game with a ball about twice the diameter of a baseball. The game
consisted in throwing the ball from hip to hip. It is not unlikely that
the ball was made of guayule rubber, which could have been obtained
from the country east of the Sierra Madre, or even of rubber from tatanini,
1 From the Nahuatl xihuitl, weed. This spelling is given by Endlich (loc. cit.).
“Jihuite” is given in Zacatecas. ‘‘Gihuete”’ occurs in a legal instrument drawn
up at Matamoras, Coahuila, under date of March 9, 1905, in which also “‘hule”’ is
given as designating guayule.
2? Endlich, loc. cit.
3 According to Juan Fritz, fide Endlich, 1906.
6 Guayule.
mariola, or other plant. The possibility that it came from the South is,
however, not excluded. Peter Martyr (1569; published in 1830), Saha-
gtin (1529), and Herrera (1492-1526) all speak of balls made of rubber
made from latex trees.'
There can therefore be little doubt that, in common with the manu-
facture of mescal, extraction of fibers, and like primitive industries, the
making of rubber balls from the guayule, just as from latex plants,
antedates the invasion of Mexico by the Spaniard. It may be mentioned
in passing that the method of extracting the rubber as above noted is
analogous to the only widely used modern method of obtaining the crude
rubber on a large scale, namely, by a purely mechanical process. The
rationale of this will be seen beyond. In this connection a recent dis-
covery of a piece of rubber which is undoubtedly of ancient origin on an
old aboriginal village-site in Arizona is of peculiar interest. Of this
discovery the following account is furnished me by Prof. R. H. Forbes:
The lump of rubber, a portion of which I recently handed you, was found in
December (or thereabouts), 1909, at the west end of the Santa Cruz Reservoir and
Land Company’s dam, 14 miles west of Sasco, Ariz. Mr. C. O. Austin, who was
present, states that this ball of rubber was contained in a small olla with articles of
stone belonging to the older prehistoric ruins of this country. The find was made
at about 3 feet below the general surface which was formed by the off-wash of an
adjacent low mountain. No traces of houses on the present level of the land,
according to Mr. Austin, were visible. One other ball of rubber was found here,
and is now in Col. W. C. Greene’s collection at Cananea. I regard this find as
genuine, as Mr. Austin is familiar with Salt River Valley ruins and his statements
are confirmed by others.
Microscopic examination of the specimen to which Professor Forbes
refers throws doubt on the view that it is guayule rubber, but a final
statement can not at present be made.
A record of this kind would be incomplete without reference to the
use of guayule as a fuel. On account of its resin content, the plant
burns with a fierce, smoky flame, after the fashion of ‘‘fat pine;’’ so
that whenever it was available it was invariably used as a fuel for the
crude Mexican adobe smelters, ruins of which are frequently seen in the
mining districts. In this way thousands of acres have been depleted
of their guayule, a wasteful process which was quickly stopped when
the value of the plant became known. It can scarcely be doubted that
many peculiarities of local distribution within restricted regions are due
to the pulling of the guayule for fuel. Thus a large smelter and a num-
ber of roasting furnaces were in operation at Cedros,”? the head fraction
of the hacienda of that name lying to the west of Mazapil, for a term
of years, and this circumstance is often referred to by the peons to
explain the absence of guayule in places where it would naturally be
expected. The case is analogous to the use of walnut for fuel and fence-
rails in the early days in the eastern United States.
1 Jumelle, 1903, quotes these authors at length.
? According to Juan Robles, whose duty it was, in 1856, to weigh the shrub
as it came into the fundicién at Cedros, guayule was paid for at the rate of 18
centavos per carga (6 arrobas=7o kilos), or about 17 pounds for 1 cent (gold)!
The women on Cedros burned guayule in their bread ovens as late as 1894 (fide G. R.
Fleming). Guayule shrub now fetches 150 pesos the ton.
Historical Account. 7
HISTORY OF MANUFACTURE.
Public attention was drawn to guayule rubber,’ apparently for the
first time in 1876, by an exhibition sent from Durango to the Centennial
Exposition at Philadelphia (Pearson, 1907). In the same year, accord-
ing to the Mexican Herald, the Natural History Society of Mexico took up
the study of the plant and reported the presence of rubber of good
quality (Delafond, 1908).
The first move toward the utilization of guayule rubber other than
by the natives appears to have been made in 1888, when a company,
the name of which is unknown to me, but probably the Mechanical Rubber
Co., of Passaic, New Jersey, sent a special agent to Mexico with instruc-
tions to ‘obtain a large quantity ”’ of ‘‘rubber-bark,” “from which it was
proposed to extract the rubber by a process of grinding and washing.”’
According to the account, the agent seems not to have clearly understood
his instructions, and shipped to New York 100,000 pounds of the entire
shrub! The company in question did not relish paying the freight on
the wood, and this item of expense deterred further investigation.
However, the shrub was decorticated, the bark and twigs ground up
finely, and ‘‘immersed in hot water * * * finally coagulating the
rubber into one mass.’’ The result was an extraction of 18 per cent rubber
(the wood of course not entering into the count), the quality of which
was regarded as equal ‘‘to the best grade of Centrals,’’ and a specimen
was reported? to have been in good condition in 1895. There seems to be
little doubt that the ‘“‘rubber-bark”’ referred to in the preceding para-
graph was guayule, though ignorance of the identification was confessed.
However, the material was collected at Hot Springs (Aguas Calientes),
Chihuahua, and was referred to in a letter by the local agent, who under-
took the collection, as ‘‘hule.’’ *
In this same year, 1888 or thereabout, a Mr. Herbert Wilson sent a
sample of the rubber to England for analysis, and at about this time
also Herr Juan Fritz employed a number of peons to chew out a suffi-
cient amount of the raw material for examination, and this he sent
for study to a German chemist, whose report was a practical condemna-
tion of the rubber as an article of commerce.
Shipments of crude shrub made to Hamburg in 1900 were treated
with caustic soda and small amounts of rubber thus recovered were
placed on the market. . In the following year 25 or 30 pounds of guayule
rubber were sent to the market from a laboratory which had been estab-
lished by Germans at San Luis Potosi. The earliest efforts seem to have
centered here, so that San Luis Potosi may be regarded as the birthplace
of the industry.
The laboratory experience at San Luis Potosi led in 1902 to the
establishment of a factory at Jimulco, by Adolf Marx, representing the
1T have been unable to obtain a transcript from the original records. An
anonymous writer in the India Rubber World, April 10, 1895, refers to this exhibit
as rubber from ‘‘a native plant of the genus Cynanchum, of the natural order
Asclepiadacez, according to Mr. Fernando Altamirano.”’
2In the India Rubber World, 10: April, 1895: ‘“‘Extraction of rubber from
minor plants” (unsigned), upon which I base the account in this paragraph.
3 [India Rubber World, loc. cit.
8 Guayule.
Compania Explotadora de Caucho Mexicano, from which factory rubber
was put on the market for the first time in 1905. In 1902, also, certain
American capitalists financed an expensive but eventually successful
series of experiments which led to the successful extraction of the crude
rubber by a mechanical process (devised by Mr. Wm. A. Lawrence), and
two years later, in 1904, the first lot of rubber thus prepared was taken
by the Manhattan Rubber Company. On December 25, 1904, 50 pounds
of crude rubber, extracted by means of the now successfully adapted
pebble-mill, were shipped to the United States, over half of the amount
being purchased by the Manhattan Rubber Company. Then followed
the building of a large factory at Torreén by the Continental-Mexican
Rubber Company (plate 3, plate 4, fig. A), in which the results of the
earlier experiments were used. This event marked the beginning of
commercial success in the extraction of rubber from the guayule shrub
by the mechanical method, which has superseded all others, and it
should be said that this phase in the development of the industry is
almost entirely due to American initiative and ingenuity.
From 1905 on a large advance in the outlay of capital followed, and
extracting plants of various sizes were established in San Luis Potosi,
Saltillo, Monterey, and Gomez Palacio, as well as at Torreén and Jimulco.
Manufacturing enterprise has lately brought the guayule industry
into Texas. On September 1, 1909, a factory! at Marathon, Texas, in
the heart of the guayule area of that State, began operations under
the Texas Rubber Company. But it should be added that the manu-
facture of guayule rubber had already to some extent been carried on
in the United States and abroad. The extent of this phase of the indus-
try is indicated in the total exportation of crude shrub from Mexico,
the statistics for which are given on p. 11.”
At the present writing, according to Mr. Henry C. Pearson,’ the out-
lay of American capital alone in Mexico amounts to $30,000,000.
METHODS. OF EXTRACTION.
A brief statement of the principal features in the methods of extrac-
tion of rubber from guayule will be of interest here, especially as they
differ widely from nearly all hitherto-used methods of preparing crude
rubber from latex plants.*- It must be understood that the rationale of
the processes lies in the fact that the rubber exists as such in the cells of
the plant, and will not escape by bleeding.
The material must, then, either be dissolved out, after preliminary
grinding, by suitable chemical agents, or must be agglomerated mechan-
ically, either with or without the assistance of a substance (caustic soda)
which will attack the cell wall. The chemical method is used successfully ,
it is understood, at Akron, Ohio, where an excellent brand of guayule
‘Previously built and operated for a short time by the Big Bend Rubber Co.
* We now read that the Japanese have entered the market, and are buying
shrub (Dec., 1909).
* India Rubber World, 40 : 383, August 1, 1909.
‘ African “ grass-rubber,’’ however, is obtained in a crude way, but purely
mechanically, from species of Landolphia (Jumelle, 1903).
LLOYD PLATE
C
A. Upper floor in a guayule factory from which pebble mills are charged.
B. Lower floor: discharge chutes and ditch from pebble mills.
C. A battery of washing and sheeting machines.
LLOYD
7 SSG0 BOSS GEOS UGRL SRBG TIE ODD : 295 GAD DUNN RUee oe | = See
~ \S
| 5 0 ee eel
aor a ae a
a Deon) +
tige. skh
ey olge- land as
: Pera
A. Stacks of guayule in bales. Continental-Mexican Rubber Co.
B. Experimental ground, with plants two years old, from stocks.
Cedros.
PLATE
Historical Account. )
rubber is produced. Although the principles involved are well known, the
precise steps are preserved secret. The process, which is based on meth-
ods of organic analysis, is not widely used, and only a small part of the
total manufacture is carried on in this way.
Of greater interest, not only in itself, but for the future economic
development of the rubber industry, is the mechanical method. This
may be described only in its broader outlines, since the steps employed
by various manufacturers are changed from time to time as experience
indicates.
The shrub is first washed so as to free it from dust and other foreign
matters which affect the specific gravity of the ‘‘worm”’ rubber by becom-
ing attached to the agglomerated particles. It is then passed between
rolls which comminute it while it is being sprinkled with water.1 The
rolls used have been supplied with knives, or have been adapted to
pulverize the material, or, as now used, the shrub may be run twice be-
tween corrugated rollers, running differentially, for the sake of even and
fairly fine grinding. The resulting mass is then placed in a pebble-mill,
which is a short cylinder of steel, lined with Belgian flint bricks, such as
is used for grinding cement, paint, charcoal, and the like (plate 3, figs.
A,B). The grinding is accomplished by means of Norwegian or Medi-
terranean flint shore-pebbles.*
The pebble-mill charge consists of one-third its volume of pebbles,
one-half of water, together with 6 to 8 bushels of shrub. The mill is
revolved at the rate of about 30 times a minute for a period lasting go
minutes to 2 hours, at the expiration of which time there results a finely
ground pulp consisting of minute agglomerations of rubber mixed with
fine particles of bagasse. This is separated from the dirty water in which
it was ground and is then run into tanks, where a skimming process sepa-
rates the rubber, which floats, from the bagasse, which sinks. A part
of the bagasse, however, does not sink at this time, namely, that con-
sisting of flakes of light yellow cork.
Nor is the rubber free from particles of wood fiber, imprisoning more
or less air, and this interferes with the complete separation of rubber and
bagasse. The complete water-logging of the bagasse may be attained
by means of a compressor, in which the skimmed rubber, with its adhe-
rent fiber, is subjected under water to a pressure of about 225 pounds
for a period of 15 minutes to 2 hours, according to the kind of shrub
being treated. Subsequent treatment in a beater-washer, an elliptical
tank, supplied with a paddle-wheel of half its transverse diameter,
prepares for the final separation of rubber and bagasse in settling-tanks.
1Tt has been suggested (Whittelsey, 1908) that decortication, previous to
grinding, would be an economy. It is interesting to recall that this was done—
on an experimental scale, albeit a generous one—in 1895 (India Rubber World,
April 1895). ;
An alternative method, recently proposed by Chute and Randel (India Rubber
World, vol. 42, p. 360, 1910), consists in grinding the shrub dry and then deresinat-
ing (the solvent to be recovered by distillation). The ground shrub, now supposedly
free from resin, is then treated as here described, beginning with the pebble-mill.
? The internal structure of this mill has been the subject of numerous patents.
Thus, steel balls, associated with various forms of projections from the interior
surface of the cylinder, have been used, but without supplanting the ‘‘ pebble-mill.”’
10 Guayule.
An alternative treatment consists in allowing the washed rubber
from the first skimming-tank following the pebble-mill to soak for a week
in settling-tanks, during which time the bagasse becomes water-logged
and sinks. The soaking is probably of value also in separating from the
rubber certain substances, probably enzymatic in character, which other-
wise would contribute to the earlier breaking down of the rubber.
The clean rubber is now passed between corrugated and smooth
rolls for the purpose of washing and sheeting (plate 3, fig. C), when the
product is ready to be put on the market. Unless further treatment
ensues, the rubber thus prepared contains about 25 per cent moisture,
together with a proportion of resin.'
Other special steps in treatment are applied to the separation of
rubber from bagasse, or in preparing special grades. For example,
boiling the skimmed rubber in a 1 to 2 per cent solution of caustic soda
has been used as an aid in the separation of rubber and fiber, and for
partial deresination by the saponification of the resin acids. By this
means the amount of resin, 25 per cent, usually present, may be reduced
to 17 or 18 per cent.” Other modifications in treatment are necessitated
by the condition of the plants when treatment is begun. Old, weathered
and dried-out shrub is not worked with the same ease nor with the same
result as fresh, while a certain amount of seasoning is an advantage. Con-
siderable losses have been entailed by storing guayule in the yard exposed
to the sun (plate 4, fig. A), as may be imagined if a million dollars’ worth
of shrub is handled in this way, even though the amount of deterioration
is small. This loss is now avoided by placing the shrub in storehouses.
THE NATURAL SUPPLY OF SHRUB.
With such large interests at stake, it soon became a matter of
moment to determine the relation of supply of the shrub to the manu-
facture, as to total supply in sight, as to its rate of reproduction under
natural conditions, and as to the possibility of its cultivation.
The first of these questions was naturally the first to be raised, and
many attempts have presumably been made to find an answer. The
earliest,and,so far as Iam aware, the only published calculation was made
by Endlich (loc. cit.),who assumed an average amount of half a ton per
hectare in virgin fields. The total area of the general guayule region
being taken as 75,000 square kilometers, and assuming that only one-
tenth of this carries the shrub, Endlich arrived at the sum of 375,000
tons total supply in Mexico, which, at the rate of 7 to 10 per cent of
rubber, represents 26,250 to 37,500 tons of rubber. This estimate was
probably quite conservative, as indicated by calculations based upon
official reports brought together in the India Rubber World.
Using the probable corrections for exports of crude rubber other
than guayule, this publication gives the total imports of guayule rubber
1 Whittelsey, 1909.
? At this writing an announcement is made (Guayule Rubber by a New Proc-
ess, India Rubber World, December, 1909) that a method (‘‘ physico-mechanical”’
—sic.) has been patented whereby crude rubber, after treatment, has the com-
position: ‘‘ Pure caoutchouc, 88 per cent; resin, 7 per cent; water, 5 per cent.”
Historical Account. tr
into the United States from June 30, 1905, to June 30, 1909, as 32,010,820
pounds. This being about 80 per cent of the total export, using the
data for 1906-1908! as a basis, we have a total exportation of crude
guayule rubber for four years of 40,013,525 pounds, which amount to
20,000 long tons in round numbers, representing, on the basis of 7 per
cent extraction of rubber with 25 per cent moisture (5.25 per cent dry
rubber), shrub, 286,000 tons; and on the basis of 15 per cent extraction
of rubber with 25 per cent moisture (11.25 per cent dry rubber), shrub,
132,000 tons.
The last two sums give us the highly probable extremes between
which the tonnage of shrub represented by crude-rubber exports falls.
To the amount must be added the amount of shrub exported, for which
figures for two and a half years are available, namely, 2745 tons. We
have, therefore, the limits of 288,745 tons and 134,745 tons.
That the larger amount of shrub is nearer the true amount taken
appears to be the case, since the extraction of rubber with 25 per cent
moisture has only recently reached 15 per cent, and this is probably
not attained by manufacturers in general. For a long time it fell below
Io per cent, so that an average extraction of 8 to 10 per cent of rubber
(25 per cent moisture) is probably near the truth. This would represent,
on the 8 per cent basis, 252,745 tons shrub; on the ro per cent basis,
202,745 tons shrub.
It is therefore probable that in the neighborhood of 225,000 tons
of shrub were disposed of up to June 1gog. This, according to Endlich,
would be somewhat over half the total original available supply. This
estimate agrees with that of some interested informed persons who hold
that one-half of the original supply is used. But estimates carefully made
for business purposes show that there were at this time at least 200,000
tons still available. Of this amount, I myself have seen at least 100,000
tons in a comparatively restricted area on three estates.
Allowing for guayule still remaining on fields which have been gone
over, and which in certain well-known cases is in considerable amount,
it seems not improbable that the total original amount reached 500,000
tons. The amount in Texas in the Big Bend country is not known and
must therefore be left out of account, but without it it does not seem
probable that the total amount of virgin shrub is sufficient to last more
than four to six years at the present rate of consumption.” It is likely
that the smaller concerns will be closed out, so that, with a reasonably
restricted output, the supply may be made to last six to eight years,
which is the period during which the solution of the cultivation of the
plant must be compassed if it is to keep the industry on its feet.
1 India Rubber World, September 1909. For 1906 to 1908 the total crude
sagt! exports were 22,693,489 pounds, while our total imports were 17,917,342
pounds.
? The recent high prices paid for crude rubbers have stimulated the manu-
facture of guayule rubber, which has brought as much as $1.25 per pound. The
imports into the United States for the year ending June 1910 were, approximately,
10,000 long tons. On the basis stated above, this quantity represents something
between 66,000 and 145,000 tons of shrub, but, in view of the improved methods,
the smaller figure lies nearer the truth. If we assume a 12 per cent extraction, we
get 83,300 tons of shrub used in the year.
12 Guayule.
As in all commercial enterprises depending upon the rate of growth
of the raw material, and more notably of lumber trees, the methods
were and still are conducted without relation to the future. When,
however, the capitalist began to see that nature had set a definite limit
to the rate of supply, it became a matter of moment to determine what
could be done to meet the demand. The method of obtaining the shrub,
when not owned outright, is by contract between the companies and the
hacendados whose lands support a growth of the desired plant. These
gentlemen at first signed contracts at a very low figure, but when they
saw the market stiffen and their acreage continually reduced, they very
naturally began to take thought for the morrow. I have conversed
with hacendados who had for some years endeavored to germinate the
seed, in the hope of solving the problem of cultivating the plant. Lack
of success, however, was the chief result of such effort, though a few
doubtless succeeded in getting plants to grow. Indeed, optimistic state-
ments as to the possibility of growing the plant profitably have been
made in some quarters,’ and it has even been claimed that the whole
problem of cultivation at a profit has been solved. As will be seen, how-
ever, in what follows, as regards the secretion of rubber, which is the
all-important point, a very great deal of caution should, in view of the
lack of evidence, have qualified any statement of this kind. It seems
more consonant with the truth, as well as with business methods (a
not invidious juxtaposition, it is hoped), to take a skeptical attitude,
which, however, need not be unduly pessimistic. It is rash at best to
attempt to foretell what solution science may bring to any problem.
ATTEMPTS At ‘CULTURE:
That hope has been entertained that the cultivation of guayule on
a profitable basis may be possible is evident. In addition to private
owners, at least two companies have spent time and money in seeking
this end, if unauthoritative statements may be relied upon. Of these
the Continental-Mexican Rubber Company essayed to make a serious
trial, and employed a scientific corps to undertake research looking to
the final solution of the question.” pes
It is not surprising that so valuable a desert plant should have
attracted the attention of interested persons of other nations whose
authority extends over desert areas in other parts of the world. No
detailed statement on this score can be made, however, beyond that the
Germans * are said to be conducting experiments in the cultivation of
guayule in East Africa. The feeling properly exists that any effort
toward the subjugation of the desert is justified. The time will come
when not only those parts of arid regions which may be brought under
irrigation, but those also which remain unmodified in this regard, will
yield their possibilities to the hand of man, and we stand at this moment
at the serious beginning of this conquest.
* See India Rubber World, May 1, 1908.
? This work has recently been taken up anew (September rgro).
3 Ross, 1908.
CHAPTER IL.
THE ENVIRONMENT.
GEOGRAPHICAL DISTRIBUTION.
The northern limit of distribution of the guayule is in the southwest-
ern part of Texas, where it occurs in Presidio, Brewster, and Pecos (near
Langtry) Counties. This area is continuous with its area of distribu-
tion in Mexico, throughout which it occurs with greater or less frequency.
The periphery of this area runs approximately as follows: from the west-
ern extremity of Presidio County in Texas, the western boundary will run
somewhat west of south till it reaches the northern boundary of Du-
rango, near Santa Barbara, Chihuahua.!' From this point the limit turns
approximately toward the southeast, running parallel with the Mexican
Central Railway at a distance of about 100 kilometers (Endlich, 1905).
Beyond the state of Durango the boundary turns still farther to the east,
curving northward again not far from the city of San Luis Potosi.2. The
rorst meridian marks roughly the eastern boundary, lying somewhat west
of it till beyond Saltillo, where the boundary then curves slightly west of
north, reaching the eastern limit in Texas at about Langtry. The north-
ern limit is marked approximately by Fort Stockton.
The guayule is thus seen to be peculiar to the Chihuahuan desert.
The belief which has sometimes been entertained that it occurs in western
Sonora, southern Arizona, and New Mexico seems not to be well founded,
and the area within which it is found is confined to the northern portion of
the central plateau, embracing an area of approximately 130,000 square
miles, or 290,000 square kilometers. Of this area, it will be understood
that only a small proportion will be found to carry guayule, and a rough
estimate of 10 per cent would certainly not be toolow. Endlich’s (190s)
estimate, 75,000 square kilometers, is probably as nearly correct as we
may make it. It may here be remarked that the very great irregularity
of distribution makes it very difficult indeed to make anything approach-
ing an accurate estimate of the amount of guayule as to acreage alone,
aside from the question of density, so that any figures which may be given
are subject to correction.
ALTITUDINAL DISTRIBUTION.
The whole region in question is, as already said, embraced within
the northern part of the central plateau (mesa central) of Mexico and
the adjacent area within which guayule is found in Texas. This area
has an altitude varying from 2,000 to 10,000 feet above sea-level. The
‘Mr. W. H. Stayton reports seeing a small amount of guayule in the Sierra
Madre east of Sahuaripa, Sonora. The amount on the eastern slope was somewhat
greater than on the western. It is now believed to occur sparingly in eastern Sonora.
*I am informed that Pringle found guayule near Pachuca, Hidalgo, which is
probably its southernmost limit.
13
14 Guayule.
range of the plant in altitude extends from the lower limit mentioned
to about 7,000 feet, or somewhat higher. As observed by Endlich (1905),
however, the most important acreage is not usually to be found much
above 6,000 or 6,500 feet.
CLIMATE.
The climatic conditions under which the guayule lives have not
only scientific interest, but very important practical bearings as well.
This will be understood upon the reflection that many proposed opera-
tions relative to the culture of the plant involve the use of water, and
whatever the theoretical possibilities may be, success on a large scale
must be conditioned very closely by the nature of the desert areas to
be utilized. The details in question will be considered in Chapters VIII
and IX. For these reasons a somewhat detailed account of the actual
climatic conditions observed at Cedros,in North Zacatecas, will be given.
RAINFALL.
Fortunately, perhaps, for our purposes, the year (1907-08) during
which observations were begun was unusually dry, and afforded, we
believe, about the most rigorous conditions which the vegetation is
subjected to without marked unfavorable results. It is to be regretted
that data for the whole of this year can not be reported, since observa-
tions could not be commenced before the month of August. Relying
upon estimates and upon general, verbal reports, and judging by analogy
with the region about the city of Zacatecas, where the precipitation for
1907 was about half (320 mm.) of the mean for 29 years (596 mm.),’ it
seems reasonable to believe that the total rainfall for 1907 was not
greater than 175 mm. (7 inches), of which 138 mm. were recorded in-
strumentally as falling during the last four months. The growing season,
as would be indicated by the scant amount of rain which fell earlier in
the year, was a practical failure as regards crops in general, and the indi-
cations of growth in the guayyule, which at this moment concern us most,
were consonant with the precipitation, which was at best very scanty.
As will be seen upon examination of table 1 and fig. 3, the rainfall for
1908 was somewhat over Io inches, which appears to be about normal,
while the effective rains fallin the summer months. In 1908 it was suf-
ficient to produce a prolonged period of relatively high atmospheric
humidity, while the replenishment of the store of water in the soil was
marked enough to produce very pronounced mesophytic conditions. In
the low-lying flats, especially where the more abundant collections of water
were formed, annual plants of weedy appearance grew densely breast-
high, and seedlings of the mariola scattered among them grew with great
rapidity to a height of 40 to 50 cm. in one season. On the low ridges and
in the hills the available stratum of the soil was full of water, and the
guayule and mariola, together with many other shrubs and annuals,
were in full bloom and making rapid growth in June. Other features
of the distribution of rains are indicated in a general way in the diagram
and are of importance as related to the period of growth of the guayule,
to be referred to beyond.
* Boletin Mensual del Obs. Astron.-Meteor. Zacatecas, Jan. 21, 1908.
The Environment. 15
TaBLE 1.—Rainfall at Cedros, September 1907 to August 1908 (fig. 3).
Date. Millimeters. | Date. Millimeters. | Date. Millimeters.
1907 | 1908 1908
Sept. 9 | 24.4 1 Mar. 9 | a July 7 9.8
Oct. 68. | eM es va : 9 aor
is | ae : 76.7 | 21 | 45.0 Sd Tats | pte o
a oe i 27 | trace 14 | 24.0
20 8
Nov. 28 | 16.2 Apr. 3 24 21 : 4
9g | trace t 81.6
Dec. 2 220 || re 70 22 ae
9 Wowie Tes 17), |\\crace eats ae
13 9.6 | 28 9.6
| May 1 3.10 29 | trace
| 18 2.6 30 | trace
1908 27 Go| 72 31 8.4
ane, 93) trace | 20, |.206.8
18 | trace | Aug. 1 4.5]
act lcteace | June 1 ge: 6 3.0
26 | trace a sin: 7 T.5 345
| 22 Qua Aa. 12 Hole.
Feb. 1 | trace \| 23 ASA: 18 2.4
6 | trace 25 6.0 20 207 ey
NortEe.—It seems probable that the rainfall for the four months of 1907 was
relatively high, and includes an amount which normally would have been distrib-
uted earlier in the year, that is, in the summer months.
I visited Cedros during April 1909. Upon arrival there it was found
that there had been no rain, save a few drops on a few occasions, be-
tween August 20, 1908, and April 5, 1909. On the latter date heavy
showers occurred over considerable areas, leaving water standing in
‘“charcos”’ for several days. This was a very persistent drought, and it
was found to have affected guayule quite unfavorably in many localities.
I am informed by Mr. G. R. Fleming that drought again persisted till
June 16, 1909, when it was broken and a very abundant rainfall ensued
during the summer of 1909.
AIR-TEMPERATURES.
Table 2 shows observed temperatures at Cedros during the time
indicated. The lacune observable in May, June, July, and August are
not as fatal to an adequate notion of the prevailing temperatures as
might be supposed. A brief study of the table as a whole will show that
the temperatures are remarkably uniform, and this is especially true of
the months for which data are lacking. The readings, therefore, which
were made nearly every day, were not recorded except as they showed
variations of several degrees.
The lowest temperatures to which guayule may be subjected are
not known. The minima at Cedros are undoubtedly higher than those
which occur in the guayule region of Texas, but as meteorological data
for that region are lacking we are compelled to judge by those of El
Paso, the nearest station. The minimum temperatures observed here
during the last twenty years range close to zero, so that we may infer
that the guayule plant can withstand lower temperatures than those
16 Guayule.
encountered in North Zacatecas.! Attempts which may be made in the
future in the cu:tivation of the plant, e.g., in New Mexico, must be made
with regard to its resistance to cold, and it is to be regretted, therefore,
that a final datum on this point can not be given.
DEGREES FAHRENHEIT
Sy
40 = at As
>
NIGHT |
| 1 = = 1 | =! =e
JAN. FEB. MAR. APR, MAY JUNE JULY AUG. SEPT. oct. NOV. DEC.
Fic. 1—Maximum and minimum day and night temperatures by months; maximum
summer and minimum winter soil-temperatures at to cm. depth. Cedros.
It will be noted upon examining fig. 1 that growing temperatures,
though sometimes low, occur even during the winter months in the day-
time. At night, however, the air-temperatures are seen to be practically
non-effective between the middle of September and the beginning of
May. This condition, judging from air-temperatures alone, may be
regarded as resulting in a functional resting-period of at least three
months; that is, the amount of growth possible in the year would be
that occurring within nine months of time, aside from the considera-
tion of rainfall. The soil-temperatures are of course higher, and are, on
account of the high insolation, frequently favorable for the absorption
of water by the roots, which would, under favorable conditions of soil-
moisture, be important in respect to the water-content of the plant,
though it might not, except when water was abundant or under other-
wise exceptional conditions, stimulate growth. The conditions as re-
gards growth, then, may be stated thus: The winter, or resting-period,
is effective during the night-time chiefly during October and on to the
end of April. The day temperatures during this period may effect growth
when water is sufficient.
1 We now have records showing that guayule can stand a temperature of 5° F.
at Marathon, Texas, and of 10° F. at Tucson, Arizona.
17
The Environment.
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zs zQ 4 z6 zQ 36 gs £6 oS oF 89 oF | «29 | “FS aN
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g¢ a al : £S z6 | #5 06 6+ IG St £3 | ov 08 St om | 6, i Ot
rae : ¢ P “a2 zs ° oS 88 gt 06 zs Lg | +b 1g Iv 19 LA eee
SS | 06 So 66 GS 26. |) 46 zo gt S6 zs zg gt LL oy Sap ofA hie
09 | o9 | So ool : oo; || SS 06 LY z6 ivan ee SIG gt gl iy || of || 2a zg
°9 a: SOs | vs | ko Ly oor | gf z6 St 16 | vr) | oR | ad 0g zt So zs 0° {°°
‘ur | Xe | UN | “xE~ | “UN | ew | UN | XP) cu | xe | cuny | xeM | cu | -xey | om | xew | cum | xem | cu | xem e
| ees ro | go alle | | = pe neal : pl ae el
*g061 4ysnany | “go6r1 Aine | *g06r aun “S061 Av "g061 fludy “gO6r YolRyy | rgooL ‘gay | *go6r-uef | ‘L061 ‘daq | *LO6OI “AON Ae
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18 Guayule.
TABLE 3.—Maximum and minimum temperatures at Cedros.
Month. Max. Min. Mean. Month. Max. Min. Mean
our: os Oh °F. ay 2s oh,
Septnw a 82 64 72 March... 98 22 70.4
OGCt).2.5% 86 53 68 Pore Soar 07 7 7 T0
INOWaraer 85 39 60 May....| 100 40 74
Dee sex2% 85 26 55.9 JuNne:. <2. | FEO 55 75
Jate nas 86.6 22.8 Wey Waaly a. 3%. 95 50 72
ebier one 30 63.8 7S | ee go 53 67
SOIL-TEMPERATURES.
A single record of soil-temperatures extending over a period of 15
months was made by means of a standard pair of thermometers. The
instruments were buried at a depth of 10 cm.1 below the surface of the
ground at station 3. The surface had a gentle slope toward NE. by E.,
6 A.M.
Fic. 2.—Air-temperatures (/.) and soil-temperatures at 2 cm. depth. November 6, 1907.
and would therefore receive neither the highest maximum nor the lowest
minimum insolation. The indices stood at 70° F. when the instruments
were buried, on December 31, 1907. They were removed April 2, 1909,
when the following readings were taken: maximum temperature, 94° F.;
minimum temperature, 52° F.
1 This is about the average depth at which the lateral roots of the guayule
are placed.
The Environment. 19
It is seen, therefore, that at the depth mentioned the lower critical
growth temperatures in the soil are probably never reached, and it is
to be inferred that the dormant condition of the vegetation is deter-
mined by other factors, namely, soil-moisture and air-temperatures, and
of these the factor of moisture is probably the more effective.
The temperatures affecting germination, however, are those of the
surface of the soil or at a very slight depth. Fig. 2 presents the curves of
air and soil temperatures for November 6, 1907, at a time when difficulty,
ultimately shown to be due to other causes than temperature, was ex-
perienced in germinating seeds in boxes. The soil-readings are for a depth
of 2 cm., and the soil was wet, but was exposed to full insolation.
The temperatures from about 9 a.m. till 10 p.m. can not be said to
be unfavorable, though their effect upon the rate of germination and
subsequent growth would be offset by the succeeding hours of cool soil.’
The cooler period is more marked during the succeeding months till
March or April (fig. 1). Inasmuch, however, as the night temperatures
are scarcely ever favorable for germination (assuming 40° Fahr. as the
lower limit) before June or after October, and even during this period
not especially so, we may conclude that the existing temperature condi-
tions at Cedros are of subsidiary importance in determining the time of
the year when germination occurs. This conclusion is supported by the
success attending germination tests made in January (Kirkwood, 1g1o),
when the temperatures ranged from 32° to 64° Fahr. At these tempera-
tures, germination did not begin so soon as when, later on, they were
somewhat higher. It therefore may be concluded that, aside from a cer-
tain rhythm which may be detected, winter dormancy both in the mature
plant and in the seed is due, in the area we are considering, rather to lack
of soil-moisture than to unfavorable soil-temperatures. This conclusion
can not, however, be applied throughout the whole of the guayule region,
since the winter temperatures in Texas are much more unfavorable.
SOIL-MOISTURE.
The residual soil-moisture during sustained periods of drought may
be reduced to a point below the minimum necessary to sustain life. This
is the chief cause of the local dying off of guayule during such periods.
Generally, however, the amount of soil-moisture, though insufficient to
stimulate growth even if other conditions are favorable, is more than
enough to sustain life, and indeed may be enough for growth when the
equilibrium between the plant and the environment is destroyed. The
results of certain experiments detailed beyond show this to be true.
Plants at station 2, quadrat 2, were pollarded in November 1907, about
5 to 8 cm. above the surface of the soil, and these had made a marked
growth by February 18, 1908, although the surrounding plants showed
no growth at all, and indeed did not until much later on. While there
had been a very small amount of rain, it was quite insufficient to account
for the growth, even in the pollarded plants, during the period between
the dates above mentioned. We may therefore conclude that usually
1 Abbe, C., 1905, p. 36.
20 Guayule.
during dormant periods the soil-moisture is considerably above the neces-
sary minimum,' but insufficient to stimulate to growth, although, on
account of lack of facilities, a quantitative statement can not be made.
This is to be regretted, because the peculiar distribution of the guayule
in the foot-slope, while Parthentum incanum extends beyond its limits
into the playa,? is probably connected either with the superior water-
holding capacity of the soil of the foot-slope or with its greater air-
content, aside from the differences observable in the topography of the
root-systems of these plants. The naked statement that the guayule
is confined to slopes which are well drained? conveys little of explanation.
RELATION OF RAINFALL AND TEMPERATURE TO GROWTH.
Whatever is said here about the behavior of the guayule in regard
to growth-rhythm must be understood to apply to the region of North
Zacatecas, where the data which appear beyond in detail were obtained.
It is believed, however, that the generalizations are approximately true
for the whole area of distribution.*
The grand period of growth falls in the warm season, when super-
ficial soil-water is normally most abundant and when the night as well
as the day temperatures are most effective. If the rainfall is subnormal,
the drought so caused at this time results in very slow growth, made
possible only by the meager amount of water that reaches the plant
from the subsoil, derived in part from the earlier and usually small rainfall
of the previous winter, together with the more immediately available
supply from insufficient rains. This is only another way of saying that,
in the region above described, water, as compared with the otherwise
usually favorable conditions, is the prime condition for growth, and we
may best see what the habits of the plant are by observing what growth
takes place in relation to the rainfall. The extreme possibilities would
be expected to be shown by plants under irrigation during every season.
The observed growth in such plants, even in the presence of abundance
of soil-moisture during November, December, and January, is exceedingly
small in amount. Had the soil-moisture been reduced, say in Septem-
ber, so as to bring on a period of dormancy in the plant during October
and November, it may well be believed that a much more marked growth
might have occurred during the period following, when in point of fact
little growth actually occurred. This behavior would be in accord with
our general knowledge of growth-rhythm.
Although I have made no observation of positive value in this
regard, it is said by supposedly competent observers that the guayule
in the field may be expected to flower at any time, and that it has been
seen to do so in every month of the year. Flowering, however, usually
involves some foliage-stem growth as well; and so the evidence favors,
or at any rate does not contradict, the view that growth may ensue at
any time of the year. Because of the unfavorable night air-tempera-
‘Cf. Livingston, 1906. 3 Escobar, 1910.
> Tolman, see Spalding, 1909. *Cf. Bray, 1906.
The Environment. at
tures of the cooler period, however, the total amount of growth will not
be great, to which result the less effective growing day temperatures
contribute. The evidence shows further that growth in January, e.g.,
will ensue upon a period of rest coupled with an unusually favorable
rainfall, spread over time enough to produce a marked rise in the avail-
able soil-moisture as far down as the shallow roots. The times at which
this conjunction of conditions may occur is indicated, negatively at
least, when it is said that no growth in field plants was observed till
May in spite of the rain, as indicated in table 1 and the accompanying
diagram (fig. 3).
Not only, indeed, did no growth occur, but the guayule plants in
the field in widely separated localities showed a marked need of water,
a condition still more evident in April tg909. On the 11th of Novem-
ber, at Jaguey, 10 miles northeast from Cedros, the leaves were in a
very much shriveled condition. Leaf-fall began toward the middle of
December, the upper leaves, which are not cast off, being at this time
in a distinctly flaccid condition. At this time the irrigated plants showed
signs of leaf-fall, but for some time only the lowermost on the season’s
growth of stem were involved, while in the field plants all the fully de-
veloped leaves fell away at the same time.
Although, as above, seen it appears probable that growth may
take place under favorable moisture conditions even in the winter,
there is little evidence (Chapter III) that the amount is ever anything
but small. The internodes are short, and thus is produced a crowding
of the leaves; which by summer growth would be spread apart, and the
structural marks between the two grand periods of growth are less ob-
vious. As will be seen later, the dependence which may be placed in
these marks as indicating the age of the plant is not materially disturbed
by this circumstance.
RELATIVE HUMIDITY.
Unfortunately no instruments were available at Cedros for the
study of relative humidity, and it is especially regretted that an atmom-
eter after Livingston’s design was not at hand. The only data obtainable,
aside from my general observations, are those issued from the Observa-
torio de la Bufa at Zacatecas. A curve of tentative value based on these
is presented in fig. 3, and, while this can be regarded as only approxi-
mate, it serves to indicate that the relative humidity is relatively high
at Cedros (though not as high as at Zacatecas), and that there 1s a some-
what prolonged summer period of high humidity. The following re-
marks accord in general with these conclusions.
Dew is frequent during the cooler months, and was sufficient to run
off the roof of the house occupied as a laboratory, the material being of
painted canvas. The dew-point is always approached closely at night
and usually passed in winter and during the rainy summer season. The
high relative humidities which occur at all times during the night, and
in certain situations during the day, at least during growing periods,
are reflected in the vegetation. Only when this factor is taken into
consideration can we explain the pronounced contrast seen between
22 Guayule.
the vegetations of the north and south facing slopes (Lloyd, 1909), and
the peculiar distribution of certain plants, notably epiphytic species.
A most instructive example is offered by Tillandsia ciliata, which is to be
found epiphytic chiefly on the ocotillo (Fouquieria splendens), on slopes,
mostly steep, where the drainage of cool air of high relative humidity
passes downward from higher levels. The ocotillo itself grows in the
more arid soil of southerly slopes. The Tillandsia (‘‘pastle’’) occurs on
other shrubs also wherever the most favorable humidity conditions are
to be found, namely, in arroyos and narrow cafiadas receiving air-drainage
from adjacent high land, and I have seen a small amount in open flats
many miles from the mountains, where, during the rainy season, water
stands for some time over large areas,! thus producing similar conditions
in less marked degree.
elena!
MILLIMETERS
Fic. 3.—Monthly precipitation at Cedros, and relative humidity at Zacatecas city.
We may therefore conclude that the atmospheric humidity in this
region is for a desert markedly favorable for vegetation, and may be
called into account to explain the denser total growth of this desert as
compared with the region immediately about Tucson, Arizona. What
biological relations between plant structure and the conditions described
above may be found is a problem for the future, the importance of which
I have elsewhere pointed out (Lloyd, 1908b). Ross (1908) refers to the
occurrence of dews in the guayule region and suggests that the dense tri-
chome structure may be related to the absorption of atmospheric mois-
ture, but offers no evidence. At the present time we may do little more
1 As in the ‘‘laguna’’ in the Camacho bolson, east from that place.
The Environment. 23
than attribute to the high vapor-tension a general dampering effect upon
evaporation, both from the plant and from the soil, but it is not improb-
able that research will discover plant-structures which are specifically
related to atmospheric humidity, especially as it has been shown (Lloyd,
1905a@) that the ocotillo and probably other plants have the ability to
take advantage of rain which has not yet reached the earth.
TOPOGRAPHY AND SOIL.
The surface of the high plateau of Mexico on which the guayule
finds its home is broken up into mountain ranges of various extent, sep-
arated by wide, flat valleys or ‘“‘bolsones.’’ The middle reaches (playas)
of these valleys are nearly level and have a deep, fine, alluvial soil,
containing a vast amount of capillary water. In this soil the mesquite
is generally found in abundance, and often of large size. Within these
flats are frequently found more or less extensive areas (alkali spots, salt
spots) where salts have accumulated and where the salt-bushes (Atriplex
sp.) only may be found.
From the periphery of these alluvial plains, extending to the foot-
hills of the mountain ridges, is a gentle slope of low gradient, the foot-
slope, characterized by a gravelly soil (plate 5), which becomes more
and more stony as the foot-hills are approached. Here the soil is fre-
quently very shallow and may be confined to the crevices of the under-
lying rock. This condition becomes still more marked in the hills proper,
where the edges of the strata are often exposed and where the vegetation is
confined to the intervening fissures. The most widely distributed plants
of the foot-slope and adjacent ridges, and therefore the most characteristic,
are the alvarda or ocotillo (Fouquierta splendens), the palma samandoca
(Samuella carnerosa Trelease), and the Cedros sotol (Dasylirion cedro-
sanum Trelease).! The gobernadora or Mexican greasewood (Covillea sp.)
is also a very common plant of the foot-slopes and ridges, but is to be
found also in the alluvial plains and is therefore less characteristic.
Of the species of Parthentum found in the region, the guayule is
confined to the foot-slopes and foot-hills,? being also abundant in hills
not above about 7,000 feet in altitude. It is therefore, like some of its
1 Dasylirion cedrosanum Trelease (n. sp.).
Subacaulescent. Leaves slightly roughened on the dorsal angles, pale, the
upper face glaucous, somewhat fibrous-brushy at tip, broad (20 mm.), 1.5 m. or
more long: prickles mostly 10 to 15 mm. apart, yellow or at length reddened at
tip, 3 to 5 mm. long, moderately heavy, upcurved or hooked, the whitish-yellow
intervening margin roughened by minute hyaline tipped denticles. Branches of
the narrow inflorescence rather elongated, about 7 by 60 mm. Fruit narrowly
elliptical, 4 to 5 by 7 to 9 mm., deeply and acutely notched, the style much shorter
than the wings.
Cedros, Zac., Mexico, Lloyd, No. 118—the type, No. 82, 1908; Kirkwood,
No. 96, 1908.
Allied to D. wheeleri and D. graminifolium, from both of which it differs in
its smaller fruit not widened upwardly and with shorter style more conspicuously
surpassed by the wings, (fig. 4, and on the extreme right of fig. A., pl. 1). ;
The type is in the Herbarium of the Missouri Botanical Garden; cotypes in
the Gray and National herbaria.
2 It is generally believed by those familiar with the plant that it affects more
particularly the south slopes, and this accords in general with my observations,
though it must not be inferred that it does not grow at all on north slopes.
24 Guayule.
associates above mentioned, an ‘“‘edaphic’’ species, found only where
the ground is stony. In the alluvial plains one meets only an occasional
isolated plant, but if the plain is traversed by a low ridge of gravelly
ground, even if the surface is raised only a few inches above the surround-
ing area, the guayule may be found. In the fine soil of the plain, on the
other hand, the mariola (Prathenium incanum H. B. K.) and the annual
species P. hysterophorus grow in abundance, though the mariola is com-
monly associated with guayule on the foot-slopes and hills. This asso-
ciation of guayule and mariola frequently misleads the inexperienced
observer in estimating the amount of guayule which may be found in a
given area.
Why the guayule does not grow in the fine alluvium is not clear,
and is a question often asked by persons familiar with the facts. Any
reasons, aside from those mentioned above, which may be assigned are
at present of only speculative value, but some reference may properly
be made to them.
Fic. 4.—Dasylirion cedrosanum Trelease. Type material in the lower row. Above, for comparison
fruits of D. wheeleri at the left and of D. graminifolium at the right. X 3/1.
Guayule is confined practically to the Cretaceous region of the Cen-
tral Plateau, and therefore to highly calcareous soil (see Chapter IX).
It may very well be that the plant is sensitive to even a slight acidity,
and therefore prefers a soil with a very small amount of humus. Certain
experimental results referred to beyond, while not conclusive, indicate that
this explanation may apply during the period of germination, but it has
been found that the absence of lime is not a hindrance to maturer plants.
It is a popular notion that the plant “rots”’ in situations where
water is relatively abundant, and that for this reason it is not to be found
in ‘‘bajillos’”’ or low places. It is true that for a considerable period in
the summer season practically mesophytic conditions prevail in many
areas within the flats, especially in the frequent slight depressions. Here
annual weeds grow in profusion, and a number of species of desert shrubs
flourish. Among these is the mariola, the seeds of which germinate
freely among the dense vegetation of shrub and weed, and in one season
«
A. Station 2, Quadrats 5 and 6, foot-slope of Sierra Zuluaga.
B.
Station 3, Quadrat |, near Cedros.
A good stand of mature plants.
ta
oO
ac
trom Qua
ants
>]
The Environment. 25
attain a height of a foot or two; but this is not true of the guayule. That,
however, the mere quantity of water or the density of the vegetation
are not the determining factors is shown by experimental evidence,
while in the field are to be found numerous instances of plants which
have germinated in the dense shade and dampness found beneath the
dead leaves of the sotol and in crowded conditions produced by other
plants, such as the lechuguilla. Indeed, these are frequently the only
conditions under which the plant gains a foothold. It therefore does
not appear probable that the abundance of water or the density of the
vegetation is the determining factor in preventing the guayule from get-
ting a start; hence we may infer that the conditions below the surface
must be understood before an explanation may be had. The edaphic
habitat of the plant suggests that the mechanical conditions of the allu-
vial soil are unfavorable, owing to meager aeration, in connection with
which the humus conditions also may have to be taken into account.
DENSIFY OF “GROWTH.
Of great importance economically as well as to the student of vege-
tational problems is the number of plants per unit of area, both abso-
lute and relative. The operations of the forester rest upon this datum
in the first instance, as this, together with the size of the individuals,
forms the basis of calculations of the available tonnage per acre. It
will readily be understood that any estimate on a large scale will involve
a necessarily large error, since it would be impossible to do more than
proceed on the basis of sample counts combined with acreage and esti-
mates of size. This can frequently be done with great accuracy by persons
who have had practical experience in taking guayule from the field,
especially if the judgment be checked by survey and sample counting
and weighing. The following tables, the data for which were obtained
by accurate measurement, will, however, serve a useful purpose in indi-
cating a method of making estimates, as well as in furnishing indications
of actual conditions. For the purpose, quadrats of too square meters
were laid out by means of a steel tape, the data obtained attaching to
the guayule plants within each such quadrat.
The weights in the following tables are field weights. For dry
weights a reduction of 20 to 25 percent is necessary. As field weight is
usually assumed, however, I have followed the usage and have not applied
the above correction.
26 Guayule.
TABLE 4.
Two adjoining quadrats, each of 100 sq. meters, on a loma or ridge extending
toward the Sierra Zuluago, about to miles north of Cedros (plate 5, fig. A). All the
plants were pulled up and sorted, each package containing plants of similar size and
habit. The packages were then grouped into classes arbitrarily, and a typical plant
for each class photographed (plate 6). The age of this plant was carefully estimated
and checked by estimating the ages of a number of similar plants. (March 29, 1908.)
| |
No. of : Average : Average
ct : Weight of f Estimated :
oi planis.in | "package. | Mematol | "age. | height o
lbs. lbs. years. com.
20011) = | peo pS
| ES ee ee ee en ee ee 28 Pell | 9.85
20 Baty 185 ED Ep ome 58
20 17 0.85
20 | 13 | 0,65
Zo! LOS On52
TED cy. cgeeatotarcencts) ot ass ie ra ens eenrentelee 2 0.45 |
Zoe) OLG O52 Se ae) 3°
20 9.5 0.47
20 I2 °.6
WielaSEy ¢ Paes 0.381 |
ae igh ae A Ried ate mente 40 | 10.25 0.25
III | par MEE SR: 7 to 10 25
40 bike) 0.25
{ 60 8.25 0.14
LW. atest 07s AId Shaman Se ots: 120 18 0.15
50 7.5 o.14 5 to7 | 20
70 II 0.16 |
f 60 | || pO
80 8 O.1
60 6 Onue
Wane ve siete ASE Ae su! Ses 100 7-75 0.08
| 183 18.25 One: aoe TS
100 Tbe 5 OnE
100 ie) ONL |
| ons 2 OPE |
ee ee ee eee
| |
MOtalth: Mota och. 1371 Bognor || leer oll Weeks aac srcdee
1685 plants in roo square meters.
Class V is made up of all sizes up to the maximum indicated. In plate
6, figs. 5 to 7, examples of three sizes and ages are shown. The weights
and estimated ages of all the plants in plate 6 are shown in table s.
TABLE 5.
Class. | Age. Height. Weight.
years. cm. oz.
Re senna ee 15 58 £85
1 EA eae ace stir 30 6.
1b) Bren aif cee 8 25 1.875
IWVe re eee 7 20 0.875
Wito dares ise 5 15 C55
Wali Seer 2 | Le 0.05
Wallen occ To Gil) wee meee
The Environment.
27
TABLE 6.—Station 8, quadrat 2 (100 square meters on low ridge just north of Cedros).
Class.
Total.....|
No. of
plants in
class.
HHA eRe PRR BHHH RR WH HAA
NN HAN ND
——— SE ———— ———————e*"*E
H
WE NHN HHA RRR ORO
-
~I
on
Total weight.
HH eH RR OH RO CON NNN NWO
eS
883
°
°
a Se
Hf f
Lal
Leal
oO
14
HWHNHNNWHR OAC FF
Individual
weight. Height. Age.
lbs. oz. cm years.
ae 4 7° 2)
gai) 60 20
3) 0 70 22
3 6 60 15 to18
3 8 60 to 68 20
2735 45 to55 16to1& |
2 4 68 20 (about) |
ae ra) 55 20
Zo 50 15 to 18
210 50 15 to 18
a GO 70 ()
ony 35 45 to 50 718 to 20
i) KO 40t045 15
4 46 15
De digi 65 17 to 18
Te PISE SS 45 to 50 15
I 00 40 7 or 8
im 13 37 7 Org
1) 16 40 ro to 12
© 97 43 12
Be) SI, 9
15 40 7to8
15 25 +O) tO LS
ea 35 bite)
II 40 8
II 38 255
Io 40 -
8 35 29 to 10
6 30 9
4 30 8
3 30 be)
2S 27 6to7
25025 23 5
| 2Peh235 27 6 to7
a5 20 (7)
1.875 27 6 to7
25 20 5
E. 3 to4
1QOld scraggly plants whose age it was impossible to determine even approximately.
2 Sl
3 Tall and slender.
4 Retofios.
5 Very small.
6 Classes I-IV, inclusive.
owly growing, densely branched plants.
28
Guayule.
TABLE 7.—Station 8, quadrat 1 (100 square meters on low ridge just north of Cedros,
July 20, 1908).
No. ot
plants in
package.
lbs.
I 6
| I 6
cc
hae z
oe 4
I 4
Lady 2
I 3
I 3
Pee es
I 3
I 2
Rea S
I 2
T 2
i 2
2 4
I 2
I nme)
5 10
(eer I
Io Io
Ame) be)
me) Tot
I I
I I
I I
6 ine)
6 4 B
21 De
5 I
eS
81 135
Total weight.
S
NW DANY Of BFA
Individual |
weight.
~
WHWHW KRARBAR QOS
LADY oS
NHHNHHN NNN ND
oo
we HR HR HOW
nN
Height.
1 Retonos.
The Environment. 29
TABLE 8.—Station 9, quadrat 1 (100 square meters, on a 25-degree northeast slope,
in the hills east of Cedros).
No. of |
| a Individual | am
Class. ee | Total weight. weight. Height.
| lbs. oz lbs. 02 cm
ie aan eh feRieg MOORE eae Ie ors ee ae 60 to 65
erergute ice ; 3 ee A Be
2 4 10 B15 50 to 60
2 4 4 2 2, 55
Il a eetisjiclie/ sw ehe)je vl wel sl © © eee, 6) (6 2 4 6 2 3 35 to 40
5 ‘ore fo) 2 3 60
I 2 fo) 2 fo) 45
5 9 GE I 14 40 to45
3 Eg Et ag 5°
I in Ke) | 1 a 5°
| 2 3 ie) I
(Ut bt CE tree conte Cio Geno | : os ; e 6 ie
| I I 4 I 4 45
| Dy it Fgh I 2 45
I I ° I ° 40
| 4 gonna 15 35
| | 10 be) 45
| | 4 2 2 8.5 | 25 t035
I 5 | as
I a5
| I 2.525
| 2 B25 1.625 |
| |) och kt T5
| at 1.375
Ee eH 25
Le. Os Se ee A eee Se aa | ei Sak.
| m | .875
| I a5
| 2 1.25 .625
| Tinh) PS Ogee as
20 5 25
| 20 Fy 06
| 20 leit
| 4 St25 03
| 8 | L2G Or5
| Wy | SSE |
| — | = ————
1 Retofios of small size. f
2 Total weight, classes I to III, inclusive.
30 Guayule.
TABLE 9.—Station 2, quadrat 7, April 3, 1909.
[The data for this table were obtained by pulling up, sorting, and weighing all the
plants on roo square meters ina guayule field from which all the plants above
40 cm. tall had already been taken. (Plate 1, fig. A.)]
No. of plants. Weight. Average weight. Height.
lbs. 032. 02. cm.
100 2 eo 4 25 t035
60 it i c 3 30
60 TO eee Boh 2S LONsS
60 10 8 2.8 20035
31 fee ee 3-7 25 to 30
60 12 ae 2.5 22 to 30
50 4 8 I.4 20 to 30
50 7 4 Bre 20t025
50 (Qe ay 2 20 tO 25
50 4 4 i. 26 20 tO 25 |
50 5 8 T7710 20 tO 25
64 3 4 0.8 18 to 20. |
61 2h ee 0.5 15 |
19 a6 0.125 aera: | = |) Mo chorroecn.: |
755 Ti) eee wes eee, ie Wl daa ttn orers:
1 All seedlings of 1908, except one of 1907.
TABLE 10.—Station 9, quadrat 2, April 14, 1909 (100 square meters, ridge of loma in
hills (El Potrero), east of Cedros).
No. of plants. Weight. Average weight. | Height. Remarks. |
lbs. 02. 02. | cm
I 4 4 68 70
I 3 48 65
I 3 be | 48 65
I 2 8 | 40 | 63 Scrubby.
I 3) 12 60 | 62
I I Te? 28 60
I 2 4 36 60
I 2 a 32 57 Rather scrubby.
I War pea 2 | 28 55 Do.
I 2 ae 36 53
I ee 92 50 Spread 100 cm.
I 2 4 36 50
2 ant Mire 22 44
4 3 12 15 | 40
I Hash: BES Bo 24 4 to 5 years seedling.
I ame et 4 I 24 Badly developed.
I | 2.5 2.5 23 Seedling, 5 years.
I | Oe 2.5 16 3 years retono.
I | 0.5 Ons II Seedling, 2 years.
I | ONS 0.5 14 Seedling, 3 years.
24 ea 6
The Environment.
31
TABLE 11.—Station 10, April 5, 1909 (quadrat of 100 square meters).
[On a southerly slope 10 kilometers north of the Cerritos de los Calzones.]
No. of plants. Weight.
lbs. oz.
I 2 4
I 52 8
3 4 4
5 6 ane
5 9 “
6 6 12
4 5 8
8 6 4
8 6 a
2 ee 8
5 6 8
5 5 12
7 5 8
ae) 6 a
8 6 8
5 3 8
16 4 ar
5 4 .+
nde) 3 ie
nae) 2 8
16 2 12
I2 ly ae
6 a5 4
a me
if
7
8
3
186 IOI 4
Remarks. |
Average weight. Height.
Oz. cm,
36 65
8 50
23 40 to 50
19 | 4gotos5o
29 45
18 | 35 to.so
22 30toO 50
10 al 35 to4o
12 35 to 4o
4 30 tO 40
14.8 30 to 4o
18 35
T2086 35
9.6 35
13 35
TeTeZ 30035
4 30 tO 35
13 25 t0 35
4.8 20to 40
4.4 25 to 30
2.75 25
5-3 20 tO 30
0.6 I5 to 20
Note. — The shrub of |
this region is rough
looking and rather badly |
attacked by insects. A |
good deal of witches’
broom, and many plants
attacked at the base by |
borers. |
Seedlings, 2 to 3 years old. |
Seedlings, 2 years old.
Seedlings, about 3 yrs. old |
Retonos.
Small plants.
1 Scant.
TABLE 12.—Station 11, near Caopas, April 6, 1909.
No. of plants.
Bo corr CH QU HH
Weight. Average weight.
lbs. 3. Oz.
2 10 38
i 286) 26
a) 07] 3
7 4 22
6 ° 16
I hae 16
6 12
5 16
& II.4
AG wt i: 8
2 8 4.4
ae 8 5
4 12 6.3
Bure ah is: ea
Ce eee by Re
2 8 3.3
2 8 i. 5
2 8 4
2 4 Dey.
Tete ais OL5
« LO 0.6 |
. 4 0.2 |
Be °.8
Io 0.8
7G 4.5
Height.
eee eee
17,100 pounds per hectare.
32 Guayule.
TABLE 13.—Station 12, foot-slope ridges south from A pizolaya,
April 10, 1909. (Plate 7, and plate 1, fig. B.)
|
| No. of plants. Weight. Average weight. | Height.
|
| Ibs. oz. os. | cm.
2 4 8 oe | 60 to 70°
5 ie) ee 32 | 60 to 70
5 RO! 4 ee as 32 | 60 to 70
= 9 8 30.4 60 to 70
5 9 8 30.4 60 to 70
5 oO” Be aez: 28.8 | 60 to 70
5 0 be ata 28.8 | 60 to 70
5 8 Me. ens 25:6 60 to 70
5 | @ 12 24.8 | 60 to 70
5 0 payer ars 22.4 | 60 to 70
5 6 8 20.8 60 to 70
I Zi atin 48 | 60
| 5 9 8 30.4 | 60
| 9 | HO") Seat re 10.7 | 60 |
5 | "2 8 8 50 |
5 | 23 “Sheth ee | | 50
9 A eae oye Fist | 50
7 3 8 8 50
5 4 4 | 13.6 40 to 50
6 4 ro) | I2 DG
5 [ee toe || 2S - 40
faa is 3, on8sae| | tree 40
| 5 4 4 13.6 40 (1 at 50)
| 10 3 cnet 4.8 40
| 6 5 4 14 35 to45 |
| ste) 2 4 | 3.6 35 to 4o |
| 12 2 4 | 3 30
25 2 by eo 3 30
20 2 4 20) 30
25 2 8 r0 30
28 I 12 I 25 to 30
20 2 12 Qe 25 to 30
ys sie) 5 30
53 OBS il Ae 18 to 23
| 4 eel lt awenting 18 to 23
Situdsht| i \sigateSecoaiih oir | 22
Add, making additional weight—
10 3 4 40 to 60 (scrappy)
SCraps seal Beate
ie 172 5 ote |
Summarizing the above results, and including data from other sources,
we have as follows:
Station. Quadrat. No. of plants, Station. | Quadrat. No. of plants.
| |
= | = 0 Amaia om = pat
4 I 50 | 8 I 81
5 I 275 9 | I 130
2 3 | 360 a! | i PS
2 4 270 9 | 2 2A)
| 2 I 285 se) 186
5 I 30 | II 279
2 15 and 6 685 re ZIT
| 8 2 a5 ||
1 Averaged.
S ” a
‘tue eee eS .,
YS oe . be D a
A. Quadrats (station 12) in a very dense growth. Apizolaya
B. The same, the guayule removed.
The Environment. 38)
Here there is a range in numbers of plants from 2,400 to 75,500
plants per hectare, but the meaning of these figures can not be under-
stood unless the size of the plants is taken into consideration. From
the point of view of business opportunism, a stand of 2,400 plants per
hectare may be better than one of much higher figures, while for one who
is looking for a basis for permanent investment other questions of rela-
tive sizes and numbers of plants arise, the answer to which involves an
explanation of the rate of reproduction in the field. This subject will
be treated in detail in Chapter IV, it being our purpose here to show
the actual condition as viewed by one who is estimating the tonnage per
unit of area.
If we refer back to table 4, we will observe that the two quadrats
contained 1,371 plants, the average weight of which was a little over 3.5
ounces. Of these, however, only 80 were large enough to be gathered,
namely, those about 1 pound or over in weight; though if the land were
being exploited smaller ones would be taken, say those weighing above
half a pound. This would include all of the plants in classes 1 and 11,
weighing in the aggregate about 58.75 pounds, or 5,875 pounds per hec-
tare, assuming the quadrats to be fair samples, or about 2.67 tons (long).
Treating the remaining tables similarly, we have the following
figures:
TABLE 14.
| a BS a
} No. plants a No. plants | Average weight
| Table No. above Weight of Weight per hectare. below : | of age ele
8 ounces. Z 8 ounces. a plants.
| eel alk =
| |
lbs. oz. lbs. | tons (long). | lbs. oz.
4 80 Fore LZ Peisseae NC) 264. “| 605 | Op ales
6 45 83 14 $399 | 3-7 30 Ir 13.8
7 69 131 7 13,140 oO = 12 els
8 | ° 43 69 8 6,950 20 go I 9.8
ite) 18 40 5 AxO2 300 |i) ED-0 6 2 5.8
It II4 83 8 8,350 347 98 On Tan
12 47 43 2 4,374 | 2 — 232 | 14.9
13 Er L3i7 ° EOS OOP wnt 200 seare 3.4
It will need but a glance at the above summary to show that, from
the business point of view, the acreage of large but comparatively few
plants is the more valuable to the purchaser who is not looking to the
future, for the reason that the cost of harvesting a small number of
large plants will be less than if the available plants are large in number
and of smaller size, and because the larger plants can be handled more
readily and therefore more cheaply. Furthermore, it is much easier
to determine the tonnage with fair accuracy where the plants are few
and large. The error due to applying data taken from small sample
areas to an extensive area within which the sample area falls, must of
necessity be large, for the number of plants as well as their character
must be considered. Taking the question of number alone, the size of
the error on this score will be appreciated when it is known that on an
area of 42.7 acres at Station 2 (plate 1) 181 bales of guayule, or at the
3
34 Guayule.
rate of about 800 pounds per acre (1,976 pounds per hectare), were actually
collected. As this was gathered under the rule that no plants less than
4o cm. in height or in spread were to be taken, some plants which would
run over 8 ounces were doubtless left, but allowing for this error probably
not more than 2,o00 pounds to the hectare could have been taken, or at
most 1 ton of 2,200 pounds. On another area of 30.8 acres of the same
general character, but of thinner stand, 53 bales or at the rate of 344
pounds per acre (about 850 pounds per hectare) were gathered.
It will thus be seen that the difficulty in estimating tonnage per
unit of area with small error is at best very great, and this, as already
said, is rendered more so by the difference in the character of the plants.
To judge of the truth of this, one has but to examine the various illus-
trations accompanying this paper. In particular, a comparison of two
prevalent types is shown in plate 8, namely, a slender and a spreading
type, but neither of extreme form.
TABLE 15.—Dimensions of narrow and spreading types of shrub, illustrated in plate 8.
Narrow type. Spreading type.
:
Plant. Weight fresh. Weight dry. | Height. || Plant.| Weight fresh. | Weight dry. | Height.
| |
lbs. 08. Ibsysos- |) toma lbs. 08. lbs. 02. cm.
A Ale (6) 2 } Og 9 he 2k 3 6 lh" Ae ea 50
B 20 jaglitst= 48 B 22 [)* pexerecg 45
C i a2 15 46 C i © site ase a5
D 8 Soo | Ss D he 5 23
E 6 4 28 E 5 Boys 21
F 3 1.5 24 F Boz. | O03" || “27
G 25 Ons 23) |
From the above data it is seen that, speaking broadly, the weight
of plants of the spreading habit is one-third to one-half greater than those
of the narrow type of similar height, so that a stand of the latter must
have a density correspondingly greater to equal in total weight a given
stand of the spreading type.
As one looks over a “‘field”’ of guayule, these apparently minor dif-
ferences of form are not at all apparent, because of the interference of
other vegetation with the vision. If the occasion presents itself when
more accurate estimates will be demanded than at present, this condi-
tion will have to be taken into account. It should be further mentioned
that the weights given above are of freshly gathered plants. If it is
desired to calculate to ‘‘air-dry”’ shrub, the proper correction should
be applied, but as this is very variable, according to the season, no con-
stant can be given. It may, however, be as great as 22 per cent in the
dry season.
The only other published calculations of this kind were made by
Endlich (1905, p. 1118), who, for the purpose of calculating the area of
guayule land necessary to support the industry, assumes the average
weight of the plant to be 500 grams, and the density of growth to be,
by weight, 500 to 800 kilograms per hectare, or from 1,000 to 1,600
The Environment. 35
plants per hectare of 500 grams average weight, taking into account the
unevenness of distribution, that is, the more or less extended areas where
guayule does not occur. The following figures are deduced from the
quadrats above detailed, taking all the plants into account :
TaBLE 16.—Number of plants in given areas.
Table Nos. No. of Average! weight. Kilograms
recording | plants per per
quadrats. hectare. Gunes Gis Pu hectare.
4 68,500 am BA ee 6,795.2
6 7,500 Ton 521.6 2,02
7 8,100 26.56 753.0 6,099
8 13,300 825 241.0 3,205
10 2,400 Dole Shaft 1,850.64
II 18,600 8.71 246.9 4,592.34
Tr 27,900 Apu 116.79 Be25on 72
13 31,100 8.86 252.97 7,867 .36
Ave..| 22,175 Tg | Byes) | A,O7 2553
From the above it is seen that the average in long tons per hectare
is 4.67, per acre 1.85.
The average weight of all the plants on the quadrats is thus seen
to be less than Endlich’s estimate by 125 grams, or one-fourth, and as
these sample areas include the very best guayule land, that is, the densest
areas with the largest plants in relation to the density, it may be con-
cluded that the present estimate is more nearly correct. In estimating
the average density over large areas, great difficulties are met. Endlich
assumed one-tenth of the area of the guayule region to be occupied by
the shrub at an average density of 500 to 800 kilograms per hectare.
This figure does not approach the indications of our data, though it
must be remembered that these do not take into account poor areas
where the shrub is very scattering or nearly absent—as the Mexican
well expresses it “‘salteadito.’’ For certain areas, e.g., one of 1,800,000
acres (728,744 hectares) which has been somewhat closely studied for
the special purpose of estimating the amount of shrub to be found there,
Endlich’s factor was found to be very small, for if only one-hundredth
of its area carried guayule in the quantity of our general average, there
would be as much as of one-tenth of it which carried shrub of the amount
of his factor. We may feel sure, however, that our average applies to
more than one-hundredth of the total area. Whether Endlich’s figure
applies better to the total guayule area of Mexico can not be said with
any certainty, but it is only fair to say that, in view of the great diffi-
culties involved, it is probably as near the truth as any that we might
venture,
36 Guayule.
BIOTIC RELATIONS.
COMPETITION.
The relation of guayule to the other plants with which it is com-
monly found associated is of great importance, especially if forestry
methods are contemplated. Both the mutual effect of each element in
the vegetation upon the guayule and the relative rate of growth must
be understood in order to judge what the final effect in the struggle for
existence is likely to be. To do this, however, involves a very consider-
able amount of sustained observation by means of the quadrat method,
first devised by Clements. Following is a census of the more important
plants found growing in association with the guayule in quadrats 5 and
6, Station 2.
Ashen a7
Scientific name. | Common name. pes aioe Gy |
Parthenium argentatum......... Guayule see ee ere mot 7ae
Acave lechesuillay 2h. de sco ect. Weel ouulll ase eye | 50
Covillea mexicana...--.. +. paces Gobernadoran-.. caren | 6
Samimuellarcarnerosaane a ieee Palma samandoca....... : 4
Dasylirion cedrosanum .......... Socoliens 5 eorex I
INCACIA TABRESIanlat any cose ee ome es Epaisachte Sets tc.ao ee ete 7
Jatropha spatulata:.. 4. tote... e Sangre de drago:... ...5t. 8 Scattered all
over.
TGSGosouky OK PUO ME AA a eee OHS |\ Oo ob clon Geooopc ode aoe 6
Lophophora williamsii........... Peyote (peyotl)..........| about 20
Opuntia mnegalarthra:sy).942. Sor Rastreroy.a40- Spiele ee 45
Also the following, from Station ro:
TABLE 18.
Scientific name. | Common name. “ oe eee
Parthenium argentatum......... | Guayulecae rare enon | 186
Parthenittniamcatntiiiene. wes ee: Marto laisse en eee ieee | 14
Opuntia stenopetala’ Wuyi Nopalcolorado..22te ee 5
Opuritia,micradasys oni Sy Hatori Se oad On 5 hese nectar Peels a A
Covillea MEXICANA. ua o.com 5 bese | MRODEE MACON. aes acne tell 8
Opuntia imbricata. :+ atc. tee. Gardenche 28 Ss Re ad I
Saliviarelmanze dinvordess: mt 125 ck. Bneorda ¢abrasicsnueise 3
Dasylirion cedrosanum.......... SOLO) = Seco cuceseeeheistate | I
fa O22 os ems entnameiiaeinh et a Stott es _ Several small
| inconspicu-
ous plants.
Samuellacarnerosals. a. -t-c. ale ai Palma samandoca.
AGAVE ASPELDIMA Sf ste eye 5 ate eee Maguey.
With few exceptions, these constitute the dominant vegetation of
the foot-slopes and the low ridges, though of course a number of other
species may be found in other localities, and indeed may be more impor-
tant elements elsewhere than has been observed to be the case in North
Zacatecas.
PLATE
LLOYD
D:)
|
(See table
pe of guayule.
ty
og
cS
type of guayule. B. Spreadin;
A. Narrow
The Environment. 37
A few of the more obvious of these are:
| Scientific name. Common name. |
| |
| ; : :
|) Bchinocactus palmer: “=. 738 4.072). Biznaga burra.
(* Potugtierta splendens... 5... 2... 54- Ocotillo or alvarda. |
Echinocactus pringlei.............| Biznaga colorada. |
Buddleia marrubiifolia............ Asafran. |
|
The above enumeration indicates that at the present time the guay-
ule in this habitat is far and away the most important plant numerically,
and is therefore dominant in the usual sense. Whether it will continue
so—whether its dominance is waxing or waning—may be indicated by
the relative numbers of guayule plants of different ages and by the inter-
action of the various elements in the vegetation.
We may therefore consider briefly each of the numerically most im-
portant species.
LECHUGUILLA (AGAVE LECHEGUILLA).
While the actual number of plants of this species found in quadrats
5 and 6 is much larger than that of any other save guayule, it is very
small compared with the number which is found on much guayule land
(e.g., plate 5, fig. B).
In common with the Agavee, the plant propagates itself chiefly by
means of stolons which lie a few centimeters below the surface. In this
way it spreads from an original plant radially, taking up the ground as
it goes, from which nothing but death dislodges it. In the course of a
few years it attains maturity, when a tall flower-stalk is developed; then
the whole individual, consisting of a single cluster of leaves attached
to a short (10 to 15 cm.) and thick (6 to 7 cm.) stem, dies. Where the
lechuguilla has occupied the ground for some time, it frequently forms
a dense growth, from which other plants, save a few annuals or emaciated
perennials, are excluded. Its manner of spreading, by which it repro-
duces itself vegetatively, enables the plant to occupy areas in which the
soil is confined to the crevices of the rocks, and in this manner it may
occupy ground which is unfit even for those desert plants with which
it is usually associated. From it is extracted the fiber ‘ixtle tula,’’or
“ixtle de lechuguilla,” which is of considerable commercial importance,
and thus the plant is of some value—not, however, sufficient to justify
it as a competitor of the guayule. The method of vegetative reproduc-
tion above noted is also characteristic of the guayule (Lloyd, 1908c),
especially when growing where the country rocks come to the surface,
but is in this plant of relatively much less importance.
The mutual behavior of these two plants under strong competition
is not very easy to describe precisely. It seems clear that, with the excep-
tion of a few plants which succeed in gaining a foothold by germinating
in the shade between plants of lechuguilla, sometimes being favored
by the protection from drying out and from cropping by animals thus
afforded, ground occupied by lechuguilla is much less favorable for the
38 Guayule.
growth of guayule than that from which lechuguilla is absent. For
although it would seem that germination and early growth are favored
by the protection offered by the lechuguilla, as a matter of observation
one finds but few young plants of guayule in such situations. One reason
for this is, probably, that the guayule seeds (achenes) find difficulty in
reaching the soil, because the leaves of the lechuguilla catch them and
hold them in their axils till they die, thus materially reducing the num-
bers which reach the ground. Aside from the consideration that the
lechuguilla takes up from the soil its quantum of water, its effect upon
guayule is unfavorable, therefore, because of its superior powers of pro-
gressively and steadily occupying the ground, and because of the loss
of guayule seed by being caught in its leaves. Lechuguilla appears to
be an increasingly dominating type in every situation where it gains
a foothold. It is common to every part of the foot-slope and in the
hills throughout the range of guayule. The great quantity of it to be
found produces in many parts of the mesa central the dominating yellow-
green coloring often seen there. When it and the guayule are associated,
the green is dotted by the gray of the latter, although other plants also
may contribute this subdued note in the coloring.
GOBERNADORA (COVILLEA TRIDENTATA) AND OCOTILLO (FOUQUIERIA SPLENDENS).
These may be considered together. Their forms are similar because
of the habit of their slender branches, which arise from near the base
and reach obliquely upward, producing the effect of an inverted cone.
They are both taller than guayule, but the shade cast by them is small
in amount, and less is cast by the ocotillo than by the gobernadora. The
only places where the ocotillo grows thickly are in certain situations on
south slopes, and here it often forms a dense thicket. When thickly grow-
ing it would interfere with the rapid harvesting of guayule because of the
thorny branches, but, excepting for the draft it makes on the soil for
water, the effect upon guayule is negligible. This applies about equally
to gobernadora, which in North Zacatecas, however, reproduces itself quite
rapidly by seed, and so may readily come to occupy too much ground.
PALMA SAMANDOCA (SAMUELLA CARNEROSA) AND SOTOL (DASYLIRION CEDROSANUM).
These are similar in form. Each plant has a single stem supporting a
large rosette of leaves. The sotol, however, rarely rises sufficiently above
the surface of the soil to free the surface from the lower dead leaves,
which cover about ro square feet of area. Both plants are valuable eco-
nomically, the palma samandoca affording a fiber of less value than the
lechuguilla, but of which a good deal is prepared, while the other is the
basis for the manufacture of the whisky-like liquor, mescal sotol, or simply
sotol. Neither of these occurs in sufficient numbers to figure in compe-
tition with the guayule within its proper habitat. Indeed, for reasons not
yet understood, when sotol grows densely, forming a chaparral, guayule is
entirely absent. One reason, if not the only important one, is that the
sotol appears not to be confined to limestone areas, but is not excluded
from them.
The Environment. 39
SANGRE DE DRAGO (JATROPHA SPATULATA).
This plant is a very characteristic xerophyte, and is found beyond
the limits of the Chihuahuan desert, westward into Sonora (MacDougal,
1908). The upper part of the plant consists of a simple, dark-brown
and somewhat fleshy stem, scarcely branched at all and slightly curved.
The leaf-producing lateral shoots are very short, and are roughened with
small scales; from them arise the bright green narrow leaves in clusters.
Reproduction takes place readily by means of seed, and the plant spreads
by underground stems which are thick and fleshy, and are, in fact, water-
storage organs. Like the lechuguilla it is a colonial form, growing in
dense patches, but is less able to occupy the ground to the exclusion of
other plants because of the slender aerial parts. Its ability to take up
large amounts of water from the superficial soil must, however, be reck-
oned with. There is little doubt that this is a dominating type.
RASTRERO (OPUNTIA MEGALARTHRA).
This is a spreading, low form of prickly pear. Though sometimes
very densely packed, making progress difficult, mechanically it interferes
comparatively little with guayule. This is to be explained by the fact
that,on account of the edgewise position of the flat, procumbent branches,
very little soil surface is actually occupied. One finds, indeed, that young
plants of guayule are frequently abundant in irregular rows beneath, or
nearly so, the branches of the opuntia. It is not unlikely that the spines
of the former aid somewhat in protecting the guayule from jack-rabbits
and other predatory animals, and so, in this particular respect, help it
along rather than hinder it. While this opuntia is a persistent type, its
occupancy of the ground is apparent rather than real.
A composite shrub (Zexmenia brevifolia), huisache ( Acacta farnesi-
ana), gatufio (Acacia greggit), and asafran (Buddleta marrubitfolta) are all
shrubby, freely branching kinds. The last resembles guayule in color,
and the novice may easily mistake the one for the other. The gatuno
and huisache are small trees with slender branches, and make but little
shade. The nature of the competition between these forms and lechuguilla
is more evident than in the case of these and guayule. They are slow-
growing and do not reproduce themselves except by seed, and this not
rapidly. Nevertheless, excepting the gatufio, they may be found growing
very plentifully in some situations and often outnumber the guayule.
Thus on north slopes the composite shrub is frequently more numerous
than the guayule.
Peyote (PEyYoTL) (LOPHOPHORA WILLIAMSII AND L. LEWINII).
These cacti are the mescal-buttons or dry whisky of the Texas In-
dians and cow-men, and have been sought after as the source of a little
understood alkaloid of marked effects upon the nervous system. The
exposed part of the plant is little more than a convex disk a few centi-
meters in diameter, of fleshy texture. The stem and root together form
a conical, fleshy mass. They are a very modest element in the vegetation,
occupying little surface, and may be disregarded from a practical point
of view.
40 Guayule.
There can be little doubt that the component elements in such a veg-
etation are in a state of ebb and flow, and, in view of the density of the
vegetation, in contrast with the condition usually met with in deserts, con-
stitute an important question economically. Here the individuals come
into actual contact above ground, where the competition is often severe,
as well, presumably, as below ground. Referring especially to guayule,
it may be accepted that, when a plant is once well started, it is seldom
killed outright by contact with its neighbors, but the occupancy of the
ground by other species which have superior methods for spreading grad-
ually reduces the available surface and water-supply for the guayule. This
plant takes advantage of surface-water by means of its superficial roots
and plants with which it is associated and which behave similarly (e.g.,
Jatropha spatulata) must come into severe competition with it in this
regard. But, assuming that, for purposes of forestry, it is desirable to
thin out other vegetation in order to favor the guayule, the question
arises as to the effect upon the germination of seed of this plant, which is
undoubtedly favored by partial shade. It may be argued that the superior
numbers of seed available and the shade of the guayule plants themselves
will suffice, and this seems probable. On the germination of seed in the
open more will be said, based upon experimental evidence (Chapter IV).
Denuded areas are under observation, and the future may be expected to
bring exact observation to bear upon the practical question of the value
of clearing land, as well as upon the theoretical aspect of the questions
above stated. (See also Chapter IX.)
PARASITISM.
Of vegetable parasites affecting the guayule only two are at present
known. Of lesser importance, so far as we may judge, is a rust hitherto
known as Uredo parthenti Speg. (fig. 5). Prof. J. C. Arthur, to whom
material was sent for identification in April, 1908, reports that the fungus
properly belongs in the genus Puccinia, and may be
called Puccinia parthentt (Speg.) Arthur, tned., for the
purpose of record.
It has been noticed that the fungus appears
chiefly on plants which are on the north slopes of ar-
royos, especially near the bottom, where the relative
humidity is most favorable, since it is here that the
highest vapor-tension exists. It has been found also
on plants growing on ridges, and especially on those
which are subject to a condition which we have called
‘““witches’ broom,’’ in which the leaves are small and
very much crowded. It appeared in the spring of
1908 also on plants which had been grown under
Fic. 5.—Teleuto and ure- - 7-4:
aospores of Puccinia iftigation at Cedros, apparently on the older leaves,
parthentt (Speg.) At which still remained attached from the previous
year. The parasite is not at all plentiful, and appears
to be absent almost entirely from guayule growing in open situations.!
: * A small seedling which germinated in the early summer of 1908 was found
in April 1909 with a single infection spot, quite in the open foot-slope (Station 3),
in which situation the fungus is seldom seen.
The Environment. 4]
Of more importance, economically, is the ‘“‘seda”’ (silk) or dodder
(Cuscuta sp.), which often grows very plentifully. The habit of this par-
asite is well known, so that no account of the plant is here necessary. It
is very readily recognized as a yellow or orange vine-like leafless organ-
ism which winds about the upper twigs and leaves of the host. It is not
confined to the guayule, being found also on hojasen (Flourensia cernua),
on mariola (Parthenium incanum), on tatalencho (Gymnosperma corym-
bosum), and other perennial plants, and probably on some summer an-
nuals. It reproduces itself by means of seeds which germinate after the
advent of the summer rains, but is to be found vegetating vigorously long
before this time. This is explained by the fact that it passes the win-
ter in rather tight, compact clusters of thread-like stems, tightly wound
about the uppermost twigs and leaves of the host. (Lloyd, 1g908d.) Thus
it is independent of seed and is a true perennial.!
The effect of the dodder upon the guayule is due to two causes.
It diverts water and foods from the host into its own tissues and thus
reduces the rate of growth, and it strangulates the twig and leaves upon
which it fastens itself. There is thus produced a dwarfing and distortion
which is reflected in the whole habit of the plant.
As soon as growth commences in the host, the dodder, which is
ready at the top of the previous year’s growth to take hold of the new
tender tissues, begins to twine about the newly forming stem and leaves
and soon overtakes and strangulates them. The effect is to produce very
slowly growing plants, and it is seen that the presence of much dodder
would materially reduce the annual accretion of growth and therefore of
rubber. In periods of severe drought the effect of the dodder is even more
marked, since it diverts the already meager water-supply and thus causes
the death of the portion of the twig at and above the zone at which the
dodder is found. Plants with twigs killed in this way, and in which
the dodder itself had succumbed, were found at the close of a sustained
drought, in April 1909. The dodder should therefore be stamped out
wherever it may be found. The best and only practical means is to har-
vest with the initial crop all the guayule affected with the parasite. In
this way the parasite will be checked, and additional checks will be re-
ceived at each harvesting by following the same rule.
Indications of another vegetable parasite were thought to be seen in
the ‘“‘witches’ broom”’ above mentioned, but material examined by Prof.
W. G. Farlow gave no clue to the cause. The densely packed leaves in-
deed favor the growth of the rust already described, but this is quite a
secondary condition. It is possible that the distortion is due to the crop-
ping of the guayule by animals, but not all plants so treated show it, else
nearly all would be affected. Plants closely in the field trimmed back
(Station 2, quadrats 1, 2) show a tendency to produce ‘“ witches’ broom,”’
indicating that constant or close browsing by animals may after all be
the cause of this condition.
1Cuscuta is sometimes a perennial as far north as the State of New York.
Stewart ez al., Bull. 305, Agri. Exp. Sta. N. Y., Nov., 1908.
42 Guayule.
ANIMAL PARASITES.
The root-system, particularly the tap-root and its larger branches,
are frequently found to be infested with two species of the Coccide,! Cero-
puto yucce (Coq.), and a species of Orthezza, distinguishable from the for-
mer by the fluted, waxy egg-case attached to the abdomen. The number
of these insects found on plants in the field is not inconsiderable, and may
be responsible for lesions in the root-tissues which affect the growth of
the plant. But of more importance is the circumstance that they occur
in greater numbers upon seedlings raised under cultural conditions in
wooden trays. Plantlets a few centimeters in height have been found
with a dozen or more large individuals on the tap-root, the diameter of
which was not as great as the breadth of the mature insects. They may
therefore easily be responsible for retardation of growth, though external
evidence of lesions has not been noted.
Field plants especially are often infested below the surface of the
soil by a scale, identified by Dr. C. L. Marlatt as Targtonta dearnessi CkIl.
This is a widely distributed species in this country. Large tap-roots are
frequently half covered by this parasite.
A gall insect attacks the leaves and inflorescence. The female punc-
tures the young leaves and stems, the peduncles, and even the bracts of
the capitula, and the resulting galls produce marked distortion. Many of
the affected leaves fail of anything approaching normal development; the
peduncles are hypertrophied unevenly and become very much contorted,
and the inflorescence fails to develop. The net result of the work of this
insect is to reduce the rate of growth very materially and to cause a prac-
tically complete abortion of the flowers and, therefore, of the seed. The
plants affected are readily recognized on account of the irregularity and
lumpiness of the terminal growths. The stems proper do not seem to be
affected, as the insect appears to commence its work toward the close
of the season of growth and to confine itself to the last-formed leaves,
which remain attached throughout the winter, and to the inclosed young
inflorescences. The increase of growth in the stem is, however, affected
indirectly, and the annual accretions frequently amount to less than 1
em., and scarcely ever to more than 2 cm., during the period of attack.
Many plants in circumscribed areas are subject to the attacks of these
insects, and it may readily become a serious menace to both the growth
of the plants and to their seeding power. The following notes have been
kindly furnished me by Dr. Mel. T. Cook:
The study of this material presented many difficulties, as must necessarily be
the case when it is not possible to make a field study.
A gall produced by Cecidomyia parthenicola on Parthenium? in New Mexico has
been described by T. D. A. Cockerell in Entomologist, July, 1900, p. 201. The gall
before me does not fully correspond with Cockerell’s species, and yet I should hesi-
tate to say that it isan entirely different species without further study, which is im-
possible with the material in hand. Dissection of the material showed two entirely
different species of larva and immature insects, cecidomyid and cynipidous, while a
study of the histology presented certain confusing and anomalous characters.
1 Kindly determined for me by Mr. J. G. Sanders, through the courtesy of
Dr. L. O. Howard.
? Parthenium incanum, presumably.
The Environment. 43
The isolated galls were small, monothalamous, and in the shape of a truncated
cone, usually on the upper surface of the leaves and standing in an oblique position
The opening of the larval chamber was through the top and was guarded by hair-
like growths or trichomes, which pointed inward. This would indicate a cecidomyid
gall, but certain preparations showed the opening closed by a thin membrane.
Whether this latter condition was real, therefore proving the presence of two species
of galls, or only apparent, was difficult to determine, owing to a tendency of the galls
to coalesce, forming irregular masses.
HISTOLOGY.
The gall in its earliest state shows the reduction of the palisade into cells of
the mesophyll type. This condition is characteristic of the origin of all leaf galls.
As the gall develops, the cells, which constitute the lining of the larval chamber, are
rich in protoplasmic content, which decreases from inner to outer surface. This is
indicated very readily by the stains and is characteristic of the more highly devel-
oped galls and usually designated as the nutritive zone. A little later certain galls
showed a reduction of the nutritive zone and the formation of a protective zone of
sclerenchyma cells just outside the nutritive zone. The presence of this protective
zone is characteristic of the galls produced by cynipidous insects, and the writer has
never found them in galls caused by cecidomyid insects.
From the above facts, it appears that we may have two species of galls, one pro-
duced by a cynipidous insect and the other by a cecidomyid, or a single gall which
has been parasitized.
Fic. 6.—The guayule barkbeetle (Pityophthorus nigricans Bland). (a) Work of beetles and larve in
barkand wood. (b) Adult beetle, greatly enlarged. Small figure at right shows natural size. (c) Egeg-
galleries of parent beetles, with intervening larval mines, all grooved on surface of wood. (From
illustrations loaned by the Bureau of Entomology, U.S. Dep. Agric.)
44 Guayule.
THE GUAYULE BORER.
In the fall of 1907 it was noticed that guayule in the stack (plate
4, fig. A), awaiting treatment for the extraction of the rubber, was being
attacked by an insect, the only signs of which were the finely-powdered
débris escaping from minute, circular openings in the bark. It was at
once evident that a borer of some kind was at work. Material was sent
to Dr. L. O. Howard, who kindly referred the matter to Dr. A. D. Hopkins,
in charge of forest insect investigations, Bureau of Entomology, U. S.
Department of Agriculture, to whom I am indebted for the accompanying
notes and drawings (fig. 6, p. 43). Dr. Hopkins writes as follows:
The beetle is Pityophthorus nigricans Bland. It has also been reported to the
Bureau of Entomology by H. Pittier, who found it injuring the same plant at Tor-
reon, Coahuila, Mexico. The insect is of specialinterest because of its habit of attack-
ing a plant of such commercial value, and on account of its being the largest repre-
sentative of the division of the genus to which it belongs. Those of one division
infest coniferous trees only, while those of the other, to which this species belongs,
infest only the broad-leaved plants and trees. The guayule barkbeetle evidently
attacks the plant after it is dead, or soon after it has been cut, and, ashas been
shown by the specimens in the forest-insect collection of the Bureau of Entomology,
may continue to breed in the same bark and wood for several years. It is evident
that the prompt utilization of the plant for the manufacture of rubber within a few
days after it is cut would prevent all losses from this source.
Inasmuch as the buyers of shrub sometimes accumulate large quan-
tities and place it in stacks until needed, and as this may represent large
investments, the amount of damage may represent no inconsiderable
loss. In order to determine what this loss might amount to, a piece of
stem of average thickness which had been attacked by the borer was
weighed as a whole. It was then decorticated and the insect débris was
carefully removed. Some of the débris had of course been lost, and thus
an error is introduced into the calculation of fully 5 per cent of the total
weight of the bark. The tunneling done by the insect was not complete,
however, and for this reason the figures may be regarded as the average
result of the damage which may occur in the space of a month or two.
TABLE 19.
mm. | grams
Length of sample piece of stem....25. | Weight of the whole........... 3.801
Diameter of the.wood............ 9.8 | Weight of wood cleaned of débris. 1.703
Thickness of the bark .....-... gS see 2.3 | Weight of bark cleaned of débris. 1.903
Total diameter of the stem....... 14.4 | Weight of material lost (with
probable correction)....... 0.2
Of whichthalfas bark vizeae ee O.1
It can be mathematically shown that the amount of destruction in
the smaller twigs in which the insects work may amount to very con-
siderably more, indeed to the extent of 40 per cent of the volume of the
bark (cortex). Inasmuch as the bark contains practically all the rubber,
it is seen that the loss may be great enough to warrant serious considera-
tion. It must be observed, however, that the comminution of the cor-
tical tissues by the beetle does not diminish the amount of rubber in
the stem except by the amount that happens to escape through the en-
trances, so that the real question is, whether the commuinution of the cor-
a
ena Yin ng oe Oa
The Environment. 45
tex and of the rubber contained in it renders the rubber unavailable in
the manufacture of the crude product or not. In order to answer this
question, a sufficient quantity of the débris was collected and subjected
to mastication. By this means it was possible to cause the partial agglom-
eration of the rubber, but it was quite impossible to separate out the
“‘bagasse’’ on account of the fineness of the particles. These have the
effect of separating the rubber so that it is in the form of a fine mesh-
work, the connecting isthmuses not appearing to be great enough to
overcome the surface-tension of the smaller masses. Microscopical ex-
amination shows the mass to be composed of minute fragments of tissue
derived from the wood and cortex embedded in the rubber. Measure-
ments of these particles showed them to be 0.02 to o.1 mm. in size, occa-
sional pieces being as large as 0.5 mm. If during mastication one is
careful to allow only a small amount of saliva to bathe the mass, it may
be held together for some time, but if it be flooded for a moment and
worked meanwhile, it will quickly disintegrate and can not be reagglom-
erated. It therefore appears that the work of the beetle, while not destroy-
ing the rubber, puts it into such condition that it is lost to the manu-
facturer who uses a mechanical method of extraction, since the minute
particles can not be made to agglomerate. When the insects have once
got a fair start in a stack-yard the amount of damage which may be
caused in a short time by their very large numbers may be great enough
to warrant the adoption of means to avoid the loss, if it is found that
stacking the guayule is necessary.
CROPPING BY GRAZING ANIMALS.
It has been pointed out that the growing guayule is browsed by an-
imals. Burros, jack-rabbits, cotton-tails, and goats are all given to this,
and as these animals are numerous a great loss is entailed. Goats are
herded habitually in the guayule fields, and these animals, with their
all-devouring appetites, eat almost everything that grows. Not the least
damage done by them is the wholesale destruction of the developing
shoots and flower-buds, reducing the crop of seeds very greatly. Goats
and burros may, however, be pastured away from the guayule fields, and
thus loss may be avoided.
The work of rabbits, where other food is available, is not serious,
though in the event of adopting forestry methods they may become a
menace to the plant. These marauders do not merely crop off the foliage
and new shoots; they lop off whole branches, which are left on the ground
to die. One jack-rabbit may therefore do a great deal more damage than
a goat in the same time. It has been noticed that they treat the gober
nadora in the same way. One frequently sees a complete circle of dead
branches about the base of a bush, all having been lopped off at one time.
CHAPTER III.
DESCRIPTION OF THE GUAYULE,
Parthenium argentatum Gray.
SEED:
The word seed is here applied in a loose sense, inasmuch as the body
to which the term is applied is, correctly speaking, an achene, a one-
seeded fruit in which the pericarp remains indehiscent and dry. What
passes as “‘seed’”’ in guayule is a mixture of achenes, sterile flowers,
involucral scales, and pedicels, and, inasmuch as the opportunities for
sophistication are nearly always at hand, and for the reason that the
peon employed for the gathering of seed will not always be diligent in
distinguishing between guayule and mariola ‘‘seed,”’ the present chapter
may appropriately begin with a description of the flower.
In the genus Parthenium, as in all the Composite, the order to which
it belongs, the flowers are arranged in heads or capitula (fig. 10). In the
guayule these are about 5 mm. in
diameter, and contain two kinds
of flowers, commonly known as
ray and disk flowers. The rays
are normally five in number, and
are readily recognized during
flowering by the open corollas,
which project radially beyond
the margin of the capitulum.
These only produce seed, each a
single one, if fertile. The disk
flowers produce pollen but are
Fic. 7.—Ray-flower with attached disk flowers and incapable be See B28c5 al
the subtending bracts. Parthenium incanum. though the pistil 1s present and
serves after the fashion of a pis-
ton to eject the pollen, as commonly occurs in the Composite. When the
fruit is ripe and the period of flowering is quite past, the capitulum
becomes dismembered in a somewhat unusual fashion. Each ray-flower,
the two adjacent disk flowers and their subtending involucral bracts,
become attached to each other by concrescence, and fall away as a whole
(fig. 7). The remainder, 7.e., all but ten of the disk flowers, also remain
attached to each other and fall away as a shriveled, conical mass. There
remain behind five involucral bracts persistently attached to the recep-
tacle which supported the whole. In collecting “‘seed”’ all of these are
taken, so that it will be seen that the bulk of the material is chaff.
Considering the fertile flower and its accompaniments, we observe
that the achene is hidden between the adjoined pair of disk flowers and
its own bract. This bract, which is quite broad and concavo-convex, is
1 Polyembryony occasionally occurs.
46
Description of the Guayule. 47
composed of three morphological elements, fused above, but more or less
loosely connected below; a rare occurrence, analogous to the condition
of some stamens. The middle element
is the narrowest, and is the bract proper
of the pistillate flower. This, to be seen,
must be dissected out.
Another peculiar feature then be-
comes apparent, namely, that the two
disk flowers can not be separated from
the achene without pulling away two
narrow strips of tissue from its margins.
(Fron et Francois, 1901.) The whole
arrangement would appeal to the tele-
ologist as an excellent adaptation for
dissemination by the wind or by water,
since the thin, light, and air-imprison-
ing tissue may serve as wings or floats
according to circumstances. Theachene
itself is crowned by the persistent but
shriveled corolla, and at either side of
this and against its ventral (upper or
inner) aspect are three short awns,' one
in each position, The achene proper
is ovate, with an acute base. It is par-
tially clothed with short appressed hairs,
but for which the pericarp would be
black or dark gray. The achene meas-
ures 2.5 mm. in length by 1.8 in breadth
when of normal size, exclusive of the
awns.
The ‘‘seed”’ of the two other spe-
cies, mariola (P.incanum) and P. hys-
terophorus” (an annual), which grows
with or near the guayule, may be
distinguished by attention to the char-
acter of the lateral awns, which may
readily be seen with a lens by viewing
them as they project beyond the bract.
In the guayule the awns are brown,
with papery, denticulate margins. In fic. s.—A, a fully germinated seedling of
the mariola these are slender, appearing dav and frst two foliage leaves, C. trans
dentieulate-or quite’ without membra= Yoesection through achenoof araytlowes
nous margins, tapering and distinctly
reflexly curved, and are usually darker in color, being black toward the
1 Taxonomic works usually indicate that there are only 2 awns, but this is
an error. There are 3 awns in Parthenium argentatum and P. incanum; 2 in the
herbaceous P. lyratum and P. hysterophorus. Engler and Prantl describe the genus
as having 2 to 3 awns, but do not indicate further details.
2 This plant grows in great profusion in the summer months in the alluvial
plains upon which the guayule lands border.
48 Guayule.
base. In P. hysterophorus and P.lyratum they are very broad, and are
membranous in the former. Figs. 7 and 9 will make these and other
characters evident.
eis!
eg
~
Zt
SS
Deepal |
os
Fic. 9.—Achenes of (1) Parthenium argentatum, (2) P. hysterophorus, (3) P. incanum, (4) P.lyratum.
SEEDLING.
When germination is complete the seedling of the guayule consists
of a short primary stem (hypocotyl), 5 to 10 mm. in length, terminating
in a long, slender tap-root. Attempts to find the end of this in the field
have been fruitless, on account of the nature of the ground and because
of its very tender character and great length and thinness. Experiments
show that it reaches a depth of at least several inches. This slender root-
let, with very few branches, is the means of keeping the plantlet supplied
with water from the soil for some months, as frequently during the first
year in the field no adequate development of lateral roots occurs. The
seed leaves (cotyledons) are nearly or entirely circular in form, and range
in size from 2.5 mm. in width by 3 mm. in length to 4.5 mm. in width
and 4.7 mm. in length, according to various conditions. At the apex of
each cotyledon is a hydathode, composed of a group of water stomata.
Other conditions being the same, seedlings grown in the shade and high
relative humidity have the largest cotyledons (plate 34, figs. 6, 9), and
the largest were seen on seedlings grown experimentally under such con-
ditions. The primary stem is about 1 mm. in diameter, and in seedlings
grown under natural conditions, 7.e., with direct sunlight, is dark red; in
LLOYD PLATE 9
A. The root-system of guayule.
B. Group of plants which started as retofos.
C. A strongly monopodial retofio.
Description of the Guayule. 49
shade plants it is green. The dark-red color extends also over the under
surface of the cotyledons, which are rather thick in sun forms, and thin-
ner in shade-grown plantlets (plate 34, figs. 4, 6).
The early foliage leaves, soon after germination and because of the
very short internodes, are closely crowded. By partial etiolation these
internodes may be caused to lengthen, and thus the structure of the pri-
mary epicotyledonary stem may be better studied. In this way points
may be made clear which otherwise would with difficulty be explained.
The first 8 leaves are usually ovate, entire, slightly acute, and taper
suddenly into the petiole (fig. 8). They are clothed, as are all the foliage
leaves, with closely set T-shaped hairs (plate 30, figs. g-11) laid parallel
to the axis of the leaf, and thus is produced that light green-gray, satiny
sheen which characterizes the plant. The first leaf is usually about 1 cm.
long by 3 mm. wide, though measurements vary a good deal. Inthe mar-
iola seedling the earliest leaves resemble those of the guayule, but differ in
being broader and lanose, a difference due to the form of the trichomes,
those of mariola being of the ‘““whip”’ form found frequently in the Com-
posite. As the hairs are much thicker on the under side of the leaf, the
species may be very readily recognized even when only one foliage leaf
has developed, though identification is difficult before this leaf appears.
The last formed of the entire-margined seedling leaves may reach,
in field plants, a length of 7 to 8 cm. and a width of 1.5 cm. The first
approach to the mature leaf form is seen in a single tooth, usually on
one margin only, at about the middle of the blade. In the next stage
the tooth may be found on both sides, and larger, while half-way between
their position and the apex a second pair of teeth appears. By basal
contraction of the blade and extension of the upper portion, the first
teeth appear to move downwards, and by enlarging attain lobate pro-
portions. The leaf is now relatively shorter and broader. An additional
pair of basal teeth may also add to the complexity. While this descrip-
tion, illustrated well in plate 18, is generally true, few plants are more
variable as regards the form of the leaf than the guayule, and this varia-
bility is, with the exception of the earliest foilage leaves to be formed,
closely connected with the amount of available soil-water. Thus we find
that in plants grown under irrigation the amount of lobing is very much
more marked than in field plants. We shall return to this subject later.
The first inflorescence is usually formed early in the history of the
plant, and may occur in the first growing season even in field plants,
though this is exceptional (plate 17). This early flowering in a shrubby
plant of long life appears to reflect its relationship to herbaceous forms,
and would not improperly be regarded as indicating that the perennial
habit of the guayule and mariola is, phylogenetically, a recently acquired
character. The inflorescence, which is a compound monochasium (fig. ro),
is terminal, and thus ends the growth of the chief shoot. In some in-
stances flowering may not occur for some years, and in this event if no
accident befalls the chief shoot it may attain a length of 15 cm. or more
before the first flower shoot appears to conclude the growth of the chief
axis. In such a case the lateral shoots make but little growth. Upon
the first occasion of flowering the growth of the branches begins; these
4
50 Guayule.
in turn terminate in inflorescences and, by ending their growth, give stim-
ulus to the growth of branches of higher orders, each in its turn. Thus
the plant becomes profusely branched, and this habit contributes mate-
rially to the amount of secretion, which is proportional to the number of
branches.
Fic. to.—The inflorescence of the guayule.
THE MATURE PLANT.
ROOT-SYSTEM.
The root-system of the guayule consists, in a plant derived from a
seedling, of a strong tap-root extending to a considerable but undeter-
mined depth in the soil. The lower end, which branches more or less,
draws upon the water-supply of the deeper layers of the soil, especially
in younger plants. Just below the surface of the soil a number of strong
lateral roots are given off, which in many instances are of extraordinary
length, reaching a distance of 150 to 200 cm. or more from the plant
(plate 9, fig. A). These serve to take up the water in the shallower lay-
ers of the soil, derived from rains sufficient to wet the soil to this depth.
Such far-reaching, shallow-placed roots are characteristic of many desert
plants. Cannon (1909) has studied and mapped the root-systems of a
number of such, and has further shown that competition between juxta-
posed plants may be eliminated by the difference in the type of root-
system, the one going deeply, while the other is chiefly shallow. The
development of two differently placed parts of the same root-system, the
one drawing on the deeper, the other on the shallower layers of the soil, is
of very great importance biologically, and is well exemplified by the little
cactus Ariocar pus kotschubeyanus, which grows in the alluvial plains of the
mesa central. The shallow roots arise from the top of the tap-root and
ascend as nearly vertically upward as may be, till they reach to within a
few millimeters of the surface of the soil, when they suddenly take a hori-
Description of the Guayule. 51
zontal position, and in this direction traverse considerable distances from
the plant. This condition is closely analogous to that in the guayule and
serves to make even clearer the significance of the arrangement in that
plant and in others, in all of which the tap-root system, while quantita-
tively inferior both as regards the number of branches and the amount of
water absorbed into the superficial system, may nevertheless be of a good
deal of importance in enabling the plant to withstand prolonged drought
when the shallower portions of the soil become very dry. This is indi-
cated by the readiness with which retofios arise from the tap-root after
the plant and lateral roots have been cut away.
RETONOS.
From these shallow lateral roots there frequently arise new adventi-
tious shoots, sometimes singly, sometimes in groups of two or more (Lloyd,
rgo8c) (plate 9). They are locally called “retofios,” though this term is
not alwaysused strictly, and may apply to shoots arising from stem tissues.
It is the same word as ‘“‘rattoon,’’ used by sugar-cane planters, but as this
is used constantly to indicate offshoots from the base of the stem, it is
inapplicable as an equivalent of retofio. Since it is the only term used in
Mexico for these shoots of root origin and as our common English equivalent
is characterized chiefly by its inelegance, we shall venture to retain the
Mexican-Spanish expression.
Retonos usually arise from the plant at a distance of 20 cm. or more.
They have been found at a meter’s distance, and doubtless may occur
still further away. The point of origin may be above, below, or at the
side of the root. As growth proceeds the proximal part of the root fails
of further secondary thickening, or at most undergoes very little thick-
ening. It ultimately becomes abstricted by decay, apparently induced
by pressure of the tissues of the retofio, and quite soon loses its physio-
logical value. The distal portion, however, thickens more rapidly, keep-
ing pace with the growing retofio, and takes on the proportions of a tap-
root, though it may always be distinguished from a true tap-root by its
curvature and position in the soil. Secondary, adventitious roots (fig. 11)
later arise from the basal portion of the stem of the retofio, thus amplify-
ing the root-system. A large root-system thus developed is shown in the
central and largest plant in plate 9, fig. B.
The author of this publication stated as follows in a previous paper:
The formation of these new plants in this manner is not spasmodic or excep-
tional, nor are they fugitive in their nature. Under certain conditions they are pro-
duced in such numbers as to entirely overshadow the numbers of seedlings; and they
as frequently grow into maturity, producing a plant which, if the origin were not
known, would not unlikely be considered a varietal type, in point of habit. The
mature plant which had its origin as a seedling has a single trunk, usually 10 cm.,
sometimes 20 to 30cm., in length; the mature plant produced vegetatively has usu-
ally a very short trunk, or a group of separate ones, more or less coalesced by growth,
though marked exceptions may occur (plate 9, fig. C).
The ratio of the number of new plants arising as seedlings and of those arising
as root-shoots varies with the habitat. Both forms may be found in any situation;
but the retofios are much more numerous on stony slopes, often outnumbering the
seedlings. The reverse relation is seen in more level places. Thus, at the foot of a
low ridge I have found seedlings plentiful, as many as 30 in a square foot (these
small and larger ones as well scattered about relatively thickly). A zone of this
52 Guayule.
character could be traced around the ridge. Just above this zone another could be
made out in which the retofos were abundant and the seedlings scarce, while
coming to the top of the ridge the seedlings again outnumbered the retofos. Thus
on that part of the slope most affected by erosion, and where there is more chance
of uncovering the shallow roots, the retofios are most abundant. It would appear,
therefore, that the exposure to light is a potent, if not the most important, factor
in inducing budding in the roots. Yet I have found that when a plant is removed
by cutting at the base so as to sever the roots and leave them in the ground, shoots
start from the root, not only where the root is accidentally exposed, but as far
back as the drying out of the root makes it necessary. A root thus severed in
January failed to bud till June in consequence of the lack of rain; when at last it
rained, the buds started out 12 cm. away from the cut end and several centimeters
deep in the soil. On the other hand, roots purposely exposed for a portion of their
length and slightly wounded had failed to start buds at the end of six months when
last examined.' So the case appears to be more complicated than at first appears.
Injury may be a factor at times, but, experimentally, I have shown that scarring
or cutting the cortex is not sufficient to insure budding, at least under field condi-
tions, for it is probable
that the exposure to a
low relative humidity in-
hibits the growth of callus
on exposed roots. It is
more probable that had
roots been injured and
left covered with soil,
positive results would
have accrued.
This occurrence of
retofios in guayule pre-
sents a very interesting
biological phenomenon.
In a habitat where the
F = peas oF wee 4: rainfall is very meager, so
IG. 11.—Retonos, showing position of adventitious roots. pr., : :
proximal portion, and dr., distal portion of mother-root. that ya we oe in which
the conditions for germi-
nation are prohibitive, and where, moreover, sudden and severe rains wash the soil
on the steeper slopes severely enough to remove seeds or expose seedlings when
young so as to prevent their further growth, it will easily be seen that the vegetative
method of reproduction presents certain very marked advantages. This is true also
where the soil is confined to the crevices of the native rock where it lies at or very
near the surface. This condition occurs very frequently in North Zacatecas, where
large areas will be seen in which the vegetation is confined to bands of outcropping
rock, where it occupies the soil beneath the edge of a stratum. Where the relation
of the strata to the surface is such that flat blocks of rock support but a thin layer
of soil, the distribution of vegetation is determined by the fissures. In the case of
guayule we have an exception, for this plant may send out a shallow lateral root
over a block of stone, above which plants may start. Very frequently we find
individuals which have grown in this position, with their roots straddling the sub-
imposed rock. Such are almost invariably retofios. Plant 1 (plate 9, fig. B), was
found so placed. There are other plants which can compete with the guayule in
this regard, such as the lechuguilla (Agave lecheguilla), which spreads out by means
of stolons, and occupies areas for itself to the exclusion of everything else. It is
clear that the habit described is of no small importance in the fight for foothold.
One can easily imagine, too, that a distinct advantage is to be had in the rate of
growth and the quickness with which the ability to flower abundantly is reached
by retonos. The rate of growth is relative to the size of the mother root; but itisa
very common thing fora retofio to grow 10 cm. and to come into flower in two months
in summer (plate 9, fig. B, ro and 11). Seedlings, on the other hand, flower only
1 Only negative results were had as late as September 1908.
An exceptionally tall (130 cm.) indiy ,
A\n exceptionally tail (!5U cm.) individual
tS
ite
=
=
LLOYD PLATE 11
A. A widely spreading (130 cm.) plant of guayule. Weight 10 lbs. 9 oz.
B. A large plant of the usual habit. Weight 8.5 lbs. Apizolaya.
te
The winter « ondition on the left.
guayule,
A biotype of
Description of the Guayule. 53
infrequently before the third year, and the amount of growth then does not more than
equal that of a single-stemmed retofio in one year. At the end of three years the
retofio makes a considerable plant (6 in the same plate), and flowers richly. The
influence which retonos would have in reforesting processes, both by their own growth
and by seedlings, can therefore be well appreciated, and probably with difficulty over-
estimated. Some basis for judgment in this regard will reward a study of the accom-
anying photograph! (plate 9, fig. B, in which the horizontal lines are to be regarded
as 10 cm. apart).
From his comparative morphological and anatomical studies on “ nor-
mal’’ parts and those of individuals (‘‘rejets’’) arising from root buds,
Dubard (1903) draws the following conclusions:
En résumé, la multiplication par bourgeons radicaux est un fait peu normal
dans le régne végétal; elle donne naissance a des rejets d’organisation inférieure, dans
la plupart des cas; chez quelques éspéces elle tend a s’établir d’une facon réguliére,
mais ne devient qu’exceptionnellement une sauvegarde effective de l’éspéce.
The inferior organization of the retofios studied by Dubard is always
in the direction of an anterior form “by virtue of hereditary ante-
cedents’’: ‘‘les rejets radicaux des diverse éspéces d’un méme genre
manifestent une convergence qui ne peut etre fortuite.”’
The retonos of the guayule are in the same case. The absence of
medullary and of cortical canals is a marked return to a more simple
structure, as is also the absence of medullary stereome, in which, and in
the absence of canals in the medulla, we see an assumption of seedling
characters. But the retofio assumes a still more ancient condition, we
may believe, in the loss of the cortical canals.
Nevertheless, the guayule, while in this measure conforming to the
observations made by Dubard, can not on any account be relegated to
a subnormal category, characterized by comparative impotence in safe-
guarding the species. The frequently strong vegetative growth; the early
maturation of flowers and seeds; the already established root-system; the
cincture of the mother root tending to separate the retofio physiologically,
if not always structurally, from the parent plant (fig. 11); its frequently
wide separation from this; its ability to gain a foothold where seedlings
must surely perish; all these facts heighten the importance of the retofio,
despite the relatively small numbers in which they are found, in enabling
the species to maintain a foothold. It seems, indeed, not unlikely that
a further classification beyond that of Dubard will be necessary—one for
those plants in which the retofio is of great importance in this regard.
A comparison at this point between the guayule and the mariola is
of special interest, because, while they are closely related species, their
methods of vegetative reproduction are quite distinct.
In the first place, the root-system in the mariola differs in that the laterals
run at a steeper angle into the soil. Occasionally retofios are formed, but, as far
as my observation goes, always close to the plant, within, say, 5cm. What always
happens, however, is this: From the basal portion of the stem, where there are
many dormant buds, as a sequence of the short internodes marking the slow initial
growth of the seedling, new, slender shoots arise, growing to a height of 30 cm.,
more or less, in two months. From the base of each such shoot an adventitious
root starts out, immediately above the point of origin of the shoot. This usually
single root develops as a tap-root, and supplies all the water for the daughter shoot,
1F, E. Lloyd, rgo8c.
54 Guayule.
which develops apace, and ultimately becomes an independent plant. The isthmus
of tissue between it and the parent plant does not enlarge much in any case, so that
it is quite easy, on taking up a bush of mariola, to separate it into several smaller
plants by merely breaking off the functionally independent elements. Thus the
habits of mariola and guayule in this regard are so different that one plant, the former,
remains a single-stemmed shrub of tree-like habit, while the mariola is of the bushy
habit. This marked difference, it will be seen, precludes the advisability, though
the possibility might remain, of grafting the guayule on the mariola, a suggestion
which has. been made on the assumption that increased growth might follow in the
scion. No economic result would follow, and for this reason: Suppose that we suc-
cessfully graft a piece of guayule on a stock of mariola. The scion grows, but at the
same time new shoots arise from the base of the stock as described, and their growth
is so rapid that in a month or two the guayule shoot is overtopped, and this ends the
usefulness of the graft for economic purposes. We might very well make a graft
for the purposes of pure science, but economically it would be a failure (Lloyd, 1908c).
Recently it has been proposed (Escobar, 1910), but with admirable
reserve, that the dissemination of guayule seed in areas where only ma-
riola grows may be attained by grafting guayule upon it. The plan ap-
pears impracticable.
METHOD OF BRANCHING.
It has been pointed out above that the monopodial growth of the
seedling is brought to a close by the development of the first inflores-
cence. Following this event, several of the uppermost branches make
a more rapid growth. These branches in turn end their growth each by
the formation of an inflorescence, when usually the two or three upper-
most buds continue to lengthen. Thus is produced a constantly divari-
cating system of stems (plate 11, fig. A), which, if uninjured, results in
a splendidly symmetrical and closely branched shrub. A very excep-
tional plant, approaching the ideal form, is seen in plate 11. Through
failure of some branches to develop, irregular forms are often seen. These
usually attain a greater height than the symmetrical plants. An unusu-
ally tall plant is shown in plate 10, in which the irregularity of growth
is illustrated, while in plate 11, fig. B, a form more frequently met, espe-
cially in very rich fields, is shown.
A comparison with the mariola is here pertinent, as there appear
to be two types of guayule in respect to the manner of branching, one of
which approaches the condition in mariola. The usual manner of exten-
sion of the branching system is by the nearly equal growth of two or three
branches just below the inflorescence (plate 14, fig. B). As will be seen, the
anatomical distinction between stem and peduncle is abrupt, and the dead
and, according to age, more or less disintegrated peduncle remains as a
spur in the angle between the uppermost branches. Often this may still
be seen after the lapse of many years. No absciss layer is formed,’ and
this again gives a suggestion of the recent departure of the shrubby type
from the herbaceous ancestor. After flowering, the dead peduncles re-
main in evidence above the foliage of the plant and form a conspicuous ©
character. Inthe mariola, on the other hand, with the same morphologi-
cal basis, a different habital form is had. The stem, as in guayule, ends
in an inflorescence, is more slender, and is beset with short branches or
1 This condition is, of course, common to many plants, and is specially preva-
lent among the Composite.
Description of the Guayule. 55
spurs, which, because of the more rapid growth of the shoot in mariola,
are more numerously developed. The transition into the peduncle is grad-
ual, and not sudden, as in guayule; this organ is, therefore, not sharply
delimited, either morphologically or anatomically, and is leafy and pro-
vided with buds well up beneath the inflorescence. In the following grow-
ing season, and this usually means in the following year, some of the
short spurs develop into leafy branches and in their turn terminate in
peduncles. These, like all the branches, are slender and tapering, and
their position, rate, and manner of growth result in a close interweaving
of stems, in striking contrast with the guayule.
BIOTYPES:.
Returning to the subject of habital types in the guayule, it has been
found that some plants have the mariola manner of growth (plates 12
and 13). Instead of an abrupt termination of the stem at the base of the
peduncle, the transition is gradual and the stems are of smaller diameter
than in the usualtype. Foliar differences are to be noted beyond. The
matter is possibly of practical importance, as the slender branches with
vaguely delimited flower-stalks would, mutatis mutandis, contribute to pro-
duce a less desirable form of plant from the point of view of production.
A phylogenetic interest also attaches to it, inasmuch as the mariola habit
is more closely comparable to the herbaceous manner of growth, as dis-
played by congeneric herbaceous species, than is the guayule habit. On
this score, as on others, the guayule is the type more widely divergent
from the theoretical herbaceous ancestor.
These differences are, indeed, quite fundamental, and may be traced
back to the earliest seedling stages (plate 13). The clearness of the dis-
tinctions is such as to indicate that we are dealing with a field mutant,
and the differences in the structure of the awns (pappus) would seem suffi-
cient ground, in the light of the taxonomy of the genus, to warrant us in
regarding the broad-leafed type as a distinct species. The two forms,
P. argentatum proper and this closely related form, be it a well-marked
species or a type of less taxonomic evaluation, are remarkably distinct,
and call to mind many similar juxtapositions of closely related species,
recognized as such, known to occur among plants, but not yet properly
appreciated as evidence in the discussion of isolation (Lloyd, 1905b).
Another difference in the habit—though not correlated, it appears,
with the manner of development of the inflorescence—is seen in what
may be termed straight and crooked limbed forms. The one is clean-
cut and smooth-limbed, each span of growth being nearly straight; the
other is rougher barked, the more slender limbs showing marked curva-
tures. The former is the more rapidly growing type, suggesting differ-
ences in the available water-supply. One frequently finds examples of
very marked growth differences in field plants, such as are shown in
plate 9, fig. A, of which the right-hand plant grew in a shallow rock crev-
ice and was unable to develop a competent root-system. The annual
accretions of growth in this plant were very short, not exceeding a centi-
meter, and this resulted in the production of a very dense, much-branched
mass of limbs, as seen in plate 9, fig. A,on the extreme right. This and
56 Guayule.
the left-hand plant in the same figure show extremes of rate of growth,
somewhere between which lies the average, which it is desirable to know
in estimating the rate of reproduction.
A still further difference in habit, which is not very readily distin-
guished from the foregoing at first glance, is one recognized by persons
engaged in the gathering of the shrub, who designate the two types in
question ‘“‘macho”’ or male and “‘hembra’”’ or female. The differences, which
are shown in plate 14, fig. B, were pointed out to me by Don José Herrera,
a gentleman who has had a great deal of practical experience in collect-
ing shrub. ‘‘Macho”’ guayule has fewer branches, and they have a larger
diameter than those of the ‘“‘hembra,’’in which the branches are much
more numerous. These terms are not here used in the sense spoken of on
page 4, to distinguish guayule from mariola, which latter is sometimes
called ‘“‘ hembra de guayule,” but merely to designate the plant with the
stronger and therefore ‘“‘macho”’ habit and that with the weaker or “‘ hem-
bra” habit. These adjectives are used analogously with respect to other
plants showing similar differences. ‘‘ Hembra” guayule makes greater bulk
when made up into bales, and for this reason those who gather shrub pre-
fer to take it if they are being paid at a rate per bale. Whether the dif-
ferences are biotypic or are due merely to environmental conditions can
not be said; nor whether there are other correlated differences, as in the
amount of rubber secreted, though such are variously claimed to obtain.
There appears to be a stronger tendency in the“ hembra’’ for the branches
to run out into inflorescences, entailing a greater amount of dying back
at the close of each growing-season, and thus it may turn out that these
differences are essentially the same as those mentioned previously.
Finally, many guayule gatherers and others think to recognize dif-
ferent kinds as to color-characters, either of the bark or of the leaves. In
Durango white guayule (‘‘blanco’’) is distinguished from dark or “‘ prieto,”’
though no other characters could be pointed out to separate the two
kinds. Indeed, when a branch was exposed to view in one position, so that
the under surface of the twigs was seen, it was pronounced “ prieto,”’
and when the upper surface of the same branch was later shown it was
called ‘‘blanco.’’ This color difference, as between the upper and lower
surfaces of the branches, is quite constant.
“Blanco” and ‘‘ceniso”’ or ashy guayule are maintained to be dif-
ferent also, though the same difficulty of seizing upon other than mere
color differences obtains. So far as I could determine, ‘“‘ceniso’’ guayule
was shrub which had been exposed to severer drought, in consequence
of shallower soil in exposed positions, as on benches, and in which the
leaves had therefore dried to a dirty-yellowish color. Prolonged study
might, however, discover that some of these differences are constant and
racial, and the matter therefore deserves more consideration.
SLZE.
The question is often asked, especially by persons interested from
the business point of view, as to the size which the guayule attains. It
may at once be said that anything like the maximum size is a matter,
or will be shortly, of academic rather than economic interest. Once the
”?
LLOYD
D. Seedlings and mature plants of these biotypes.
A-C. Seedlings of typical and atypical guayule.
owering.
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Description of the Guayule. 57
virgin guayule has been removed, big plants will no more be seen. The
largest plants which have been reported weighed in the neighborhood of
10 kilograms. Overseers of field experience insist that they have seen
and weighed such. Endlich (1905) quotes Marse as having seen a plant
weighing 6.5 kilos; but a plant weighing over 5 kilos is exceptional. Of
three large plants which are illustrated in this paper, that in plate 12,
fig. A, weighed 10.56 pounds (fresh weight), was 75 cm. tall, and 125 cm.
wide. Plants over a meter in height are seldom met with, and are nearly
always more or less stag-headed and moribund (plate 10). They have
usually lost a good many limbs, and for many years have not been mak-
ing any net gain in weight. Endlich places the average weight of virgin
guayule at 500 or 600 grams. As will develop in the discussion in the
following chapter, plants of this size, which would be 40 to 50 cm. or
more tall, will in the future be considered large plants.
SURFACE CHARACTERS OF THE STEM AND METHOD
OF DETERMINING AGE.
The importance and difficulty of determining accurately the age of a
particular guayule plant has prompted careful study of the appearance of
the surface of the stem at various ages (plate 14, fig. B). This appearance
is due to (1) the primary superficial characters (epidermis, leaf-scars) and
(2) the succeeding secondary cork. Secondary changes in the cork are
produced by weathering. As marks also aiding in the determination of
age may be mentioned the dead but persistent peduncles and the number
of divarications of the stem, as related to the formation of inflorescences.
Data relating to the rate of growth of seedlings, the marks of which are
usually quite obliterated in plants taller than 10 or 15 cm., must also be
considered.
FIELD PLANTS.
Let us suppose that we are examining a plant at the close of the
growing season of, say, 1908. The characters seen in the accretions for
the years mentioned will be as follows:
1908. Leaves still adherent. The epidermis is intact and densely
clothed with appressed T-shaped hairs, producing the greenish-gray color
uniform with the leaves. If the length of the year’s growth is exceptional,
say above 1o cm., the basal part may show slight longitudinal fissures.
Diameter at base 3 mm. or less, rarely more.
1907. Epidermis still adherent, but more or less fissured, showing
yellow cork. The hairs have been partially removed by attrition and
withering, but most of them remain, preserving a gray color. Epidermis
light brown. Leafless, but scars present. Often with short spurs, or un-
developed branches with each a few leaves. Diameter usually between 3
and 4 mm.
1906. Color gray, slightly slaty brownish, generally fissured, the fis-
sures shallow, disclosing a gray-colored cork (weathered), with small areas
of epidermis remaining between. Diameter about 5 mm.
1905. The growth for this and earlier years is dark gray, becoming
darker with age. The fissures are shallow, becoming deep only with an
age of over 10 years. The fissuring is deeper, and lenticels are more abun-
58 Guayule.
dant on the lower surface of stems which are not in a vertical position.
This is because of the thicker development of bark on this side. On old
stems the fissures attain a depth of a few millimeters and become long. On
very old stems the base may become transversely fissured also (plate 10).
In using the above marks as a means of judging the age of a plant,
one may with considerable accuracy judge of the amount of growth for
3 or 4 years, and the average of these will come very near to the truth.
Some difficulty may be experienced as the result of reduplicated growth
in one year confusing the evidence offered by the leaf-scars, which are
crowded fairly closely in the region where the internodes of the winter
buds occur. These are of the tropical type, there being no specialized
scale-leaves, and consist merely of a few terminal leaves of small size
which persist till the following season of growth.
The natural wounding which results in fissures, especially as the stem
grows older, as well as the accidental wounding which frequently occurs,
usually sets free more or less of the resin,' of which large amounts are found
in the cortex, as in the pith. The escaping resin collects as drops on the
wound and, as it increases in amount, falls on the ground. Under every
guayule plant of any size, therefore, a good deal of resin in the form of
limpid masses of irregular size may be found. Should it turn out that the
resin is of particular value (Chute, 1909), as for a special varnish, consider-
able amounts could be collected by peons.
IRRIGATED PLANTS.
In irrigated plants secondary thickening begins within a short dis-
tance (5 to 15 mm.) of the growing-point, and proceeds at a rapid rate.
The fissures are very long and straight, and long patches of epidermis
are left which may be still visible 30 to 40 cm. from the apex. The color
for two years remains a clean, pale yellow, modified by the gray of the
adherent hairs wherever patches of epidermis remain (plate 21, fig. A).
The diameter, which remains nearly the same throughout the length of
a year’s growth in a field plant, making the growth cylindrical, increases
rapidly in irrigated plants, so that the basal diameter may be three times
that of the tip in the first year and eight times at the end of the second
year. The early fissuring and the coloring are correlated with this rapid
secondary thickening.
Che cea Ves
The leaves of seedlings have already been described. In the adult
plant the form of the leaf varies according to the amount of water avail-
able and its position on the twig. In general the water-factor determines
the amount of lobing. This is apparent in field plants as well as in those
grown under irrigation, and the relation is made manifest, in field forms
especially, in the sequence of leaf-form seen during the growing and the
subsequent resting period, consequent on drought and cooler tempera-
tures. The guayule may be called semi-deciduous, as it sheds a part of
the leaves only, namely, those which are produced between the more elon-
1 Loss of resin by secondary thickening is for the most part prevented by
plugging of the resin-canals (Chapter V).
Description of the Guayule. 59
gated internodes. Those which are still crowded together in the terminal
bud-cluster remain and form the basal leaves of the subsequent season’s
growth. These leaves are the last to be developed, that is, at the close of
the growing-season. Since the length of the season is determined chiefly
by the decrease of soil-water, the shape of these last-formed leaves seems
to be conditioned by this circumstance. This is evidenced by the fact
that irrigated plants, to which water is available, continue to form lobed
leaves (plate 21), and even those which compose the terminal bud are, in
some plants, as deeply lobed as the rest.
The winter leaves, as we may call those which persist in the terminal
bud, are from 1 to 3 cm. long by 3 to 7 mm. broad, elongate-ovate, taper-
ing into the petiole, entire, or with one or two very much reduced teeth,
acute. The summer leaves are 6 to 7 cm. long by 2 to 2.5 broad when full-
sized, and are deeply lobed midway the length of the blade. A large
amount of variation is met with in these leaves, however, the form depart-
ing from the proportion given to a long, slender, merely toothed leaf, 7 by
o.7cm. The summer leaves persist, in field plants, till December or later,
at which time they begin to fall. By the middle of February all the leaves
excepting the terminal bud-leaves have fallen, leaving the gray twigs bare,
each surmounted by its leaf-cluster (plate 14, fig. B). Leaf-fall appears
to be a function of drought rather than temperature. Long before falling
the leaves show marked shriveling and curling, and fall away as much by
drying off as by the action of an absciss layer (see Chapter V), which is
imperfectly formed. In irrigated plants leaf-fall is much less prompt,
proceeding from the base of the previous season’s growth upward, the pro-
cess not being completed much before the following April.
THE INFLORESCENCE, AND: THE, FLOWERING-PERIOD.
The growth-period of guayule is indeterminate and is largely a re-
sponse to moisture conditions, within certain relatively wide limits (Chap-
ter IV). Similarly, the formation of flower-buds occurs as a function of
this growth and is not related to temperature or other seasonal conditions.
Thus, if the growth is small in amount only that flower-bud which hap-
pens to be ready to expand will be developed. If the amount is great a
second or even third series of flower-buds may be developed and come into
fruition, though it is seldom that more than two series mature in one year.
When the summer rains commence the resting buds, with their frequently
inclosed and partially developed flower-buds, soon begin to grow, and
forthwith the first series of flowers is developed.
According to my data for 1908 there was practically no growth at all
till somewhat later than May 22. By June 9,in more favorable situations,
as in arroyo beds, plants were found in full flower, and by about the mid-
dle of the month flowering was well started on the ridges of the foot-slopes
and in the hills. In certain unfavorable localities, e.g., on low ridges in
the plains west of Cedros, the peduncles had attained, by July 22, only
half their normal growth. The flowering of the hill plants continued for
a month, seed ripening and new flowers coming on, when, by the middle
of August, the vigorous flowering-period was entirely closed. By Sep-
tember 9, up to which time there was more or less spasmodic flowering,
60 Guayule.
the period was at anend. This does not mean, however, that fresh flower-
buds were not available and ready to develop, but that the water-supply
was insufficient to support the heavy foliage and to enable the full devel-
opment of the flowers as well.
The end of the flowering season is shown as much by the abortion of
the immature capitula as by any other behavior. This is but the extreme
expression of a very general phenomenon, that of the unequal development
of the inflorescence in adjoining situations. When water is abundant
the inflorescence is widely spreading, the result of the development of
the pedicels (fig. 10), while where the water-supply is meager, but not
insufficient for the development of the flowers, the pedicels may remain
very short and thus produce a crowded mass of capitula. This condition
is usually met with in the field (plate 2), and between this and complete
abortion of the flowers every degree of failure to flower is seen, the result
of reduced water-supply.
While the grand flowering-period falls normally in the summer, the
exigencies of rainfall may so modify the rhythm of the plant that it will
occur in, possibly, any month of the year. Under irrigation flowering
starts in March,’ and there is sustained a profusion of flowers through
April (plate 14, fig. A) and May. It then dwindles, a second period of
low maximum occurring in August, to be continued irregularly and with
less perfectly developed flowers into November. In the field abundant
flowers were observed in October in Durango (Hacienda de los Sombre-
retillos) and in Sierra Ramirez, Zacatecas. Up to this time of the same
year (1907) no flowers had been produced in the Sierra Mojada on the
Hacienda Santa Inez, in Durango, where the guayule plants, forming an
almost pure culture, were in a shriveled condition for lack of water.
Under favorable conditions the development of the inflorescence takes
about two weeks. The flowers emit a delightful fragrance which attracts
many small insects. Among these visitors mosquitoes were observed,
extracting the nectar from the ray-flowers.
THE, PRODUCTION (OF SEED,
Though the maximum number of seeds which may be produced by
each capitulum is only 5, the total number yielded by a moderate-sized
plant may amount to many thousands. The percentage of viable seed,
however, runs small. Ina field-plant with well-developed heads less than
5 per cent of well-filled achenes were found. In other plants as high as
25 per cent were found filled. In irrigated plants the percentage rises con-
siderably higher, namely, to about 35 per cent. When the achenes are
fully ripe the bracts become brown in color and fall away from the pedi-
cels quite easily. The collection of seed (Chapter IX), which must be
done by hand if at all, should begin to give the best results at the close of
the first period of flowering. Properly done, the flowers are stripped from
the peduncle, which need not be removed from the plant. The nature of
the ‘“‘seed’’ has already been discussed.
1In 1909 flowering did not begin in these plants before the middle of April.
Inquiry developed that they had not been irrigated freely, if at all, though of
course the soil was much better supplied with moisture than that of the field.
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CHAPTER IV.
REPRODUCTION.
METHODS OF REPRODUCTION.
It is the purpose of the present chapter to compare the two methods
of reproduction, sexual and vegetative, with reference to final efficiency
in reproducing the species. It need scarcely be said that, in speaking of
sexual reproduction, we are using the term to indicate the origin of the
seed. It will be at once accepted that accurate knowledge of the topic here
to be considered is of vital importance in deriving estimates of the rate
at which guayule fields may be expected to produce a crop of that plant.
From what has been said in the foregoing chapter it will be seen
that, taking different kinds of habitats into account, an average rate of
reproduction will be maintained by means of the retofio and seed methods
combined. The relative efficiency of the two methods depends upon
widely different considerations, and these, as we shall now see, have rela-
tion to numbers of individuals, rate of growth, and the time of the year at
which they begin this growth.
RETONOS, NORMAL AND INDUCED.
We may speak of two kinds of retonos, normal and induced. By
normal we mean those which arise spontaneously upon the lateral,
superficially placed, horizontal roots, remaining for some time attached
to the plants from which they spring (plate 9). Induced retofios (plate
15) will then be those which arise as the result of mutilation, that is,
from roots, primary or of a higher order, after the plant has been cut
away. This is done on a number of haciendas in the harvesting of the
shrub, whereas the plants used to be, and by many still are, pulled up by
the roots. This pulling results, of course, in breaking away many of the
roots, but the chief portion of the tap-root is removed, as also are consid-
erable lengths of the other roots. As we shall see, the difference in effect
upon reproduction is merely quantitative, as in both cases retofios may
arise, but in very different numbers. In order to test this with as great
accuracy as possible, quadrats of too square meters were cleared of the
guayule both by the cutting and pulling methods, and the results were
noted. These, for the quadrats observed, afford accurate data, which
must be understood as of indicatory value only. It may well be believed
that different meteorological conditions would have modified the results
very considerably. Thus, if the experiments had been started just at
the beginning of the summer rainy season more hopeful results might
have been had, but we shall see that cutting at this time is for other
reasons an unfortunate practice, and the evil resulting would offset the
value of the data thus obtained. It is well, therefore, for economic rea-
sons, that the data collated shall be well within bounds. In addition
to experimentally obtained data, others derived from observation are
given, and have already been discussed in part in Chapter III.
61
62 Guayule.
~
GENERAL OBSERVATIONS.
It is generally believed that, after a field has been harvested of its
guayule, it will reproduce itself in a short period of years, the length of
which is a matter of opinion. Estimates on this point vary from 5 to
to years.' As, however, this difference in length of reproductive period,
which we may call the period of rotation, involves so large an error in
returns on investment, an effort to get at the facts is eminently justified.
From the botanical point of view, the rate of reproduction and of growth
of desert plants has been so little studied that data bearing on these
questions are of great importance, especially as the eye of civilization
is being turned on the desert as a field in which must be developed the
natural resources peculiar to it.
NorRMAL RETONOS.
The number of plants which arise as retofios within a given area is,
with probably few exceptions, small.
TABLE 20.—Comparative numbers of seedlings and retohos in given areas.
aries [Number of small plants
Locality. (below 8 oz.).
Station. Quadrat. | Seedlings. | Retofios.
Siew as 2 26 | 4
Slee I 14 | 2
Oucess oe I 86 4
Oe an ek 2 5 I
LOS ere a go 8 |
16 Oo 20 232 ° |
TD a Staats ss 200 fo) |
|
These numbers are accurate as far as they go, but they do not tell
what proportion of all the plants of the quadrats mentioned arose as
retofios. In the vicinity of Station 2 plants of this sort could easily be
found, and all but one of those in plate 9, fig. B, were obtained in a restricted
area nearby, especially on the steeper slopes. But for all this, the total
numbers of plants which have arisen as seedlings, taking all the areas
into consideration, must far outnumber retonio plants. On irrigated plants
2 years old, some 150 in number, not a single retono was observed, a
fact which may perhaps be correlated with the weaker development of
shallow lateral roots in such plants. Only one instance (plate 46, fig. B) of
a retonio starting under irrigation has come to my notice. Numerous ad-
ventitious buds were distributed on the mother-root, evidently having
started after the plant was pollarded. This was done, not at the time of
transplanting, but some time later, when it was discovered that the plant
was not responding. The importance of normal retofios, therefore, is not
to be seen in the numbers but in other qualities (Chapter III).
1 At the present writing we read in a recent number of the India Rubber World
(March 1909), that a new crop of guayule may be expected in “‘a few years.” We
may suppose that heavily interested investors have obtained accurate information
upon which they base their operations, but none, so far as we are aware, have been
given publicity.
Reproduction. 63
Normal retofios usually begin their growth with the oncoming of
rain, especially in spring and early summer. In this regard they act
merely as expressions of growth and have no special peculiarities. Start-
ing as they do from the shallow-lying roots, they make an etiolated growth
of a few centimeters before emerging from the soil. Their rate of growth
depends upon the size of the root from which they spring and the num-
bers arising at one point. If the root is slender growth is relatively slow,
and subsequently depends on the rate of secondary growth of its distal
portion; if large, the retofio grows rapidly and may in a month or two
attain a height of ro or 15 cm., a rate scarcely to be met with in the case of
seedlings. A notion of the rate of growth may be had from the follow-
ing table of measurements, based upon the specimens in plate 9, fig. B,
the numbers referring to those similarly numbered in the figure.
TABLE 21.—Size, age and weight of plants which arose as retonos
(referring to plants in plate g, fig. B.)
No. of : :
Plant. | pore | Heit of Dicer of | Weight, fresh. Age.
|
cm. mm. bss. toe: yrs mos.
23
27 .
I 4 43 Ap Zo 8 to 9
| 16
Oe I 28 20 8 8
RG 7 20 Ala ty WO) (8) 35 4
Ais: 5 16 5 to 8.5 6 4
ale 3 14 5 to 8 25875 3
(5c 3 20 Foy UCPC) B25 3
Ge I 18 TS 3 4
Sie 4 14 4 20m 56 2
Q. 2 13 3 ‘ Baden 2
50) 4 10 ai WOAary | 9: 4
“aig 3 6 2Ve5) i Fone 2 to 3
|
1Dry weight 1 Ib. 5 oz. 3Induced by cutting away the plant, January 1908.
2Dry weight. 4 Grew in season of 1908.
It is at once apparent that, as compared with the rate of growth of
seedlings, that of rotofios is much more rapid. It takes at least 15 years
to produce a plant of 2 pounds weight from the seed. Plant No. 1, in the
above table, made its weight in certainly not more than g years, possibly
in 8. This is brought about by (1) the more numerous stems arising from
the base and (2) the more rapid elongation of the stems, due to the ad-
vantage had in the already established root-system. Table 20 affords
comparative data as between seedlings and retofios. Incidental advan-
tages accruing from this purely vegetative method of reproduction are (1)
relative certainty of success because of the previous establishment of the
parent plant, with relative independence of an initial good season in order
to start, and (2) the rapidity with which the plants arrive at a condition
to flower abundantly; e.g., plant No. 11, a few months old, produced fully
too seeds. These, in a desert especially, are no mean advantages. Thus,
they would enable a single guayule plant to compete with such a plant
as the lechuguilla, assuming that it had so fully occupied the ground that
64 Guayule.
seeds could not get started, by maintaining a foothold till the dying off of
lechuguilla plants, say as the result of flowering, allowed seedlings once
more to take hold.
INDUCED RETONOS.
In order to determine the number of retonos formed after pulling up
(usually called ‘‘cortando’’) and after cutting away guayule plants, the
following experiments were made:
Experiment 115.—Station 2, quadrat 4. Jan. 6,1908. 250 plants, all
under 4o cm. in height, were pulled up by hand, leaving in the ground only
such roots as were broken off by chance. Feb. 18, no growth; Mar. 29,
no growth; Apr. 28, 5 roots produced retofios; July 28, 9 clumps of shoots
from as many roots started. Sept. 12, none additional. Apr. 3, 1909, 6
additional roots had started.
The following measurements were made of dried material collected
on April 3, 1909:
TABLE 22.—Station 2, quadrat 4. Induced retonos.
|
| f
epic alee Diameter of No. of : Diameter of
ters on | Height of stem.| sem at base, || St on, | Heluht of stein.) “stem at base
|
| |
cm. mm. | cm mm,
2 E35 | ig 0 | I 6 3
2 13 | 3, 10 5 atog | 3 tor4
2 rs | is 9 6 || gs t0ge5 | etog
I 9 | 10 | 3 4 to8 2 to 4
ue? | 10.5 | 8 3 4.5 t07 |. tow
Grain 6 | 3 to4 eee 1 to7 eo: to 4
I 8 5 | 2 9, 10 | 4) 45
fe) 20) 7 2to4 | |
The average amount of growth in stem-length was 8 cm.; in diameter
4.4mm. All of the new growths produced flowers, and were in normal
condition when examined at the close of a long drought period. One of
them is shown in plate 15, fig. A.
Experiment 114.—Station 2, quadrat 3, Jan. 6, 1908. Of 338 plants,
all but 88 small ones (i.e., 250 plants) were cut off with a “talacho”’
from 1 to 5 cm. below the surface of the ground. No growth till after Mar.
29. Apr. 28, 1908, 40 clumps of new shoots well started, each clump of
2to 6 shoots. Stems 4to6cm. long, with leaves of the same length. The
severed roots died back about 2 cm. before the new shoots started. Depth
of soil at which the shoots started, 2.4 cm. July 28, 59 clumps of new
shoots. Sept. 12, none additional. Length of longest stems, 10 cm. On
Apr. 3, 1909, 6 clumps were removed and measured, the data from which
are given in table 23.
TABLE 23.—Station 2, quadrat 3. Induced retonos.
es re | Length. Diameter. ae aaa Length. Diameter.
| |
cm. mm. | cm, mm,
2 12 5 | 4 6 1 to5
12 | oe ee ce | I 6 =i A
2 || 9 ital Zi I 10
Ave. | 8.3 5 Ave. | 8.3 5
1From a tap-root only 4.5 mm. in diameter.
Reproduction. 65
Close by this quadrat a retofio (plate 15, fig. B) was collected, which
had sprung from a lateral root 5 mm. in diameter. The chief shoot had
1o branches. Total height, 8.5 cm.; diameter at base, 5 mm. Number
of inflorescences 8, producing 80 to 120 seeds.
Station 2, quadrats 5and6. Apr. 3, 1909. The following samples were
taken at random, supplying the attached data for growth in 1908:
From a broken-off tap-root 7 mm. in diameter, 2 new shoots, 12.5
and 13 cm. long by 6.5 and 7 mm. in diameter, respectively.
From a broken-off tap-root 6 mm. in diameter, two new shoots 6.5
and 3 cm. long by 3.5 and 1 mm. in diameter, respectively.
From a lateral root, a clump of 5 stems, each 2 cm. long.
Experiment 110.—Station 3 (one quadrat). Dec. 31, I907. 30
plants, 30 to 60 cm. tall, cut off with a talacho. No growth observed on
May 1 following, till which date there was no rain. July 16, 3 roots
had started. A number of roots, including the 3 which had started,
were taken up for examination and the data tabulated as follows:
TABLE 24.—Station 3 (exp. 110).
Diameter
No. Order. Position. Died back. of root
where cut.
cm, mm
¥ | wecondaty. 0.6... Nearly horizontal....| 25 TO.5
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Ce \Perimatyv renee ak Vertical? 4.0 e.o an 12 iLO
Gia ioe DO n, 25 ty 3 Ereea al ante rape es ease? Biss Giaorek 3 EO
7 | eccondary s,s. wo FIQni zontal coos <3) ofa-< BO A asocsus rat ani a falas eae
Bake AeDON Sree eee ASC meat eer cin aroha svaust eet ay 2.6; merely started 4.0
9 LADO AIHA, e032 Pine Voie kes Pea ee 13; started; shoot 6.5
2.5 mm. long.
10 MU) erect ace oe cas 5 EOLIZOntalls sees = 2 TOMS ercr Me yaus rete:
1 Arising from No. 6 at 2 cm. from the top, where cut.
2Started (in 1907 ?) shoot 7.5 cm. long.
Experiment 121.—Station 4, quadrat 1. 50 plants in alk These
Were cut away as in the other experiments, Jan. 14, 1908. May 6, no
growth whatever apparent. A rain-gage was placed at this station on
Jan. 14. May 6: rainfall registered to this date, 1.52 cm.
Apr. 5, 1909, 6 clumps of retofios. This appearance of new growth
followed on further rainfall, as evidenced by the rain-gage of Station 5,
a short distance away.
Experiment 125.—Station 5, quadrat 1. Jan. 15, 1908. 275 plants
cut off below level of ground with a talacho. May 6, 30 roots have started,
sending up 1 to 5 shoots each, but smaller than those at Station 2. Between
Jan. 15 and May 6, rainfall 353 mm.
Apr. 5, 1909. 43 clumps of retofios. The increase in numbers was
the result of the additional rainfall, as indicated by the rain-gage, which
was still in position, though standing somewhat obliquely. Evidently
some of the water had been lost, as the oil had disappeared. The amount
remaining, 700 c.c., indicated a total precipitation of at least 850 mm.
Length of new stems, 7 to 15 cm., with diameter of 2 to 9 mm.
Experiment 111.— Station 3. Dec. 31, 1907. too square meters.
30 plants cut off at surface of ground. No new growths till after May r.
July 15, one retofio. Apr. 2, 1909, two retofios in all.
5
66 Guayule.
The percentages of removed plants represented by new shoots in
all the above experiments are as follows:
Per cent. Per cent.
Experiment 115 (pulling)...... 6 | Experiment 1ro (cutting)...... ie)
Experiment 111 (cutting)...... 6 | Experiment 121 (cutting)...... 12
Experiment 114 (cutting)...... 193 | Experiment 125 (cutting)...... 15
From the above data the following conclusions may be drawn:
Retofios are formed much more easily from the stock left after cut-
ting at or near the level of the ground. The probability that the plants
removed will be represented by new growths after cutting is much greater
when a portion of the stem at the top of the tap-root is left. This is due,
of course, to the presence of numerous dormant buds.
The promptness with which retofios start after cutting away the
plants depends, in the absence of sufficient soil-moisture, upon the rain-
fall. It is worthy of note (1) that these retofios may start slowly before
the advent of rain, and (2) that the roots may die back at least 13 cm.
before starting. Root No. 1, experiment 11o, had died back 25 cm. dur-
ing six and a half months, that is, at the rate of about 4 cm. per month,
and it finally failed to start. It was a very dry period, and this long
tenacity of life illustrates in a striking way the physiological resistance
of these roots in desert conditions.
While this degree of hardiness would serve very effectively to pre-
serve the species under unfavorable circumstances, it is evident from our
figures that the number of new plants produced is not as great as is gen-
erally supposed. The best result obtained (exp. 114) indicates that under
the conditions surrounding this experiment scarcely more than 25 per
cent of the original stand may be expected. It is a matter for satisfac-
tion, however, that even under the most drastic treatment a field of
guayule may be expected to reéstablish itself in the course of time, since
the new growths will in a short time be able to produce seed and these
will contribute to the repopulation of the area.
In April t909, two areas were visited from which the guayule had
been removed by pulling up the shrub. It appeared that only the larger
plants had been removed, and that both places still contained the natural
growth of smaller plants. The point of interest in this connection is that
in one of the areas, the Lomerio de Zorrillos, it was very easy to find
broken-off roots which had started to grow again, and retofios of various
sizes up to 8 cm. were found. In the other area, in the Sierra de Ramirez,
the ground was very hard and the peons found difficulty in pulling the
plants out. Instead, they had twisted them off just above the surface,
and from the butts remaining, with very few exceptions, new shoots had
grown during the season of 1908, these measuring from 3 to 8 cm. in
height. This parallels the behavior of plants cut off at some distance
above the surface of the ground.
Experiment 60.—Station 2, quadrat 1. 25 square meters. Nov. 5,
1907. 140 plants cut off at a height of 8 to 10 cm. above the sur-
face of ground.
1 The total number was not determined in April 1909, but would doubtless
have indicated a larger percentage.
Reproduction. 67
Jan. 6, 1908. Many buds 2 mm. long.
Feb. 18. All but 5 plants budded. Longest leaves, 25 mm.
Mar. 29. Little change. Longest leaves, 30 mm.
July 28. 5 plants dead. Longest stems of new shoots, 7 cm.
Sept. 12. 12dead altogether. Newshoots 1oto 15 cm.long. Plenty
of flowers. Some plants have the appearance of witches’ broom.
Apr. 3, 1909. 13 dead. Maximum stem-growth, 20 cm.; minimum,
3 to 5 cm. New shoots in several cases killed by drought (plate
16, figs. A to C).
Experiment 56.—Station 1, quadrat 2 (25 square meters). All plants cut
off as in experiment 60, Nov. 5, 1907.
Jan. 3, 1908. No growth.
May 29. New stems (upwards of 15 mm. long, 4 mm. diameter)
on the majority of cut stems.
Apr. 3, 1909. Maximum growth, ro cm. stem-length.
Experiment 126.—Station 5, quadrat 2. Jan.15,1908. All plants cut
at 15 cm. above ground.
May 6. Nearly all well budded.
Apr. 5, 1909. New shoots 1o to 15 mm. long; flowered well in 1908.
From the rainfall data it appears conclusive that the best time to cut
guayule, with reference to reproduction by retofios, is just before and dur-
ing the rainy season. As we shall see, however (Chapter V), this is the
period of active growth, and the rate at which the accumulation of rub-
ber takes place is such as to indicate that the practice of removing guay-
ule at this time is not advisable. Therefore, other considerations aside
(such as competition with other plants), the removal of guayule even dur-
ing the most trying seasons will not exterminate the plant, except on re-
stricted areas which may be rehabilitated by spreading through seed. It is
scarcely to be doubted that even in the quadrat of experiment 121 a few
retofios made their appearance after the last date of observation, which,
unavoidably, was before the summer rains. Furthermore, we are able to
say from observation that the conditions at this station were more rigorous
than at Station 2, where an earlier start was made by the retofos.
The rate of growth of induced retonos will be seen to exceed the
initial growth of seedlings. The stem-growth for the growing-season of
1908, as shown by observations taken on the above experiments in April
1909, was upwards of 15 cm., the average amount of growth falling some-
where near to8cm. The retofio in plate 9, fig. B, plant No. 10, made a
stem-length of to cm. in about three months, and would probably have
made more growth had it been allowed to remain.
As between the pulling and cutting methods of gathering guayule,
there can be no two ideas as to the relative effect upon the rate of repro-
duction by means of retofios. In adjoining quadrats (experiments 114
and 115),in which it so happened that the same number of plants was
removed, in the one by cutting and in the other by pulling, the clumps
of retofios were as 59 to 15. This is explained by the fact that the roots
left in the ground when the shrub is pulled up are not only fewer in num-
1 After this was written this quadrat was visited on April 5, 1909, and it is of
interest to note that the belief expressed was substantiated. See experiment 121,
above, on page 65.
68 Guayule.
ber but smaller than those left when the shrub is cut. The larger break
off furtherin the ground andare therefore less favorably placed for starting
afresh. The disadvantage of the cutting method in the eyes of those who
are in pursuit of the greatest possible initial return is that less tonnage
per acre is obtained, a loss, however, which would be made good many
times in new plantsif the roots were properly cut and allowed to remain.
SEED.
VIABILITY.
The seeds of guayule appear to have a fairly long period of vitality,
a conclusion, however, which is inferential and has not been demonstrated
by direct experiment. The view is based on the following experiment
(exp. 78): On November 23, 1907, a lot of trays (such as are shown in
plate 45) were filled with paper tubes of 1 square inch transverse section.
The trays were then filled with soil made up of half and half garden soil
and old dry manure from a horse corral. In the top of each tube were
sown 20 to 30 seeds. The trays were watered abundantly by subirriga-
tion, it being the purpose to try the method of using the trays with paper
tubes for wholesale germination. So far as this was concerned, the experi-
ment was a failure, but it served to contribute to our knowledge of seed
vitality. The very dry season made it very difficult to keep the surface
soil moist, and as a result of alternate drying and wetting the upper part
of the soil became caked and there was considerable efflorescence of salts.
The soil below became soggy and sour, and fungi permeated the soil and
the paper of the tubes. Very few seeds germinated, not more than one
or two in each tray, partly, as was later determined, because of the char-
acter of the soil, and partly because of the prevailing low temperatures.
The trays lay thus, occasionally wet by showers, till the following July,
when a large number of seeds started to germinate. In one tray 138 tubes
had seedlings, from one to eight in each. By July 25 the seedlings had
developed two foliage leaves, and by August 28 a stem-growth of 5 cm.
was not exceptional, with leaves 5 cm. long. Some plants had at this
date as many as seven foliage leaves. Thus it will be seen that the seeds
which germinated did so after six months’ exposure to conditions about
as bad as could be imagined, being alternately wet and dry, in a sour
soil, and open to the attacks of fungi. The germination in trays favorably
placed with respect to shade was upwards of 13 per cent of viable seed,
as nearly as may be calculated.'! It may therefore be concluded that the
seed of guayule, being neither very short-lived nor very sensitive to unto-
ward conditions, is, from a biological point of view, quite efficient for the
preservation of the species.”
It is, in the nature of the case, well-nigh impossible to determine
the percentage of germination of nature-strewn seed, but one successful
experiment affords us exact data (experiment 192). On May 30, after a
1 Critical germination tests to determine the viability of seed have been
made by Kirkwood (19 10a), who finds the germinations to scarcely exceed 14 per cent,
and that after eight months there is a marked drop in viability.
? Ready germination from seed collected during the summer of 1908 was
obtained in July 1909, at Auburn, Ala.
PLATE 16
A--C. New growths after pollarding. A, February 18, 1908;
B, March 29, 1908; C, Apmnil 5, 1909.
D. Seedlings in limestone soil; E, in “garden” soil.
LLOYD PLATE 17
A. Minimum, average, and maximum seedlings. (Station 2, quadrat 4.)
B. Irrigated plant, two years old, from a stock. April 1909. Cedros.
y Men'S
ACY
2 : vith
‘ Ht) past! att oh
ve
ys
a Pes
r
aA
ln EP OR
Vi
aj!
Reproduction. 69
rain of 16.8 mm. during the preceding night, 4 ounces of seed (including
chaff) were sown at Station 7, in 5 rows, each a meter long. The ground
was previously cleared of all plants and thus loosened, and, the seed
having been left uncovered, the seedlings were exposed to full insolation.
On September g following, 119 seedlings were counted.! These compared
favorably in appearance and size with other seedlings found growing
spontaneously in the surrounding area. The seed was sown more thickly
than would occur in nature, and the number of seedlings was also much
greater, and far too great for their normal development.?
Comparison of these results with those obtained by observation of
germination in irrigated ground affords considerable interest. About
150 plants, placed in a small patch of ground by Mr. C. T. Andrews in
the spring of 1907, flowered freely during that and the following year.
A very large number of seeds must have been disseminated, notwith-
standing a good deal had been gathered, of which fully 30 per cent were
viable. During the summer of 1908, at the time (June) when seed was
germinating in the surrounding region under natural conditions, some
seedlings were observed. About 50 were counted, but in the whole
area (0.1 acre) there could hardly have been more than a few hundred
at the outside. Nor did they grow as well as field seedlings, perhaps
because of the rapid drying of the superficial layers of soil. The percent-
age of germination here must therefore have been exceedingly small,
and much less than that which occurred in experiment 192 above de-
scribed, and also than that which takes place in nature, if we may judge
by the numbers of seedlings actually found in the field in the summer
of 1908. The following observations are pertinent here:
(1) Station 3. June 1908. In areas of 1 square meter, representative
counts of 8 and 14. April 1909, 23 living seedlings of 1908 were
found on the whole quadrat (100 square meters).
(2) The region about Stations 7 and 8. On June 24 a large number of
seedlings was seen.
(3) Station 2, quadrats 5 and 6. Sept. 12. Four seedlings 10 cm. apart.
Nearby 6 seedlings 10 cm. apart. Several counts showed about
20 plantlets per square meter. None on previous visit to this
station, July 28.
(4) In 1 square foot on the same area, 6 well-grown seedlings. Sept.
12, 1908.
(5) In a wire-fenced quadrat which was cleared of all plants (other
than guayule) by Mr. C. T. Andrews early in 1907, 5 miles north
of Cedros in an open plain, leaving one tall guayule plant in the
middle, no seedlings appeared till after June. In September 29
seedlings were found within 6 feet of the plant, chiefly in one
direction. One mariola seedling was found.
(6) Station 8, quadrat t (too square meters). 24 seedlings, Sept. 1908.
(7) In 4 square feet, on a loma north of Cedros, near Station 8, Aug. 8,
31 seedlings, all of 1908 except one of 1907. This number in-
cluded one of Parthenium hysterophorus.
+The largest of these had a stem (epicotyl) 1 cm. long, with leaves 4.7 cm.
long by 1.5 cm. broad.
*In April 1909 it was found that all the seedlings had been destroyed by goats.
70 Guayule.
(8) Near this place 22 seedlings were collected from two areas, each of
12 square inches; 1 of 1906, 8 of 1907, and 13 of 1908.
(9) Station 2, quadrat 4. April 1909. 281 living seedlings, all of which
germinated during the growing-season of 1908, were collected on
Ioo square meters (plate 17, fig. A).
(to) Endlich reports finding ‘‘as many as 50 young plants around full-
grown trees” (335, 1905, Eng. tr.). Such a large number is not
common, but it is not unusual to find 25 seedlings with two foliage
leaves about the base of a single plant.
From such observations it is clear that in particular areas one may
find by chance many more seedlings than could by any fortune develop
into mature plants. Other areas, however, are quite bare of them. Again,
many seedlings which get started die in the course of time, and there can
be no doubt that the percentage of deaths is great. Counting seedlings,
therefore, is not a dependable method of determining the rate of repopu-
lation. For this purpose it is necessary to make a census of sample quad-
rats, making as careful estimates as possible of the sizes and ages of the
plants. The datain tables 4 to 13 afford such a census. They are summa-
rized in table 25 and are further displayed graphically in fig. 12.
TABLE 25.—Classitfication of guayule plants from seed according to weight on various
’ quadrats indicated.
Quadra. efibs on |e Reger atten [toatl Selby Mason ibe ts
}
Maplerae 215 Nees ° fo) fo) 10 | 100 585
Ge Fxesee lee og ° 7 13 15 II Aly |
[OOS es B63 obb!O pie 7 5 144 40 | 4 TOnp ss
soho Ha haee i este pr fe) 2 12 20 | 9 86
LO eons a aA ee ° ° o(?) | o(?) | 250(?) 755
IN) oiaa o eaenoresd <) o-c 2 3 5 | 4 | 4 6
aH rae nee ol Cer GueNe OCF G ° ° I 28 | 59 go
TQ eapence ee oneness fe) fe) I 23 ues 232
DQiaehs cre Glarsirmysper ° I 12 | 45 | Be 166
1 It is to be recalled that the larger plants had previously been removed from this quadrat. The
estimate marked doubtful is based on the figures of adjoining quadrats, and can only be approximate.
It is clear that the ratios between small and large plants, as shown
in table 25, indicate very different degrees of efficacy in reproduction
commencing from the seed. This method is the best available in the
absence of actual counts of seedlings year by year, obviously not practi-
cable. A few such counts, for future comparison, are given in table 26.
TABLE 26.
| a | When oe
| SHIMON a GRRENElEtiE Ss sot ped id peo 24 | 1908
WhStation 3. "Guadrair Nee. 2eaone. A 23 | Apr. 2; 1909
| Station 2, quadratiwceassa test Jee 281 | Apr. 3, 1909 |
| |
These few data, the difficulty of obtaining which, on account of vari-
ous circumstances, was very great, have only suggestive significance. It is
Reproduction. at
obvious that in the last station reproduction by seedlings is relatively very
good, especially as the counts were made at the close of a long drought.
A condition such as this might, in the light of table 25, be expected to
lead to a good stand of guayule. From a consideration of the curves
based upon table 25, some further points of interest are discovered. There
is a large falling off in numbers of plants between the average weight
of about 4and 12 ounces. This, as seen in the curves on pages 87 and 88,
is the period, approximately, of maximum rate of growth, viz, between
8 to ro and 13 to 15 years of age, during which time there is a loss of
total weight of about one-fourth to one-third, as nearly as we may calcu-
late. From the nature of the conditions, many of which are undetermi-
nable, such calculations can be only loosely approximate, but it can hardly
be doubted that, if the rate of reproduction by seed from plants, say
from 6™to 8 ounces in weight, can be depended upon quantitatively as
600
500
RELATIVE NUMBER OF PLANTS
0-4 eon = 2 3 3-4 4-
POUNDS
Fic. 12.—The relative numbers of various-sized plants on different quadrats. The numbers
at the ends of curves refer to the tables corresponding.
indicated in the table under consideration, it is an economic loss to allow
plants larger than these to remain. From this point of view alone it
may not pay to allow the plants to remain after the age indicated by
the weight of 6 to 8 ounces has been attained, as the numbers which die off
are great enough to cause a considerable falling off of total weight.
The data show also that the initial monetary return from a harvest-
ing of guayule may be as great or greater from a stand of a few large
individuals, but the areas with large numbers of smaller plants give
promise of future returns.
An important desideratum is to determine how to improve these
conditions. Here, let us say, is a good field of guayule, as regards first
returns. The bulk of the weight is in large plants, and the small ones
are too few for a ready reseeding of the area after depletion. It is hardly
too much to say that vast areas are in this condition. What may be
72 Guayule.
done to increase their productivity is still a question for experimental
determination, but seeding in favorable years by means of seed from
densely grown areas would be distinctly beneficial. The importance of
seed is so great that in the harvesting of shrub the practice of leaving
large plants for the purpose of producing seed should in all circumstances
be initiated. As a practical question of economics, the difficulties of
time and distance in the desert are so great, not to mention those arising
in connection with climatic irregularities, that any attempts to better
conditions over wide areas are fraught with expense which may not be
considered as warranted by those interested.
COMPARATIVE ABILITY TO GERMINATE IN THE FIELD.
The ability to germinate promptly, to attain a condition of physio-
logical resistance, is of prime importance to desert plants, and very much
more important to them than to plants which are more favorably placed
with reference to water-supply (Ganong, 1907; Lloyd, 1g09a). So far as
the question of germination is concerned the evidence is not forthcoming
that desert plants exhibit more indifference to initial water-supply than
others (Livingston, 1906). For the rest, as for further elucidation of
this problem, much comparative study is necessary. There seems to be
little doubt, however, that the rate at which physiological resistance is
acquired and the amount of this resistance are very different in different
plants. For example, the seedlings of many succulents soon acquire
the characters of the parents, the cacti (Ganong, 1898) being notable
examples of this. This must be of no small weight as a factor in enabling
young plants to withstand the rigors of drought, though this very cir-
cumstance in the cacti opens them to the attacks of animals (MacDougal,
1910), so that millions of seedlings are eaten, affording both food and
water to desert animals.
As has been shown, and as will be further developed in the following
chapter, the guayule seedling offers no exception to the rule that desert
plants need an abundance of water during the period of germination.
Observation in the field indicates further that marked readiness in ger-
mination is not in any way indicative of adaptation to desert conditions.
A field test of the germinating ability of guayule in comparison with
that of alfilaria (Station 7, May 30, 1908, exp. 139) showed that about
3 per cent of the seed of the latter germinated, while about o.2 or 0.3 per
cent of guayule succeeded in getting a foothold in the same place under
the same conditions. These figures are probably too low for both plants,
inasmuch as ants were observed carrying off seed on each occasion that
the station was visited. This test, however, may indicate the direction
in which research may contribute toward the explanation of the success
which the alfilaria has had in invading desert territory.
A further observation was made at Camacho, on the Mexican Central
Railway, on the Hacienda de Cedros, a point for the shipment of guayule,
where a stack-ground had been kept supplied with shrub from the neigh-
boring region. It is customary to bring in the shrub in loose bundles on
the backs of burros or in carts of various sizes and kinds to these ship-
ping-points, there to be made up into bales for handling on the railroad.
LLOYD
B, Apnil 13, 1908
A, May 25, 1908;
Seedlings growing in different soils:
R061 *S AeyAl ‘a $OQ61 ‘Q eunl ‘Wy :s]los quoleyIp ur sumo sBul[pesg
‘ Ah
4
Le,
pr)
7 \x q
gx
y
//
PLATE 20
“elo cm
4
A. (1) Root-cutting; (2 to 4) Sectional root-stem cuttings (Exp. 146).
B. Seedlings grown in different soils, August 1908.
Reproduction. 73
Countless numbers of seeds are therefore strewn upon the ground, and
indeed the new plants of guayule which spring up on these stack-grounds
sometimes afford valuable data on the rate of growth, although decep-
tive notions as to the numbers of plants which may be expected are some-
times acquired. The conditions of a stack-ground are, at Camacho at
least, a rather severe test, as it lies out in the open, dry plain, exposed to
full sunlight. At the same time, the surface of the soil is mulched by the
débris of broken-off guayule twigs, and thus the conditions are amelior-
ated. Shipments, which had been made for a year or longer, ceased in
the fall of 1907, and the spot was under occasional observation for some
time before and from that time on, till the following September. Al-
though it is known that much guayule was brought in in flowering con-
dition and that seed must have been dropped in large quantities, the
conditions for germination, especially the meager rainfall, were not favor-
able for guayule, though the seeds of the plants in the following list
were found in all conditions of development in June and July of 1908: '
Helianthus sp. 15 to 18 inches tall and many in flower.
Amaranthus, 2 species. Plants 3 inches tall.
Cassia (“‘coco’’). Many mature plants in flower.
Prosopis seedlings with the plumule well developed.
Euphorbia of 2 species. Mats 1o inches in diameter.
Solanum sp.
A cucurbitaceous vine.
Chenopodium, a species with broad deltoid leaves.
Grasses of 4 species.
Spheralcea, mature plants.
In addition to these seedlings, the roots of Prosopis and Covillea,
which had been cut off in preparing the ground for stacking, had sent up
shoots from 10 to 20 inches in length. That not a single guayule plant
sprang up is at first surprising, not to say disconcerting, but in the light
of experimental evidence it becomes clear that the guayule germinates
only under highly favorable conditions. For some time it has a low de-
gree of resistance, and is in point of fact of distinctly mesophytic charac-
ter. It is only when due regard to this is had that the maximum rate of
germination may be expected under cultural conditions.
HABITATS OF SEEDLINGS.
The particular preference of the guayule for certain germination
habitats is of importance in its bearing on the effect of clearing land of
other plants. It has been repeatedly observed by the investigators at
the Desert Botanical Laboratory, and by myself in Zacatecas, that there
are usually to be found many more plants of smaller size growing in the
partial shade of shrubs than elsewhere, and it is to the protective effect
of this shade that the many curious juxtapositions of perenrfial plants
may be referred. An example of this is the frequently seen saguaro
(Carnegeia gigantea), standing in a position indicating that it germinated
in the shade of a palo verde (Parkinsonia microphylla) or some other
shrubby species.
As regards the guayule, Endlich (1905) speaks of “‘ the large numbers
of young plants sometimes found surrounding the older trees * * * in the
1 Every annual had disappeared by April 1909.
74 Guayule.
territory around Jimulco, for instance, as many as 50 young plants have
been found around full-grown trees.’’ But,on the other hand, speaking
of the occurrence of young plants in supposedly very unfavorable spots,
Endlich explains this by saying that “it is likely that they have been
developed from such seeds as were either stamped into the ground by
goats (as these animals are the ones which commonly graze in the guay-
ule territories), or had been dropped by these animals and thus found
favorable conditions of development in the animal excrements. It
would, in fact, be difficult to find any other explanation for the enormous
growth of the guayule plant in small, isolated places (having usually
the size of the resting-places of the herds of goats) * * * .”
As to the supposedly favorable conditions afforded by animal excre-
ment, it may well be doubted that these are more so than the soil itself
affords. Experiments have shown that soil at all rich in humus derived
from manure is distinctly unfavorable for healthy germination. Even
“garden’’ soil at Cedros, with no addition of manure, is less favorable
than the unaltered lime-charged soil of the normal guayule habitat (plate
16, figures D and E). Even after thorough leaching from exposure the
possible advantages are hardly important, and, at all events, in such
situations the seeds and seedlings have no advantage of shade, as the
herding-spots of goats are usually bare of vegetation. Nor can the
stamping into the soil by these animals have any value, as the seeds ger-
minate well only with very shallow soil covering, as much as 2 mm.
depth being enough to show a marked decrease in germination.’ It would
seem, therefore, that if Endlich’s observations are correct as to the occur-
rence of guayule seedlings in such situations, it is safe to infer that the
rainfall conditions are, on occasion, such as to make ready germination
and early growth possible for a good percentage of seeds even in open
bare spots where no advantage of shade is offered. My own observations,
at any rate, sustain this view. Experiment 139 (see p. 72) is a case in
point, and the results were supported by general observation during the
summer of 1908, when there was a fairly generous if not a maximum field-
germination. The net result of this season is indicated by the numbers
of seedlings observed in April 1909 (see p. 70). These are known to
have germinated at or during the growing-season of 1908, and had suc-
cessfully sustained prolonged drought till the time of observation. At
no other point was there seen a better crop of seedlings at the age of
these, and they germinated without the least protection, as the quadrat
had been completely denuded.’
Nevertheless, when seedlings are observed in the field at other than
favorable seasons, it is frequently noticed that the larger numbers are
in the protective shade of other plants; but this is not peculiar to the
guayule alone. The explanation, we believe, is not that the guayule
seedling is ombrophile, but that the eliminating effect of the drought
period subsequent to a period of germination is more drastic elsewhere
1 Kirkwood, roto.
* By contrast, it should be said that at Station 1 only very few seedlings
were found on a large area denuded of all plants save small guayule. As goats
had been pastured here, however, it is impossible to draw any conclusions.
Reproduction. 75
than in the shade. Thus, in February 1908, small seedlings with 1 to 5
foliage leaves could be found beneath the shade of an occasional larger
guayule plant, but in a precarious condition, some dead, others moribund,
and plainly the survivors of the crop of seedlings of late in 1907, the chief
part of which had succumbed to the very severe conditions already noted
as having prevailed at Cedros at that time. As bearing upon this ques-
tion, we may note the meager occurrence of Opuntia leptocaulis in south-
ern Arizona, where it is scarcely to be found except protected by some
plant, while it grows in the open in great abundance in Zacatecas. It
appears evident that in Arizona the conditions for its persistence, except
when it is more or less protected by other plants, are too severe. No such
relation has been observed in Zacatecas, and it would seem that the cli-
matic conditions there are distinctly more favorable for this plant.
It would therefore appear safe, if desirable, to clear guayule fields
of the major part of other vegetation. An occasional year may be ex-
pected when the rate of germination will go far toward producing a good
stand of young plants. Those already growing will offer protection to
the younger brood, and the larger area available for guayule plants will
in part compensate for the loss of shade given by other vegetation. It
would not be advisable, however, to remove the occasional palma saman-
doca (Samuella carnerosa), which produces fiber, or the large barrel cacti
(‘‘bisnaga burra ”’ and “‘ bisnaga colorada’’), as they are heavy plants and
neither spread with appreciable rapidity nor occupy more than a negli-
gible fraction of the ground (plate 1, fig. A). This principle of practice
is, however, in the nature of a compromise, and rests upon an estimated
balance of circumstances. A more correct estimate of probabilities could
be based only upon longer observation under experimental conditions.
RATE OF REPRODUCTION AND OF GROWTH.
RATE OF GROWTH DURING GERMINATION.
This period may be divided into a period of tissue expansion and
one of induration. At the close of expansion, which begins in about a
week’s time after sufficient rain, and occupies a second week, the seedling
is tender, the hypocotyl white and translucent, and the cotyledons green
(fig. 8). The cuticle then thickens, and red color is developed in the
epidermis of the hypocotyl and under surface of the cotyledons, while the
latter become darker green and more indurated. This occupies a third
week, when, if no untoward circumstance interferes, the first foliage
leaves develop. Even under the best of conditions this period of three
weeks will scarcely be shortened.
The further seedling development is a direct function, other things
being equal, of the rainfall, the maximum potentiality, it may safely be
said, never being exerted by field plants. This apparently extremest
limit of growth for a seedling was reached by one of two particular indi-
viduals under cultivation, and constantly supplied with an abundance
of water. The height of this plant when the rhythm-limit was reached,
as indicated by cessation of growth, was 25 cm., and it had a spread of
22cm. It was a fully-developed specimen, in which each branch reached
76 Guayule.
its proportionate size. It flowered freely and produced fully 2000 seeds
(exp. 139@; plates 18 to 20). The time occupied in its growth was about
four months.
We may now offer data (table 27) derived by field observation during
the growing season of 1908, which was a favorable year, though not per-
haps exceptionally so. The rate of growth during germination is indi-
cated by the measurements of seedlings from Station 3, collected July
15, 1908. They were two to three weeks old.
TABLE 27.—RKate of growth during period of germination.
| Hypocotyl. Cotyledon.
|
Length. [ibreseios Length. | Breadth.
|
mm. | mm. | mm, | mm,
6.5 lL aeies Sc Siealeans3
6.5 Or.5 ase || Pa
To Sone ato Worn) 23.45
1gat te) | if 56) Arsyat | 4.0
Table 28 contains data based upon the individual examination of 112
seedlings collected in the field on the dates mentioned. The measure-
ments are exclusive of the hypocotyl, which measures about Io mm. on
the average.
TABLE 28.—Amount of growth of seedlings in the first season of growth.
Height includ- |
We aeall Maneuieavess | Length of stem. |
Collected. seed- | | Locality. Notes.
lings. l | | | |
| Max.| Min. | Ave. | Max.| Min. | Ave. |
H | ! | | ——a
mm. | mm.) mm.| mm. | mm. | mm. |
Feb., 1908| 11 17 7 | 10 2 ° r+ Loma north | Germinated Nov. (?),
| G5 | of Cedros. 1907. 2 to § foliage
| leaves. Cotyledons
| | long gone. Plants of
| | | very slow growth.
June 2, 1908 I i eaNet (Xe You lnearichs| Perens ZO NS a: | ‘Station 2. . 22] An exceptional and
| very large seedling
for this date. Inflo-
| | rescence 2,5 cm. long.
Aug. 8, 1908 28 «| 33 6 20 2 ° 1+ | Loma north | Cotyledonsstill attach-
of Cedros. ed. 1 to 7 foliage
leaves.
Sept. 8, 1908 24 65 Io 35 I2 1 | 5.7 | Sta.8, quad- | Good healthy speci-
2 (ean | mens.
Sept. 12, 1908 6 75 47 62 17 5) |g). OF | Sotabion) 2be . a. Do.
Sept. 12, 1908 II 90 50 73 50 7 \24 Station 2..... 2inflower. Max.stem
diam. 3 mm.
Sept.12,1908| 31 75 30 45 20 B 9 Bare quadrat | Good healthy plants.
| | in plain be-
! ' | tween Ce-
Grangea vier sis checsanttal repre vee | eaesiever | emeeaerall Geueasic 14.8 | dros and
| | | Sta. 2.
|
The seedlings in table 28 were not selected, but were, in each
case, all the seedlings found in a given area. Taking those collected in
September—which, judging by the behavior of guayule plants in gen-
eral, was near the close of the growing-season—we have an average rate
Reproduction. 77
of growthof about 14.4 mm. in stem-length (epicotyl), aside from the small
secondary branches. With few exceptions, the seedlings of a month pre-
vious (August 8) were very small, as indicated in the table, but neverthe-
less the size attained by them, judging from experience in their culture,
must have been the result of at least six weeks’ growth.
This was not the close of the growing-season, but I was fortunately
able to complement the above data by measurements of seedlings, already
mentioned in other connections, which had passed completely through
the growing-season of 1908 and been collected! in April 1909, ina state
of dormancy. The measurements of 311 seedlings were made by caliper.
Tables 29 and 30 give the data for two quadrats; a third, having 281
seedlings, 4 of which are seen in plate 17, fig. A, is not given in detail.
Combining the averages obtained from tables 29 and 30 with the data
for Station 3, quadrat 4, obtained at the same time as those of Station 2,
quadrat 7, we obtain table 31.
TABLE 29.—Growth of seedlings which germinated about June 1, 1908, and examined
April 2, 1909. Station 3. All within 100 square meters.
Length eae a
Nee ORE Demet Remarks,
pocotyl 10 mm.)
mn. mm.
I 35 Biss Flowered, flower bitten off; branched.
2 19 B se Branches 1 to 3 mm.
3 9 2.0 Unbranched.
‘A 10.5 2.2 Lateral buds just started.
5 II 2.0 Unbranched.
6 II 2.0 Do.
7 5 2.8 Two buds at base of hypocotyl; otherwise
unbranched.
8 8 2.0 Slender branch 5 mm. long at base of hypo-
cotyl; otherwise unbranched.
9 6 owas | Unbranched.
10 4 2.0 Do.
II ee 2.0 Do.
12 52 2.0 Do.
13 5.0 2.0 Do.
14 4.2 2.0 Slightly damaged; unbranched.
15 4 1.8 Unbranched.
16
C7 | BAe Meee, Do
18
19 4.0 TF .2 Do.
20 3.0 I.2 Do.
21 2.5 EO Do.
22 r7.20 °.8 Unbranched; etiolated.
23 8.0 Tae. Unbranched; slightly etiolated.
Ave. 8.1 Eo
23 seedlings; average length of main stem, excluding Nos. 22 and 23, which are
not normal, 7.65 mm.; average diameter of main stem at base, 2 mm.
N.B.—The exact age of the above seedlings does not exceed 10 months. Of this
period, 64 months were without rain, beginning with the middle of September. All
the seedlings were alive at the time of collection.
1In company with Mr. G. E. Pell, of New York.
78 Guayule.
TABLE 30.—Growth of seedlings (all unbranched) which germinated about June 1.
1908; collected April 3, 1909. Station 2, quadrat 7, 100 square meters.
Length of main
N stem exclusive Diameter at
) of hypocotyl base.
(about ro mm.),
mm, mm.
I Anes 2.0
2 5.0 eV lo,
3 sac) 2.0
4 6.0 2) aie)
5 3-5 1.5
6 6.0 133
7 5.0 T.0
Ave 5.0 Tey
TABLE 31.
Length of stem. Diameter of stem.
No. of seedlings |. Ps Be, ee Be a
in quadrat. |
Max Min Ave Max Min Ave
: , - = |
mm mm. mm. mm, mm mm |
23 35 a5 8.1 Shersy 0.8 1.8
7 6 3.5 5.0 20 1.0 re
281 55 Tas 12.6 6.0 0.8 2.8
gui
Ave. 32 2.5 8.5 3.8 0.9 a a
|
It will be seen that the average maximum amount of growth for the
whole of the growing-season of 1908, as indicated by the data obtained in
April 1909, is 8.5 mm., stem-length. This, however (as shown by table
28), is less than the amount determined by the measurement of seedlings,
germinated in 1908 but collected on September 8 to 12 of that year,
namely, 14.8 mm. The difference in favor of the earlier collections may
perhaps be explained by the fact that care was not taken to take every
seedling in a given area. To do this requires a minute search, which was
given only in April 1909. It is not improbable also that other seeds
germinated later in the season, though this is not likely. It is therefore
safer to conclude that the average amount of growth in length of the
epicotyledonary stem for the season of 1908, taking all seedlings into con-
sideration, isnot more than 1 cm. If we should consider only those which
germinated at one time, at the beginning of the growing-season, this
amount would probably turn out to be somewhat greater. Under the
conditions for the period in question the maximum amount of growth
was 5.5 cm.; the minimum, 1.5 mm. Seedlings of these dimensions, and
two illustrating the average growth of 281 seedlings (Station 2, quadrat
4), are reproduced in plate 17, fig. A. Measurement of the main shoot alone
throws out of account the growth of branches, so that a fuller conception
of the amount of development possible for a seedling under natural con-
ditions may be had only by seeing the plants themselves.
Reproduction. 79
RATE OF GROWTH IN MATURER PLANTS BEYOND THE
SEEDLING STAGE.
In general forestry practice the use of formule is directed toward
‘estimating the amount of lumber in the trunk. The deduction of these
formule is easier in the case of coniferous trees because of the continuous
growth of the chief shoot. Special problems demand formule based
upon other data than the rate of growth of wood, e.g., in the business of
producing cork from Quercus suber. When forestry practice is directed
toward the culture of camphor trees, for example, in which the whole
bulk of the plant is to be used, the desideratum will be to determine the
rate of increase of weight. This is the case with guayule, since the whole
of the plant is used in the process of extraction of crude rubber. But
the rate of increase in weight can not be determined without introducing
the time element, so that we must first determine the rate of stem elonga-
tion in order to arrive at a general average of growth. But plants of the
same age are not invariably, or even quite usually, of the same weight,
since the relation of a plant to its environment results in more or in less
bushiness, in partial death and consequent loss of branches, in unusually
slow or rapid growth, or in total loss of plant by death. In estimating
the weight of shrub per unit of area for some future time it is evident that
all these factors are disturbing elements, the values of which may not be
easily determined. About the best we can do, therefore, is (1) to determine
the average rate of growth in length of stem, and (2) to determine the rate
of increase in weight for critical periods. The data indicate that there isa
period of relatively highest growth-rate, expressed in stem length or height,
and a period of relatively greatest increase in total weight of the plant.
RATE OF GROWTH IN TERMS OF STEM-LENGTH.
It has already been shown that the first season’s growth results in
an average stem-length approximating 1cm. A stem of this size has no
branches. During the second season’s growth the stem may simply
lengthen, or it may also produce a number of short branches. This it
is more certain to do if the chief shoot produces an inflorescence. It
may otherwise merely elongate strictly for a number of years, resulting
in a very slow increase in weight, since the weight is affected chiefly by
the number of branches. At best the total weight assumed by a plant
in the first 7 to ro years is small, seldom exceeding a few ounces.
RATE OF GROWTH IN EARLIER YEARS AFTER GERMINATION.
To determine precisely the age of a given seedling is more difficult
than would seem at first glance if it has been exposed to the weather for
more than a year. Furthermore, the rate of growth in many individuals
is so slow that the marks become well-nigh effaced, if not quite so. In
obtaining the following measurements, only plants which showed the
markings plainly enough to be seen clearly have been used. This has
very naturally thrown out those of very slow growth, in which the diffi-
culties are greatest, and thus the resulting average datum is probably
too great. By way of orientation two extreme cases may be cited. One
is a seedling of two seasons’ growth, which germinated in 1907, making
80 Guayule.
in that year 3 cm. and in the following year 11 cm., a total of 14 cm. in
the two years. This is the largest field plant for its age that I have seen.
In contrast is cited a seedling of slow growth, fully 7 years of age, entirely
without branches, and only 6 cm. in height. The average rate of growth
falls between these extremes, but nearer the lower. For the sake of brev-
ity, as it would serve no useful purpose to introduce large tables of fig-
ures, the summaries of measurements alone are given.
The average rate of growth of 30 seedlings from 2 to 5 years old during
particular years is as follows:
TABLE 32.
| ||
| Age. 1908. 1907. 1906. 1905. 1904. Average amount of growth.
|
mm. mm. mm. mm. mm. |
|In first year, 17 mm.
2 15 16 ¥ sf sta | In second year, 20 mm.
In third year, 37 mm.
2 45 22 20 We oe |
4 cr 30 24 15 ve | In fourth year, 51 mm.
31 mm. |
The average amount of growth in seven seedlings for the last three
years, 1906-1908, is 26 mm.
Some ten seedlings for each of the localities mentioned below were
measured, giving average amount of growth for two to four years, as
follows:
mm. mm.
Sierra Candelariasc ace mee eit 22 | Cerritos de los Calzones........ 20
Station 4 (Sierra Guadaloupe) «247 | "Cedros.22% Faerie eee nee 34
Station 5 (Sierra Guadaloupe) . 18 | Apizolaya.........8c2.068>o- 42
Station 2 (Sierra Zuluaga)..... 31 | Lomerio de los Zorrillos....... 49
Station 2 (Sierra Zuluaga)..... AG) ean CAnAClO Une re ers ieeincihe ctor 26
Station 1 (Jaguey)............ 30 Average rate for all....... 30
It will be seen that these figures, made at different times on material
from different localities, check each other fairly well. As said before, the
average rate of growth thus deduced is probably somewhat high. The
rate undoubtedly increases toward the fifth year, and a somewhat more
rapid rate is then maintained during a few years, say from the fourth to
the seventh, during which the total height of the plant increases at a
greater rate than before or after. Usually during the second or more fre-
quently the third year a set of branches start their growth, and with this -
the weight increases more rapidly. What this weight may amount to in
four years is shown by 3 thrifty plants taken on the Lomerio de Zorrillos.
These made growth as follows:
TABLE 33.
Dry
weight.
mm.
Io
20
grams.
12
30
30
Reproduction. 81
_Hence we may conclude that the weight gained in four years’ growth
can scarcely exceed 1 ounce, and probably seldom amounts to that.
The following are measurements (in millimeters) from rapidly grow-
ing plants from Station 2, collected in January, 1908:
TABLE 34.
Plant. | 1902. | 1903. 1904. | 1905 1906. | 1907. | Notes.
mm. mm. mm mm. mm. £
40 30 94 32 | 40 Average height from base of 1904
40 25 76 Sus || Bxe growth, 2romm. Weight 17gms.
2 { 16 84 65 | 45 Average height of twigs 194
16 80 100 | 50 “mm. Weight 12 gms.
3 | eee 34 | 67 | ro | Habit strict, with short branches
| above. Weight 5 gms.
4 20 | 30 | 40 | 70 25 | 25 _ | Not less than 6, possibly 7, years
| | old. Height 200 mm. Dry weight
| | 47 gms. |
It is of interest that plant 4, though a slower grower in height than
1, made weight about twice as fast. This is due to the larger number
of twigs. Plant 4 may be regarded as an expression of the best results
which may be expected in this station. We may therefore conclude that
the weight of 4-year-old plants will not on the whole exceed 15 grams or
0.5 ounce, and that the maximum weight for a 6-year plant will not
exceed, say, 45 grams or I.5 ounces.
RATE OF GROWTH IN MEDIUM-SIZED PLANTS.
As in the case of seedlings, the annual accretions of growth have
been measured only when sufficiently clear for certain recognition. The
last 2 to 5 or more years’ growth was measured, according to the visi-
bility of the markings. Several hundred measurements were made in
all, of which the summaries and averages alone are given in table 35.
TABLE 35.—Average amount of growth per year in the localities indicated.
Average
Average
Locality. amount of | Locality. amount of
growth. || growth.
: : mm. | a ian ae ; ory mm.
Sierra Candelaria. :. i 2/1... Aa Bedras: Jee Oe re Ss | 56
Station 2s(Papuey)io-: 2626.55 . 30 Pipiaalcyeigdh fcr set lvet=2 1% hea
2 (Sra. Zuluaga)..... an | Lomerio de los Zorrillos...... | 49
4 (Sra. Guadaloupe). 4I Webmearnaclomenet. 5.4 a: - += = | 28
5 (Sra. Guadaloupe). 38 | acismte ee oes et atk a (> “Se
Cerritos de los Calzones..... : 182 el te
| Average of all........ | 43
i
1 NotEe.—The plants in this locality showed very rapid growth in 1906, explainable by the rain and
by their having been previously cropped back. The branches were few in number, so that the plants,
though relatively tall, were very light in weight. This figure would therefore better be thrown out of
account, in which case the average falls to 38 mm. per year. The datum for Station 2 has been checked
up by a later count, 26 measurements giving an average of 32 mm., and at this point it may be said
that the data above given are collated from measurements made at different times, results being used
as checks, the one on the other.
6
82 Guayule.
In addition to data obtained by observation of external marks, a
number of measurements of field plants were made by the usual labora-
tory method of marking the stem with India ink. The results of these
observations are here given:
Station 2, quadrat 3. 6 twigs marked at the tip with a drop of ink, Jan. 6,
1908. Growth commenced Apr. 28. Last observation Apr. 3, 1909. Measure-
ments as follows, in mm.: 60, 75, 70, 75, 65, 50. Average amount of growth for
season, 66 mm.
Station 1. 5 plants marked Jan. 3, 1908. Last observation made Apr. 3,
1909. The mark had been destroyed on 2 plants. The total amounts of growth
for the 3 remaining were 30, 18, and 35 mm., making an average for the 3 of 28 mm.
All growth was subsequent to May 20.
Station 3. Dec. 31, 1907. 3 marked plants showed an average growth of
1to2cm. A seedling slightly pruned showed 2 cm. new growth by July 15. The
rate of growth in all plants at this station was small in 1907.
Station 6, a low gravelly ridge in the playa, Burrita. 4 plants marked Oct. 11,
1907. On Jan. 11, 1908, 2 plants showed 1 mm. and 2 plants 2 mm. growth each.
The total amount of growth till Aug. 21, 1908, was 13, 20, 20, and 10 mm., or an
average amount of 18 mm. This is a locality of conspicuously slow growth.
The average amounts of growth observed in marked plants for the
season of 1907 were, therefore, 66, 28, 20, and 18 mm., making a grand
average of growth of 31 mm. The average is lower than the one above
deduced from measurements of field plants, but as three of the stations
suffered severely from drought in 1907 the rate of growth was probably
rather low. Our data on the whole indicate that the rate of growth of
guayule in the field lies somewhere between 30 and 40 mm. annually.
This general conclusion can scarcely be said to be too optimistic. It will
no doubt be questioned by those who entertain different ideas of the
rate of growth of this plant. The belief is current in many quarters in
Mexico that growth is much more rapid, it being a common saying that
after guayule has been cut the crop is reéstablished in five years. Such
surprising statements were made to me regarding one locality in particu-
lar that I made special effort to obtain evidence. Although an attempt
to visit the place, some leagues to the west of Escalon in Chihuahua, was
frustrated, I succeeded, through the courtesy of some friends, in getting
a number of plants, which, though of somewhat more rapid growth than
usual, are not remarkable in a special degree. The plants were clean-
limbed and smooth-barked, the effect of this more rapid growth. They
bear evidence of a heavier rainfall as compared with plants from Zacate-
cas, but this appearance is due in part to the fact that they are of two
different types; in one the foliage shoot is abruptly terminated at the
base of the peduncle; in the other the shoots taper out into the peduncle
after the fashion in mariola. The branches in the latter are thin, die back
readily and often for a good distance, and in these plants have some of
the characteristics seen in the stems of irrigated plants. I give measure-
ments of the few plants, which came to me for study,in detail (table 36).
The average amount of growth of each plant for the years indicated
is: ‘plant’1, 36°mm.; plant-2, 41 mm; plant 3, 37 mm; and for all the
twigs on 3 well-developed plants of the first-mentioned type, viz, with
abruptly ending foliage-shoots, it is 37 mm. The data are instructive in
that they point to a ‘‘fat’’ year preceding two ‘“‘lean”’ years, namely, 1907
and 1908. The rate of growth, however, compares very closely with
that derived from material from other localities.
Reproduction. 83
TABLE 36.
| Amount of increase in stem length for—
Plant No. Be
oO.
1905. 1906. 1907. 1908.
mm, mm. mm, mm,
[ I 62 41 8 8 |
5 8 10
No. 1, 85 cm. tall, dry weight 5 lbs. - 3 a4 a |
5 0z., symmetrical, well developed. | 3 45 57 ae an
5 26 II 16
TSE PUCRS! AIR GOP Lace, Je ie ees ear ea 53 41 14 II
I easy) 10 20
2 ee 86 15
: 3 ane an 28
No. 2, 60 cm. tall, dry weight 2.5 lbs., | 4 80 22 18
irregularly developed. 5 55 16 20
| 6 65 10 10
7 80 23 13
| 8 57 8 be)
PAM CT AC CHER T raters hella tae ce 76 gin 17
I 50 60 17 Di
No. 3, 35 cm. tall, dry weight 9.5 oz., |} ? 5° ee ate ze |
ro yrs. old, well developed, symmet- | ; 3 ae f |
Tical. 5 =e me < us |
|
PACVICTIAD Cray Att ar atere arte nine elec e eee © | 48 70 15 7
TABLE 37.
Amount of increase in stem-length
Plant No Branch Hon
i No.
1906. 1907. 1908.
mm. mm, mm mm.
No. 4, seedling 23 cm. tall,weight 10 gms. I S7 go 24
I 60 48 go
No. 5, 38 cm. tall, weight 48 gms....... 2 36 46
3 45 79
PNVCTAG Cac Wi. eee sens etttty ay < ofa '« lscerstol: 60 43 69
I 50 15 50
2 60 30 60
3 Bc 23 40°
4 Se 15 60
OR Set re ie eee one 5 65 12 6
6 20 8 30
7 30 15 45
8 80 30 30
9 35 25 75
AVELAG Cin kit eae Tse ee Coos: 47 19 44
— nt
Gere iam all VEAUSS. calc. 6. «shoe bays | ere leven + a7 |
84 Guayule.
Table 37 gives measurements for the years indicated of 3 “‘spindling”’
plants, which grew rapidly in height but did not develop branches and
therefore weight.
Plant 7 was 50 cm. tall and weighed 153 grams, ragged, but showing
abnormal development on certain shoots. The last three years of its
growth showed accretions, a side-shoot starting low down, of 110, 200,
and 60 mm. The upper shoots appeared quite similar to those of the
other plants, but were more or less damaged, so that one could not get
satisfactory measurements.
The conclusion one is forced to draw from a survey of the above
tables is that in a certain proportion of the plants in the locality referred
to the rate of growth per year approaches closely to 50 mm. In these
plants, however, the branches are thin, and the plants are not well devel-
oped nor heavy for their size, so that, economically considered, there is
nothing gained. Whether the differences in rate of growth are connected
with racial differences in the plants is discussed elsewhere.
RATE OF GROWTH IN IRRIGATED PLANTS.
A considerable number of plants were under observation for the
whole of two growing-periods, during which time they were freely irri-
gated! and grew rapidly, at a rate close to the rhythmic maximum. The
average rate of growth for the two years was very close to 25 cm. per
year, so that a spread of a meter was attained by nearly all of the plants.
The character of the growth is described elsewhere, but the fact here
stated indicates very clearly that plants in the field may never be ex-
pected to reach this maximum. The greatest growth of stem-length in
field plants for one year, 200 mm., was seen in a very few twigs and in
shoots favorably placed, the rest of the plant failing to behave similarly.
The weight attained in two seasons by irrigated plants growing
from small butts after transplanting is upwards of 2 pounds, or slightly
over. The fresh weight of a large plant was 4.5 pounds. Another col-
lected at the same time weighed fresh 3.5 pounds, and shrank in drying
to 1 pound ro ounces. The dry weight of two others was 28 and 32 ounces.
On the other hand, plants under limited irrigation were grown at
Caopas. I have examined three sample individuals of these, a large, a
medium-sized, and a small plant. All of these failed to start promptly,
and had been pollarded. The amount of growth made by them is shown
in table 38.
TABLE 38.
= a i : as 1 w
Se eae Amount cf growth.
, pollarde
Size of plant. ahove (=== ee cot ae
ground. 1908. 1909. Total.
| cm. cm. cm. cm.
LEBER 2 dee Canine ett ee oer 40 II | 9 20
(gies chieeandts, gated. ely, ler oe ReneS 30 £6. tO 18 | LO tore? | 25 tego
| Srna Ee te a ychaage Meee nce ce 5 13 8 21
*In 1907, through the winter until the following April. They were not irri-
gated later, but received rain in the summer. They had a sufficient amount of
soil-moisture for continuous growth.
j aan
LLOYD
A branch in the height of flowering, second season.
B.
A. A branch, one year’s growth under irrigation.
Reproduction. 85
The smaller amount of growth in 1909 was due to the absence of
irrigation, as elsewhere explained. It will be noted that the medium-
sized plant responded best, which in general comports with our observa-
.tions of the rate of growth of field plants.
GENERAL CONCLUSIONS.
The maximum rate of growth of guayule under irrigation is in the
neighborhood of 25 cm. per year stem-length. The amount of growth
between the field average and the maximum average for irrigated plants
may be closely regulated by irrigation, to which the plant readily responds.
FIELD PLANTS.
When it is borne in mind that the total height of a plant is, except
in young seedlings of strict habit, always less than the sum of its longer
annual growths, because of (a) partial dying back and (b) the branching
habit; and when also it is remembered that numerous plants suffer from
untoward conditions, either by the depredations of parasites or from
poor soil-conditions, it is not far from the truth to say that the average
annual rate of increase in height is 3 cm. A plant 30 cm. in height would
therefore be 10 years old. Plant 3, above described (p. 83), which has
undoubtedly a higher rate than 30 mm. per year, is, as certainly as may
be estimated, 10 years old. As has been said previously, however, the
important desideratum is to determine the period of life during which
the increase in weight is most rapid, aside, of course, from the very young
seedling stages, when the ratio of increase may be rapid, but the total
weight very little. For the purpose of arriving at this information, I
have assumed the rate 3 cm. per annum as a constant factor. A large
number of plants have been weighed and measured, and the data thus
derived have been correlated so as to obtain curves of increase in weight
according to size (fig. 13). For the data the reader is referred to tables
4 tO 13.
The curves have not been constructed for plants over 40 cm. in height
for two reasons: the number of plants beyond this size is very much smaller,
and, again, their age is too great to admit them to a practical considera-
tion of rotation periods. Observations from which, in part, the tables of
data used in the construction of the curves have been derived, all go to
show that the first pronounced gain in weight is entered upon after the
plant has reached a height of 30cm. The average weight of plants of this
height is somewhat over 5 ounces, ranging chiefly between 2.5 and 8.5
ounces. The average weight of plants 40 cm. tall is, on the other hand,
15 ounces or more. That is, the average weight is trebled in making the
Io cm. advance in height beyond 30 cm. This is shown in the positions
of the curves, which, however, present more irregularities than one would
wish, in spite of the fact that they are based on measurements of several
hundred plants. The greatest fluctuations in the curves are caused by the
introduction of exceptional individuals, for where larger numbers are used
the curves are more uniform. The exceptional individuals may be either
“spindling’’ or unusually well-developed in point of ramification.
86 Guayule.
AGE AND HEIGHT.
For the purpose of controlling the above conclusion I have, as clearly
as possible, made estimates of the ages of plants of various sizes, making
no assumption as to the rate of growth, but being guided solely by the
marks in each individual. The results are compiled in table 39.
TABLE 39.—Size, weight, and closely estimated ages of guayule plants.
| Weight.
Series. | Height. | Age. Remarks.
| | | Fresh. Dry.
— |
| om. oz. oz. yrs |
I a5 TOR 75 II Very well developed, symmetrical.
30 5.0 8 Medium, rather undersized.
28 4.5 7(8) | Normal.
25 4.0 i] Do.
17 0.875 on] Do.
II 5a 18 Tr 26 I4 | Y-shaped, symmetrical. ‘‘Macho.’’
AS 8 4.0 IO oO.
Ill 50 12 O57 | HOMO 12 | Y-shaped, symmetrical. ‘‘ Hembra.”
30 6 B25 7 |My tors Do.
IV 66 32 24.0 | 19 to20 Y-shaped, narrow-leafed type.
40 a AS Io to 12 Do.
V 40 Io 6.0 15(?) | Slow-growing, broad-leafed type.
35 5 3.06 Haig) a Do.
VI 50 54 36 17 to 20 Teely branched, spreading, in full
eaf.
45 44 29 16 to 18; (Notre. Plants of series VI are the
33 16 14 to torr, heaviest for their height of any,
Ar 6 5 9 except very occasional plants such
20 5 Shays 8 as that in plate 8, fig. B.)
15 Tas 0.93 6
VII 65 64 43 20 V-shaped, half-spreading type, dense-
ly branched, symmetrical (plate 8,
fig. B).
55 32 18 I5 to 16
50° 18 15 13 tO 15
33 8 BES 9
28 6 4 8
24 3 1.5 7
22 1.25 0.5 6
VIII 55 18.5 ste 15 to 16 Y-shaped (plate 8, fig. A).
30 6 ae ee Normal shape for age.
25 I 9 Do.
20 0.875 7 Do.
14 0.5 5 Do.
ARS Oper 2 Do.
These data have been charted in the accompanying curves, corre-
lating age and height (fig. 15, upper diagram). It is an obvious objection
to the value of these curves that they are based, necessarily, on compara-
tively few plants, but their value is enhanced by the individual treat-
ment, since the estimate of age was made with great care. A fairly close
correlation emerges, however, from the diagram, from which we see that
plants of 10 years of age have a height of about 30 cm., and those of 15
years about 40 cm.
An increase in height of 10 cm. over 30 or 35 cm. is correlated (judg-
ing from the data of table 39) with a doubling at least of the weight
Reproduction. 87
WEIGHT IN OUNCES
10 15 20 35 40
HEIGHT iN CENTIMETERS
Fic. 13.—Curves correlating height and weight in the plants recorded in tables 5 to 7 and 9 to 13, inclu-
sive. The approximate averages are indicated in the curve of averages.
WEIGHT IN OUNCES
r) 10 15 20 25 30 35 40 45 50 55 60 65
HEIGHT IN CENTIMETERS
Fic. 14.—Curves correlating height and weight of the plants in table 39.
88 Guayule.
Baeneaecsegenirer
A aA ts Hl lL Baa E27 nA ew
eae | Ae
a as
Bea
&
\
\
\
SN
‘
>
o
HEIGHT IN CENTIMETERS
w
a
Pee ERS Cg S| ae
" 3 15 16 17 19 19 zo
AGE IN YEARS
ese ee i
seca s
WEIGHT IN OUNCES
NY
Er ao Bees seam Sete dl
Hep ge
7 aa
ACL le
SEE EEE EA
FH eee
Bim
a
a
|
AGE IN YEARS
Fic. 15.—Upper diagram: Curves correlating age and height of plants in table 39. Lower diagram:
Age and weight correlated. The same plants.
Reproduction. 89
(fig. 14). Fig. 13, on the other hand, indicates a greater increase, to
nearly three times the weight at 30 cm. The average weight of a plant
30 cm. in height is, according to fig. 14, about 7 ounces, but as the plants
considered in this curve are normally developed or indeed considerably
above the average, the average weight of a 30 cm. plant is probably
nearer to that indicated in fig. 13, viz, 5 ounces. The mere ratio of
change in weight is not peculiar to these dimensions alone. What appears
from the data is that the weight of plants up to the height of 30 cm. is
not great enough for economical harvesting. The increase in size and
weight, however, is as great in the subsequent five years as in the previous
ten, so that taking the crop at the end of ten years would give results
only half as great as the returns of a fifteen-year rotation period.
This conclusion is shown graphically by fig. 15, lower diagram, which
indicates that the weight of a plant advances from about 6 ounces at
ten years of age to 15 ounces at fifteen years. A considerable minus error
in the estimation of ages might be allowed, and yet the increase indicated
in the preceding paragraph would still be shown. It is fair to state, how-
ever, that there is little chance for such error, as I have taken the pre-
caution of being conservative when there was doubt.
Fig. 15, upper diagram, indicates that the estimate of rate of growth
used throughout, viz, 3 cm. per year, is very nearly correct. We may
therefore conclude:
(1) That the average rate of growth of guayule per annum is about
quem.
(2) That the amount of increase in weight between the tenth and
fifteenth years of its age is at least as great as that occurring during the
first ten years; and that this further justifies, from an economic point
of view, a fifteen rather than a ten year rotation period, aside from con-
siderations which might be drawn from loss by death (p. 71), could we
ascertain this accurately enough.
CHAPTER V.
ANATOMY AND HISTOLOGY.
While the anatomy of the Composite has been studied in much
detail, beginning with von Sachs, followed by van Tieghem, Vesque,
Vuillemin, Col, and less voluminously by other writers, that of the genus
Parthenium had, up to 1901, received no examination. In that year the
plant which supplies the object of the present treatise came to the atten-
tion of the French botanists MM. Fron et Francois (1901), who gave a
brief account of the more obvious features of the anatomy of the stem
and of the structure of the fruit. A more extensive paper was published
in 1908 by Dr. H. Ross, who visited Mexico in 1907 and examined the
guayule in the field, chiefly about Saltillo. In this paper an anatomical
study of guayule was supplemented by brief reference to two other species,
P. incanum and P. tomentosum. To both these contributions, as also to
those of more general import, reference will presently be made.
ROOT.
PRIMARY STRUCTURE.
The primary root is diarch. The two bundles of protohadrome, of
spiral vessels, become early united by a centripetal development of vessels
forming a primary plate, on either side of which stand the two protolep-
tome strands. At this time the stele has a continuous pericambium and
is surrounded by a well-marked endodermis, which may be recognized by
the bands of Caspary and by the starch-content of the cells (plate 22, figs.
6-8). The starch-grains are relatively large and are compound. Their
persistence is variable, traces being visible for some months in some
instances, é.g.,in a root 4 mm. in diameter; in other cases they may have
disappeared in a few weeks. Thus in a root 0.46 mm. in diameter, in
which radial thickening of the endodermis had just commenced, starch in
these cells fluctuates, there being now more and now less, apparently
according to the draft upon it by the tissues. Without the endodermis
lie three layers of cortical cells with extensive intercellular spaces, which,
however, do not occur between the outer layer of cortical cells (the hypo-
dermis) and the epidermis.
SECONDARY STRUCTURE.
The epidermis begins very early to break down, so that in a root less
than o.5 mm. in diameter the earliest peridermal divisions have set in.
These do not usually occur in the outermost cortical cells, which here
take on, in a weak fashion, the characters of an exodermis, as described
for Cephalanthus and Tecoma by Holm (1907), but in the second hypo-
dermal layer (plate 22, fig. 7). At this time growth commences in the
cortex, both radial and periclinal divisions occurring (plate 22, fig. 8).
Growth of the endodermis is concurrent (plate 22, fig. 6). Both radial and
90
Anatomy and Histology. 91
tangential increase in size results in (1) extension of the radial dimensions
of the so-placed walls. This extension is confined to that part of the wall
between its outer limit and the band of Caspary, leaving this band in the
same position as before (plate 22, figs. 6,8). With this fact in mind the
endodermis may be identified for a long time, indeed frequently till it
is well-nigh expelled by secondary thickening. (2) Tangential growth is
accompanied by cell-divisions in the radial direction, Casparian bands
being formed in the new walls (plate 22, fig.6). In particular positions,
namely, opposite the leptome bundles, the earliest! resin-canals appear.?
These do not belong to the primary structure of the root, but arise second-
arily in the endodermis (plate 22, figs. 1 to 5). Their mode of development
is as follows: 3 or 4 adjacent cells divide by periclinal walls, thus bringing
it about that two or three places occur where 4 cells lie with their angles
adjacent. Here the walls split apart, making a simple, prismatic, inter-
cellular space without demonstrably different contents; the adjacent cells
divide radially, so that each canal has now 4 cells contingent upon and
peculiar toit. Later, further divisions, roughly parallel to the early canal-
walls, result in the canal consisting structurally of two layers of cells
constituting a prismatic tube. It is seen that the endodermal canals, for
so they will be called, are arranged in two groups, of usually two or often
three or occasionally even four canals each, each group being placed oppo-
site a primary leptome bundle. This relation was first described by von
Sachs for Helianthus. The structure of these canals does not change,
though there ensues some displacement of the cells in roots which have
thickened abnormally without losing the outer primary tissues. The figures
displaying the cell lineage will make this clear (plate 22, figs. 1 to 5).
The growth of the endodermis may be followed till it is thrown off
by the formation of cork within it. During its history it enlarges from
a cylinder, of o.1 mm. inside diameter, of 18 to 20 cells, to one of 3 mm.
diameter, composed of hundreds of cells, or even to larger dimensions,
before being finally cut out. Throughout the greater portion only radial
divisions occur, though the cells increase in radial depth. In the region
of the canals, however, the endodermal cells divide in a general periclinal
direction, giving rise to two or even more irregular series of cells.
In the ultimate condition of the endodermis and of the secondary
cortex the walls of the cells are reticulately thickened (plate 22, fig. 15),
so that in a root 2 mm. in diameter, of a field seedling, the endodermis
may be followed all the way around with great ease, provided that the
rubber has been previously extracted. The reticulations are the result
of the oval form of the broad, shallow pores, which are somewhat crowded.
They are more strongly developed in plants grown under normal condi-
tions, as appears from the fact that in an irrigated seedling, with a root
1 Ross (1908, p. 25), in stating that there are only a few canals in the primary
cortex of the root, does not make it clear that he refers to canals of endodermal
origin.
aa Exceptions occasionally occur in which the canals in one half-circle of the
hypocotyl do not approach on entering the root, and conversely, cases occur in
which the grouping of the canals occurs in the hypocotyl, on one side of it. In
other words, the root-structure is taken on at a higher level on one side than on
the other.
92 Guayule.
4 mm. in diameter, the reticulations were much less marked or absent.
The same condition is found in the definitive stem.
This large size of the cylinder of tissue inclosed within the endoder-
mis is attained only under an abundant water-supply and other condi-
tions insuring rapid growth. Under such circumstances the structure of
the canal itself passes beyond the normal definitive stage, and the cells,
usually and normally eight in number in transverse section, may suffer
further divisions as shown in plate 22, fig. 5, to an extent sufficient, to-
gether with some displacement, to render it somewhat difficult to exactly
delimit the structure. Ina root of this size, viz, 4mm. in diameter, the
endodermis may still be readily recognizable (though irregular in charac-
ter, being in part of two rows of cells) by the starch-content or by the
Casparian spots, or both. The position of the endodermis is always clearly
shown, other signs failing, by the primary canals.
The physiological changes in the endodermis are of particular inter-
est. Reference has been made to the variableness of the starch-content
of its component cells. When grown under irrigation the starch may be
seen in much larger plants than in those which have grown under normal
or field conditions. In these growth is less rapid and the extension of the
tissues correspondingly less marked. In these also the secretion of rubber
ensues earlier and is correlated with the occurrence of drought. In such
plants the cells of the endodermis, together with others to be noted below,
secrete rubber, so that, in a small seedling with a tap-root 2 mm. in diam-
eter, the cells of the endodermis will be found engorged with this sub-
stance. The cells of the canals are especially noteworthy in this respect.
By taking advantage of the effect of water upon the rate of secretion, it
may be shown that the secretion of rubber in the endodermis takes place
first in the cells of the resin-canals (plate 41, fig. 6). Thus, in the root,
4mm. in diameter, of a seedling which grew with great rapidity, the canal
calls were half-filled with small droplets of secretion which reacted to al-
kanet. The specimen had previously been freed from alcohol-soluble sub-
stances, and there can, I think, be no doubt of the nature of the material
in question.’
The behavior of the pericambium in the region included between the
primary leptome and the endodermis differs from its behavior elsewhere.
One finds, in a root 1.2 mm. in diameter, that the pericambial cells have
enlarged radially and have in some cases undergone periclinal divisions
and the daughter-cells further radial divisions (plate 23, fig. 4). The peri-
clinal divisions suggest initial cork-divisions, but this is not the case, as
both the radial divisions and the further behavior of the cells show. With
a slight increase in thickening in the root, sufficient to attain 1.5 mm. in
diameter, the cell-walls are a little thickened and a rearrangement has
taken place. The cells have apparently been compressed between the pri-
mary leptome and the endodermis, and, under suitable conditions, as in
the specimen from which plate 22, fig. 5, was made, have secreted rubber,
1I have noticed that reaction to alkanet, which is the same in all the cells
at first, becomes in the canal-cells darker with time, the preparation having been
kept in darkness. I have attributed this, with some doubt, to the greater proto-
plasmic content of these cells. Great care was taken to extract very thoroughly
with absolute alcohol, and, in a part of my preparations, with caustic potash also.
Anatomy and Histology. 93
though less than in endodermal cells with which they are in immediate
contact. Further development sees the collapse of the pericambium cells
(plate 22, fig. 6), and, as seen elsewhere, the primary stereome occurs in
the primary leptome just within the pericambium.
The primary cortical cells outside of the endodermis are also capable
of secreting rubber. That they do so at all is contingent on the rate of
growth of the seedling. If this is rapid enough to remove the cortex
before drought sets in, no appreciable secretion will have occurred. If,
however, the rate of growth is lower, so that for the greater part of a year
the tissue in question remains functional, the inner cells at least may be
found densely filled with rubber. In the root, 5 mm. in diameter, of a
field seedling fully a year old, the following measurements (along a radius)
were made, from which an idea of the amount of primary cortex remaining
active may be had: Wood, 1.4 mm.; secondary cortex, 0.64 mm.; pri-
mary cortex, 0.15 mm.; cork, 0.27 mm.
Earty SECONDARY CHANGES IN THE STELE: (HADROME).
With the completion of the primary hadrome plate there ensues a
centrifugal development of this tissue by the direct transformation of the
protogenic cells adjacent to the middle part of the plate. The increase of
hadrome extends along all radii except those lying near the plane of the
primary plate, but usually rather less rapidly toward the primary lep-
tome, so that in transverse section there appear two wings, so to speak,
of hadrome. This is protogenic, but is added to quite soon by the activity
of a cambium which first becomes apparent within and close to the pri-
mary leptome bundles (plate 22, fig. 10), and extends toward and finally
around the outer edges of the primary hadrome (plate 22, figs. 9 to Io).
Up to this point in the development of the stele nothing exceptional
is seen. The only question which has been raised is in regard to the pre-
cise origin ' of the earlier formed secondary hadrome elements, whether
this is by means of the cambium which arises on the inner surface of the
primary phloem, or is directly from protogenic elements lying adjacent to
the primary hadrome plate. The evidence from the material here under
discussion is that the latter is the case.
Now, however, a behavior ensues which is somewhat unusual. Two
independent mestome strands of (at first) a single radial series of vessels
and a very small leptome strand arise, each, usually, in immediate con-
tact with the primary trachea (plate 22, figs. 9 to rr). The emergence of
a secondary root disturbs the exact position so that the earliest vessels
may be somewhat removed from the primary hadrome. A similar condi-
tion has been observed by me in Lamium amplexicaule, and by Petersen *
in other Labiate.
It is necessary to note that although these mestome bundles are ver-
tically below the two lateral cotyledonary traces, we shall presently see
that they are independent of these and have no connection whatever with
them.
1 De Bary, Comparative Anatomy of the Phanerogams.
21 have not seen this paper, but am informed by Dr. Holm.
94 Guayule.
The appearance of the stele is now suggestive of a tetrarch structure
and is as follows: In a plane at right angles to the primary hadrome
plate lie the two leptome bundles, between which and the hadrome plate
the primary cambium lies. Ina plane coincident with the hadrome plate
and just beyond its edges lie two small secondary mestome strands,
formed independently. These, which I shall here call the intercalated
strands, lie, therefore, in a plane between the two broad primary medul-
lary rays (plate 22, fig. 10). The isolated condition of the intercalated
bundles is, however, very transient, since the parenchyma rays between
the axial hadrome strand and the small intercalated bundles are soon
bridged over, the whole, save as mentioned in the following paragraph,
coalescing to form a single axial strand of hadrome. Additional second-
ary bundles are intercalated between those already present, at first from
the four angles of the hadrome wings, so that in a tap-root 1.2 mm. in
diameter before me (plate 40, fig. 1) there appear 8 bundles, though it
must be said that the appearance of the stele in roots of the same size is
not by any means uniform.
The closure of the hadrome wings by meeting the xylem of the inter-
calated strands is not complete, and thus are left two islands (analogous
to medullary spots) of unlignified cells about the edges of the primary ha-
drome plate (plate 22, fig. 11). The outlines of these islands are quite
irregular, and they may ultimately become compressed or lignified, so
that it is only with difficulty that they may be recognized. In thin roots,
as especially in the fibrous laterals, the wood cylinder is very compact,
and may have no parenchyma rays. In such also the secondary changes
in the cortex are less extensive, and the pericycle is much compressed.
LATER SECONDARY CHANGES: (CORTEX).
With age the walls of the cortical cells become somewhat thickened
and pitted. The intercellular spaces are very regular in shape, and regu-
larly disposed. In a tangential section they appear very uniformly len-
ticular (plate 28, fig. 5).
STEREOME AND SECONDARY CANALS.
Aside from the secondary increase of wood and bast, the appearance
of stereome and of secondary canals has to be mentioned. That stereome
which appears in connection with primary tissues only may properly be
spoken of as primary. Of this there are but two slender bundles (plate
23, fig. 6), which consist each of a few (less than a dozen) slender, very
thick-walled elements buried in a mass of material derived from the pri-
mary leptome by the swelling of the cell-walls till the lumina become
indistinguishable.
The method of origin of these sclerenchyma cells is difficult to deter-
mine, and will be discussed beyond.
The pericambium appears to be continuous, but so far as I am aware
the formation of stereome does not involve the cells of this layer. The
configuration of the cells of the adjacent secondary cortex is at this time
(a root 2 mm. in diameter) very curious, the walls having been distorted,
as if by stretching in a radial direction from the primary sclerenchyma as
a center. This result would seem, however, to be due to compression by
Anatomy and Histology. 95
the growth of a mass of secondary stereome which arises within the pri-
mary stereome and is removed from it, in a root of 2 mm. diameter (plate
23, fig. 6), by about 35 microns toward the center.
This secondary stereome strand is larger than the primary strand
~and becomes an obvious structural feature. For this reason, and on ac-
count of its close proximity and its relative position to the primary canals,
it may very easily be mistaken for the primary strand. Its cells, however,
are larger, and it arises in connection with secondary leptome, and not in
relation to the proto-leptome. For this reason its position is more variable
than that of the primary strand and may suffer tangential displacement
(due to unequal development of the root), as shown in plate 23, fig.1; and
further, for less obvious reasons, the stereome may not occur at all.
Other secondary stereome strands develop, if at all, always in con-
nection with the leptome, as stated for the stem by Fron and Frangois
(1901) and by Ross (1908). The particular mode of origin will be discussed
later. Each series is circular (plate 23, figs. 1 and 2), as all the members of
a series arise normally at the same time, though the series may be more or
less discontinuous, owing to unequal development as between the mem-
bers of the series. In seedlings grown rapidly under irrigation the amount
of stereome development is usually notably less than in field seedlings or
in others grown slowly.
Secondary resin-canals (plate 22, fig. 13) arise within the secondary
leptome in close proximity to the cambium, and in the manner described
by Ross for the canals of the stem. They consist at first of two tangential
rows of cells scarcely distinguishable from the cambium from which they
arose, though quite early they may be recognized by their larger size and
the dense protoplasmic contents which at first show by their reactions
merely their protoplasmic nature. Ross (1908, p. 260), however, says
of these canal-cells: ‘‘Die den Kanal auskleidenden Zellen sind durch
dichtes Protoplasma ausgezeichnet, das sich mit Chlorzinkiod dunkel-
braungelb, mit Alkannin intensiv rot farbt, wahrend sonst das Leptom
hauptsachlich starkereichen Zellinhalt fuhrt.”’
My own observations differ from those of Ross in that the cell con-
tents, when very young, do not react to alkanet as described by him.
Very soon after the two cell-rows begin to split away, minute globules of
a secretion begin to appear, and these indeed take on the intensive red
color of the reagent. This is considerably in advance of the same appear-
ances in the adjacent cortical cells. Preparations treated with alcohol to
dissolve out the resin or oil, which might be said to occur, show this very
clearly, and further treatment of the same preparations with benzole
shows that these intensively staining masses are dissolved out by that
agent, and, in the absence of evidence to the contrary, must be regarded
as rubber. In the secondary canal-cells, therefore, as in the wall-cells of
the primary canals of the root, occurs the earliest appearance of rubber,
the secretive activity extending progressively from them to the surround-
ing tissues, and more rapidly in the primary cortex. It is worthy of
especial note that rubber occurs in the wall-cells of canals which normally
contain, in the meatus, the resin characteristic of the guayule plant. This
point calls for discussion, which will follow later.
96 Guayule.
The leptome of the root does not show any starch-content in the
sieve part, though it occurs apparently erratically in its parenchyma and
in the cortical cells adjacent also to the leptome and to the resin-canals.
It is also to be found in the endodermal cells close to the primary canals,
and occasionally elsewhere, in a 4 mm. diameter root of a field plant.
If, however, we examine a plant grown with an abundance of water,
in which the secretion of rubber has taken place only in minute quantities
and this in the wall-cells of the resin-canals, an important physiological
relation between starch and the secretory canal-cells is indicated. Ina
root 4 mm. in diameter, which developed in about three months, the dis-
tribution of starch and its quantity are very striking. It is present abun-
dantly (a) in a broad, irregular radial band of cortical cells extending
from the primary resin-canals, (b) in a narrow and somewhat irregular
circular band midway the secondary cortex, and (c) in marked quantities
in the cortical cells adjacent to the definitive resin-canals. It is not pres-
ent in the leptome adjacent to the young resin-canals. It would therefore
seem probable that the presence of starch in marked quantities near the
resin-canals is related either to the secretion of rubber by the wall-cells
especially, or to the secretion of resin. The familiar case of Pinus, in
which starch occurs near the resin-canals, suggests the latter.
The earliest appearance of rubber, which is secreted by the paren-
chyma of the cortical rays and of the cortex, aside from the cells of the
resin-canals as above noted, is to be seen in the innermost cells of the rays,
and synchronously in the outermost cells of the primary cortex, or, if that
is absent before secretion begins, of the secondary cortex. This fact is
beautifully shown in the tap-root of a seedling from the field, probably
less than one year old, collected on July 14, 1908, and measuring 2 mm.
in diameter. In this specimen the cells of the primary cortex were com-
pletely filled, as also the outer cells of the secondary cortex, there being
progressively less and less secretion toward the center of the root. The
opposite relation was shown by the parenchyma rays, in the cells of which
the amounts of rubber were found to be progressively less and less, as one
proceeded from the center outward (plate 23, figs. 3, 7; plate 4o, figs. 2 to
4). Ina still younger seedling, perhaps three months old, about 1.2 mm.
in diameter, rubber is to be seen only in the cortical cells adjacent to the
primary canals and in the few innermost cells of the cortical rays. The
amount is so small here that, while it may readily be seen with the eye,
the photograph does not differentiate it.
HYPOCOTYL.
PRIMARY STRUCTURE.
The primary cortex consists of six layers of cells, including the
endodermis. The epidermis becomes rather strongly cuticularized and
many of the cells are papillate, or, more correctly speaking, form short,
round-ended trichomes, which are usually one-celled, though two-celled
trichomes are found in a few instances (plate 23, fig. 9). The angles of:the
cortical cells adjacent to the inner faces of the epidermal cells are collen-
chymatized, but in deeper layers this character is not present. Chloro-
Anatomy and Histology. 97
plasts, few in number, however, are present in the cortical cells. The
endodermis is well marked and contains a good many large starch grains.
The Casparian spots are readily recognized.
The stele (0.18 mm. in diameter in a hypocotyl 0.53 mm. in diameter)
is, at an early age, tetrarch above the zone of transition to the root. The
four bundles are received into the hypocotyl in pairs, one pair from each
cotyledon, in which they constitute the median trace. After reaching the
lower part of the lamina they unite, as they do in the lower part of the
hypocotyl (in the “‘collet’’), to form a single bundle.
In addition to the median paired cotyledonary traces, there are
delivered into the hypocotyl four lateral traces which meet in pairs to
constitute two bundles which pass inward and downward. Each takes a
position, the one on one side of the stele, the other on the other, in a ver-
tical plane at right angles to that which divides both cotyledons. They
are the “‘faisceaux latéraux ”’ of Dangeard (1889, p. 85). So far, then, this
plant satisfies the ‘‘cas secondaire”’ of his root type with two bundles,
found in the Composite and certain Ranunculacee. According to Dan-
geard, however, the behavior of these bundles is, to use his own words,
as follows: ‘“‘Les premiers (f. médian) se comportent comme dans le cas
général;' les latéraux s’anastomosent plus ou moins longuement avant de
rejoindre le médian vers le bas.”’
If I interpret Dangeard’s statement correctly, we should find that
the lateral traces (plate 24, figs. 2 to 5, 12) anastomose with the median.
This, however, I do not believe to be the case. By following the figures it
will be seen that the lateral traces are to be seen in the upper part of the
hypocotyl, but end rather soon. The broad medullary ray between the
pairs of median bundles is then unoccupied, and remains so till the cauline
bundles encroach upon it. Between these cauline bundles, at the proper
level, the slender end of the lateral cotyledonary trace may be seen, quite
single and separate from them (plate 25, fig. 10). In the diagram (plate
24, fig. 13) the fused lateral traces are represented as being much shorter
than in that given by Dangeard for Catananche lutea.
In types with a tetrarch root-structure this trace passes downward
and articulates directly with two of the primary hadrome strands. This,
e.g., occurs in Caulophyllum thalictroides (Butters, 1909) and in numerous
other plants cited by Dangeard (l.c.).
The intervals between the lateral and median bundles are occupied
by two cauline traces, or, more properly speaking, by one (lateral pro-
phyllonary) and a half (of the corresponding median prophyllonary)
traces. There thus appear in the higher levels of the hypocotyl:
(1) 8 cotyledonary traces, viz, 2 pairs of half-median traces; 2 pairs
of lateral traces.
(2) 8 prophyllonary traces, viz, 4 half-traces, a half-trace on each
side of the cotyledonary laterals; 4 lateral traces, one on each side of the
cotyledonary median pairs.
Passing down, the 14 cauline bundles in each cotyledonary median-
lateral interval fuse with each other and then with the adjacent median
trace. Below the level of this fusion the tetrarch structure is assumed,
1That is, as above described.
98 Guayule.
the paired cotyledonary median bundles becoming somewhat separated.
The separation is, I believe, due to the rapid enlargement of the adjacent
parenchyma-cells, so that the secondary elements become, in the lower
part of the hypocotyl, definitely dissociated, leaving the primary ele-
ments occupying the position of the primary hadrome elements of the
root. The primary hadrome plate of the root lies, then, in the plane of the
cotyledons (Dangeard, /.c., p. 87). In making the approach to the root
the leptome masses revolve, two in one direction and two in the other,
until they meet, two and two, to form the diametrically opposed leptome
masses of the root! (plate 24, fig. 2); above, these same leptome masses
pass entirely into the cotyledons, with the corresponding hadrome masses,
and not into the stem. The continuity of vascular tissues between the
stem and root is established secondarily.
The above account of the structure is incomplete in that the presence
of an originally single tracheal vessel, extending from within the cotyle-
don downward through the hypocotyl into the root, has not been pointed
out. This trachea (trachée primitive of Vuillemin, 1884, p. 183) consti-
tutes a center of development, identical in the hypocotyl and tap-root,
for the primary hadrome. It is unnecessary to recount the arrangement
of hadrome in these organs, but it is pertinent to insist on the initial
centripetal formation of new hadrome elements. The dissociation of the
hadrome elements in the hypocotyl—strictly speaking, only in the upper
portion—is due, as Vuillemin has stated (1884c, p. 181), to the rapid
development of parenchyma, and is analogous to the secondary splitting
apart of the wood cylinder in the same organ by the growth of the con-
junctiva. In consequence of this interpretation Vuillemin speaks of the
paired bundles as ‘‘les deux moitiés du faisceau,’’ which are secondarily
separated by ‘“‘a medullary ray.’’ The peculiar orientation of the paired
bundles represented (but frequently not referred to) by many observers
(van Tieghem, Gerard, Dangeard, Goldsmith, Ramaley) is thus, properly
I believe, explained.’
Primary RESIN—CANALS.
These arise in the endodermis, as in the root, as a single canal directly
opposite each primary leptome strand (plate 25, figs. 1 to 6). The struc-
ture of the canal is similar to that in the root, and consists in its definitive
form of eight cells in transverse section. The course of development is
not so regular as in the primary-root canals, the meatus being ultimately
more cylindrical. Not infrequently the earlier divisions do not all take
place, so that three instead of four cells line the meatus (plate 25, fig. 2).
As these canals pass into the root they pair off, each pair coming to occupy
the position already described, namely, opposite each of the two primary
leptome bundles. When more than two canals are encountered in this
position it is the result of branching. Occasionally both branch either
once, or frequently twice, giving rise at length to four or even six canals,
though more frequently three or four only occur.
1The above account may be applied to Parthenium incanum, Lactuca, and
Helianthus in its main outlines, and is a type, I believe, of wider applicability than
usually supposed.
As for the rest of Vuillemin’s views, regarding the nature of the hypocotyl
and cotyledons, I will say only that they appear to me somewhat strained, and
far less in accord with the course of development than those of Dangeard (1889).
Anatomy and Histology. 99
STEREOME.
The primary stereome arises early in the hypocotyl, as four slender
bundles just within the canals, within the outermost part of the primary
leptome strands. Occasionally, also, endodermal cells adjacent to the
canal may undergo sclerosis, both in the hypocotyl, stem, and leaf (plate
Sh MOS.h 5 GEE):
That unmodified pericycle cells lying just within the endodermis
and opposite the leptome become sclerosed seems possible (plate 25, fig. 6),
but doubtful. I find that the pericycle is quite frequently interrupted
(plate 22, fig. 12), in which event the stereome must arise in the primary
leptome. Its further development is contributed to chiefly by elongated
elements in the leptome, and a few elements are sometimes derived also
from the leptome parenchyma. Nearly all the elements (except those of
parenchymatous origin) which play this part enlarge greatly (plate 25,
fig. 4) and cause marked displacement in the surrounding tissues. Vuille-
min (1884a) has described stereome arising, in the Composite, in the sec-
ondary, but not in the primary leptome, in Achillea, Artemisia, Anthemis,
and Leontopodium. From my own studies I am forced to the conclusion
that this takes place in the primary leptome.
SECONDARY STRUCTURE.
The prophyllonary bundles, above referred to, arise in the intervals
between the cotyledonary bundles, before the establishment of inter-
fascicular cambium (plate 24, figs. 2 to 5, 12). This, when complete,
incloses the cotyledonary hadrome, and there is thus established the basis
for the imposition, on the primary stele, of secondary, true stem topog-
raphy. It may be pointed out, however, that the cambium does not
lay down secondary hadrome in all cases in immediate contact with the
primary elements. Thus, in the radii of the median cotyledonary traces
(the elements of which do not of course pass beyond the cotyledonary
node) secondary hadrome arises which descends from the epicotyl (plate
25, figs. 10 to 10b). Between these there is frequently a hiatus which
delimits them readily to the eye, if the secondary changes have not pro-
ceeded too far. Nevertheless, though the morphological separateness of
the primary and secondary hadrome—and also leptome—is clear, the
peculiar topography, the curved outline of the secondary hadrome as
seen in transverse section, indicates an as yet entirely unanswered ques-
tion as to the immediate cause of this. As it is purposed to compare
ecological types, further detail will be considered in the following para-
graphs.
FIELD PLANTS.
The pith in a specimen about 1.8 mm. in diameter displays at an
appropriate level two gaps (plate 25, fig. 7), each in the position of a
primary medullary ray, containing the primary bundles! constituting the
lateral leaf-traces, while its transverse outline still reflects the vascular
topography of the primary condition. Surrounding the pith is a closed
compact column of hadrome which is broken up radially into broad wedges
These undergo little or no secondary thickening, except in a restricted region
below the cotyledonary collar.
100 Guayule.
by secondary parenchyma rays (plate 25, fig. 7). The primary rays are
for the most part entirely closed, though two of these are suggested by
the topography of the pith, as above indicated. No more resin-canals
have appeared. The primary stereome bundles have extended inward
by the transformation of the primary leptome, and the primary resin-
canals are still present. Small secondary stereome strands are present
on the outside of several other bundles, as indicated in plate 25, fig. ro.
The endodermis is recognizable by its starch-content. The primary cortex
is much reduced, its tissue having been sacrificed to the development of
a thick cork, the original peridermal divisions of which take place in the
outermost cortical layer of cells (plate 23, fig. 8).
The seedling in question (plate 25, fig. 7) was less than a year old,
probably four to six months. The epicotyl was 8 mm. long, with a few
small leaves, and was collected on July 24, 1908.
An etiolated seedling (plate 25, fig. 9) of the same diameter, with an
epicotyl 2 cm. long and about three months old, shows a similar topog-
raphy, save quantitatively. There is a weaker and more irregular devel-
opment both of hadrome and of leptome. There is no additional stereome
beyond the four primary strands. The primary cortex is thicker and the
cork thinner. This seedling was supplied with abundant water and the
shade of a muslin cloth, with the effect of producing responses correlated
with a relative reduction of transpiration and to loss of water from the sur-
face of the stem. The greater leaf-area, together with a more slender axis,
results, however, in a greater transpiration stream relative to the diameter
of the wood cylinder, with histological results to be noted beyond.
An irrigated plant (plate 25, fig. 8) of slow growth, one which was
plentifully supplied with water, exposed to full illumination, but limited
in the spread of its roots, is very instructive in this connection. Under
these conditions we must assume a strong transpiration stream, at least
stronger materially than is usually the case in field plants. The specimen
had a diameter of 2.5 mm. and was not more than three months old, and
on this account alone was therefore a trifle larger and more advanced in
development than the preceding. In its cork development it resembles
the field plant, and has suffered the same reduction of the primary cortex.
In fact, in both cases one of the primary-canals is just cut out by the peri-
derm. The deeper medullary rays communicate with the pith, indicating
secondary enlargement of the latter. The amount of wood as compared
with the field plant is much greater relative to age, but somewhat less
relative to radial measurement, and there is a relatively larger growth of
the secondary cortex. Most remarkable is the large and irregular devel-
opment of stereome. This irregularity is constantly associated with a
plentiful water-supply and is an expression of a general tangential dis-
placement of cortical tissues, as revealed by the later positions taken by
the primary resin-canals and the obliquity of the leptome masses, the
position of which predetermines that of the secondary stereome.
Aside from the total quantity of hadrome, these three ecological
types present histological peculiarities which are related to the transpira-
tion stream. The number and size of the vessels in the field plant (plate
26, fig. 2) are scarcely inferior to those in the irrigated plants (plate 26,
Anatomy and Histology. 101
fig. 4), while the etiolated plant (plate 26, fig. 3) has vessels somewhat
fewer, but of more uniform size and notably larger. The mechanical ele-
ments of the wood are thicker-walled and somewhat smaller in the field
plant (plate 26, fig. 5), and are nearly isodiametric. They are of much
the same character in the other two, except that they appear more com-
pressed tangentially, especially in the irrigated plant (plate 26, figs. 6, 7).
The stereome also presents differences which are still more striking,
aside from the relative amounts already spoken of. In the field (plate
26, fig. 8) and etiolated (plate 26, fig. 9) plants the cells are closely set
together, but are smaller on the whole, and in the field plant have smaller
lumina. In the irrigated plant (plate 26, fig. to) the shape and size vary
greatly, the lumina are very small, and the intercellular material is much
more extensive. The whole appearance leads to the impression that there
is a good deal of distortion during development, so that the fibers are
pushed about and disarranged, the tissue becoming less compact. If my
view of the origin of the stereome is correct, the explanation of this con-
dition may lie in a less complete transformation of the sieve-tissue into
stereome. The collapse of the unsclerified cells would cause displace-
ment, and the irregularities due to change in position and unequal growth
of the stereomatic cells would ensue. The more slowly growing tissues
are the more regular and the more compact. The stronger development
of mechanical elements in irrigated plants, both in the cortex and stele,
appears to be correlated with the larger growth of shoot, while the larger
vessels of the etiolated plant indicate the greater proportion of transpir-
ing surface (the leaf-surface) to the diameter of the stem.
LATER SECONDARY STRUCTURE.
As the hypocotyl approaches a diameter of 3 mm. a total movement
outward of the whole vascular system (including the entire wood cylin-
der) takes place, a result of the enlargement of the pith and adjacent
parenchyma-rays tissue (plate 26, fig. 1; plate 28, fig. 3). The inner edges
of the hadrome plates or wedges become more or less bent, because their
edges are held together unequally by the original solid mass of early sec-
ondary hadrome, which splits usually in four places, corresponding appar-
ently with the primary parenchyma rays. These, therefore, are at first
closed and later opened secondarily, as shown in the figure (plate 25, fig.
8), in which the rupture of the xylem cylinder is beginning. In a field
plant this expansion of the pith is also accompanied by a considerable
tangential growth of the medullary rays. This circumstance, together
with the relatively slower rate of growth of wood, brings about the result
that in field plants (plate 25, fig. 7) the amount of wood is relatively less
than in irrigated plants (plate 25, fig. 8), and the medullary rays are wider.
The thickening of the parenchyma rays is shown most strikingly in an
etiolated seedling, the consequent rupture of the wood! in which is shown
in plate 26, fig. 1.
As to the cortex, the growth has continued in all of its parts in such
a manner as to still keep the primary cortical canals included within the
1 The separation of the young hadrome in succulent roots in this manner is
well known.
102 Guayule.
living part. Two series of secondary canals have arisen in the hypocotyl
of the size under consideration, whether the growth has been rapid or
slow under irrigation (plate 29, figs. 3, 4), or slow in the field (plate 28,
fig. 3); but the total number of canals is greater in the irrigated plants,
as would be expected in view of the more numerous wood plates. The
radial depth of the cork has not increased in any appreciable amount in
the field plant, but is more uniform than in a rapidly growing plant, in
which it is relatively much thinner (plate 29, fig. 3).
It is of interest to extend our comparison to rapidly and slowly grow-
ing irrigated plants. The chief point of difference is seen in the much
greater tangential development of sieve-tissue, and, later, of stereome,
relatively to the size of the plant in slowly growing specimens (plate 29,
fig. 4). This statement may be extended also to the mechanical elements
of the wood, in which the libriform cells are of smaller diameter, have
smaller lumina and are more cylindrical, implying a greater amount of
intercellular cementing substances. The vessels too are of smaller diame-
ter, and, though this is compensated for by their greater numbers, the
capacity of the vessels in the more rapidly grown plant is considerably
greater (plate 27, figs.6,7). The phloem presents analogous differences,
having in the slowly growing plant a structure denser and much more
extended tangentially than in the rapidly grown plant, and in this, as in
the character of the wood, resembling more closely the field plant (plate
28, fig. 3). A still further difference, of more fundamental character
morphologically, is the development, in slowly growing irrigated plants,
of stereids in the pith (plate 29, fig. 4). So far as I have been able to
observe, the stereids occur under no other condition in the hypocotyl,
though, as will be shown, it occurs normally in the pith in the definitive
stem (plate 29, figs. 5, 6).
The observations on the structure of the wood in the seedlings studied,
regarding especially the water-carrying elements, are of peculiar interest
as they stand in relation to those of Cannon (1905), who studied compar-
atively irrigated and non-irrigated desert woody plants of eight species.
His general conclusions, undoubtedly supported by his observations, are
that ‘there can be no mistaking the fact that branches of irrigated plants
(even if semi-irrigated only) are poorer in conductive tissue than branches
of the same diameter of non-irrigated plants,’’ but he says at the same
time that this ‘‘is an unexpected condition.”’ Further, “irrigated plants
organize each year a larger amount of wood—which contains a relatively
large amount of non-conductive tissue—so that it happens that non-irri-
gated and older stems have more vessels than irrigated and younger”’ of
the same diameter.
For the reason that I found, to my surprise also, that some of my
observations coincide with Cannon’s, I venture to cite certain concrete
instances, and state these, together with those already presented, in brief
fashion, by way of instituting a comparison of our results:
1. In field plants (the seedlings above studied) the vessels are as large
as in irrigated plants of slow growth, or larger, and are slightly more
numerous. The stems are of nearly equal diameter (plate 26, figs. 2, 4;
plate 27, figs. 6, 8).
Anatomy and Histology. 103
2. On the contrary, in irrigated seedlings of very rapid growth the
vessels are much larger, though not quite so numerous, as in the plants
mentioned under (1); but the total amount of wood is considerably greater
relative to the diameter of the stem (plate 27, fig. 7).
3. The terminal twig of a field plant of very large size, in which the
amount of growth in any twig was very small in one season, is contrasted
with an irrigated twig of rapid growth. The wood cylinders are equal in
diameter; the vessels are somewhat larger in the secondary xylem of the
field plant. But in the protohadrome the vessels are larger in the irri-
gated plant (plate 27, figs. 4,5). Both twigs of the same and last season’s
growth.
4. Two twigs of about the same diameter of wood cylinder, one a field
twig two years old, the other irrigated, one year old. The total number
of vessels is greater in the field plant, and there are more large and more
smaller vessels. In the protohadrome, however, the reverse as regards
size is true. But the number of vessels in either year’s hadrome in the
field plant is probably the same as, or is less than, that in the irrigated
plant (plate 27, figs. 9, ro).
5. On the contrary, in another irrigated stem 6.5 mm. in diameter,
the number and size of the vessels are enormously superior to the number
and size in a field plant (plate 27, figs. 2, 3).
6. The protohadrome in a field seedling of usual growth compared
with that of an irrigated plant, before secondary xylem has appeared in
either case. In the irrigated plant, in which growth is rapid, the elements
in question are much larger (plate 26, figs. 11, 13).
7. The protohadrome in a peduncle, through which there is, relative
to its size, it can hardly be doubted, a very large transpiration stream, is
composed of very large elements (plate 26, fig. 12).
8. In an etiolated seedling (plate 26, fig. 3), in which the size of the
stem remains small in relation to the total transpiring area, the size of the
conducting elements is greater, and their numbers scarcely less, than in a
field or irrigated seedling of approximately the same size of stem.
g. In the tap-root of very rapidly grown seedlings the vessels are
much larger and the amount of mechanical tissue much less.
These observations are in part antagonistic, in appearance at any
rate, to those of Cannon, and in part agree with them. They must there-
fore be harmonized among themselves as well as with Cannon’s. In at-
tempting to cover all the cases with one explanation, we must not forget
that the problem indicated is a complex one, inasmuch as the ratios of
mechanical tissues in the two types enter into it. It will, however, suffice
to speak of the conducting elements alone at the present moment.
In stems of guayule of a given diameter in field and irrigated plants,
the amount of wood is greater in the latter. In wood cylinders ' of equal
diameter the same holds true. This is due to (a) the smaller amount of
cortex in irrigated plants and (b) the narrower medullary rays. We may
assume that the growth in thickness of the stem is correlated with the
growth of the shoot above. In the same period, the total amount of con-
1 Wood, medullary rays, and pith taken as a whole.
104 Guayule.
ducting tissue formed in irrigated wood is undoubtedly much greater than
in that of field plants, but the amount of mechanical tissue is also greater.
Putting these facts together, it seems reasonable to conclude that the
capacity of the conducting elements 1s correlated with the maximum trans pi-
ration stream. The relative numbers, and therefore their size, depend
primarily upon other conditions productive of the development of me-
chanical elements. On comparing the shoots of field and irrigated plants,
it is clear that the mechanical conditions in the latter are those under
which mechanical tissue would be developed. The mere weight of the
foliage alone would be expected to insure such responses.
ADVANCED SECONDARY CONDITION OF THE HyYpocoryL.
In a more advanced stage of growth nothing of especial note, be-
yond that already pointed out, presents itself for discussion. One point,
however, is worth noting, namely, that the daughter and granddaughter
cells of the cortex remain arranged in tetrads chiefly, thus giving the
whole tissue the appearance of consisting of pairs and tetrads of cells.
The original, but enlarged, intercellular spaces are very much in evidence
(plate 28, fig. 4). Regularly shaped and disposed spaces, such as have been.
described for the root, do not occur in the stem.
AGE AND STRUGTURE.IN. THE SEEDLING.
Both popular and scientific discussion frequently turn on the corre-
lation of age and structure in the guayule. Inasmuch as the hypocotyl is
the oldest portion of the stem, it is worth while to indicate the structure
of field plants of known age. A seedling from Station 2, which was less
than one year old when collected in April 1909, with a stem 5 cm. long
and 4.6 mm. in diameter at the base, shows in the hypocotyl the struc-
ture represented in plate 30, fig. 1. The living cortex (primary) is very
sharply delimited from the cork on account of the rubber-content of the
living cells. It will be seen that the specimen closely resembles the slowly
grown irrigated plant above described, while in point of fact it is a plant
of rapid growth for field conditions, being much above the average size for
the locality in which it was collected. It is seen from this, what will in
any event be understood, that all field plants are not alike, the water-
supply varying at different times in different habitats, thus inducing at
times growth quite similar to that which usually occurs under more
favorable conditions. This seedling has, in addition to the four primary
canals, three series of secondary canals. One below the average size, of
the same age, with an epicotyl 8 mm. long, 2.4 mm. in diameter, has only
the four primary canals. These are finally thrown out when a diameter
of 6 to 7 mm. is attained, and therewith the whole of the primary cortex
is lost.
In a seedling of the same diameter three years old, it is possible to see
three annual rings of wood, marked by the larger pores of the first growth
of each season. There are in the same stem, aside from the four primary
canals, three series of resin-canals, one in the primary and two in the
secondary cortex, so that there are marks of three zones of cortex, the
primary and two secondary, corresponding apparently with the three
Anatomy and Histology. 105
seasons of growth. Comparing the two cases, we find that the structure
attained in the cortex by a seedling of one season may be the same as that
attained in three years by one of slower growth, while the number of
growth-periodsis reflected, albeit frequently only indistinctly, by the wood.
It is, however, generally true that the ring-structure may be made out.
EPRICOTYL.
Seedlings partially etiolated by being grown under a muslin screen,
in which the internodes have lengthened, render the analysis of the tis-
sues easier. The lowermost internodes of such seedlings receive primarily
six bundles (plate 24, fig. 5) from the hypocotyl, but the number is at
once increased, so that immediately above the base eight or even more
bundles may be counted (plate 24, fig. 6). The increase is more marked
in plants with short internodes, and the primary condition is quickly
masked. The development of the stereome which arises in the primary
leptome is in the primary numerical relation, there being at first six
bundles, opposite the median and lateral leaf-traces of the first two foliage
leaves. These relations are shown very beautifully in a section taken
from a seedling which had developed one-sidedly, and this is figured in
plate 30, fig. 2. The relations of the primary cortical canals received from
the hypocotyl are well shown also in this section. One pair of these
accompanies the median leaf-trace of the first leaf, the other pair that of
the second leaf. The third petiole may receive two or one, and this is true
of all the earlier leaves as far as the tenth node, approximately. The
primary condition, that in which two lateral canals occur, may recur even
in later stages of development, but only infrequently. As they pass into
the leaf one becomes smaller and ends blindly (plate 38, figs. 3 to 9),
while the other extends into the leaf-blade. In this there is a striking
similarity between the earlier foliage leaves and the cotyledons, constitut-
ing a morphological argument against the theory that the cotyledons are
not primitively leaves. The absence of medullary! stereome, mentioned
above in the paragraphs dealing with the hypocotyl, will be noted, and
this condition, as in the case of retofios, persists until the level of the
tenth internode or thereabout. Sections of field seedlings with short inter-
nodes at a distance of several millimeters from the insertion of the coty-
ledons show no medullary stereome, and this is true also of medullary
canals.
The same section (plate 30, figs. 3, 4) serves, in addition, to show
very clearly the origin of the periderm, which in the definitive stem, as in
the earlier internodes, occurs in the first or outermost cortical layer of cells
(as shown by Ross, 1908). Fron and Francois (1901) state differently,
and their drawing depicts the earliest suberogenous divisions in the
second layer of cells; in this, however, they areinerror. Their drawing is
taken from a section through the base of a petiole, as the position of the
leaf-traces, so labeled, shows. In such a section, it is true, the earlier
divisions will be seen in the second, third, or even fourth layer. I have
introduced two figures taken from portions of the tissue in question on
1T use this in a purely descriptive sense. ‘‘Perimedullary stereome’”’ has been
used. The origin of this stereome is dealt with beyond (p. 110).
106 Guayule.
opposite sides of the same section. In the position opposite the first leaf-
trace the divisions occur in the second layer; at the other end of the
diameter, in the outermost. The periderm figured by Fron and Francois
is therefore the leaf absciss layer. Leaf fall in the guayule is consum-
mated only slowly, and, as compared with more familiar examples of the
temperate regions, is imperfect in its time relations. The layer is not
sharply defined, and the disintegration of the tissue is irregular, the result
of the uneven and irregular character of the component cells of the
absciss layer.
The epidermis, both of stem and leaves, in the epicotyl is clothed
with a single type of trichome found throughout (plate 30, figs. 5 to 11).
There are two derived kinds, a T-shaped hair predominating, with a few
scattered hairs of a type seen in Chrysoma (Lloyd, 1901) and in other
Composite, viz, the whip-hair, but in which, in the guayule, the terminal
cell remains undeveloped. The trichome does not, therefore, become
flagellate, as in the related species, the mariola (Parthenium incanum),and
in many Composite (Vesque, 1885). In certain places, as in the axils of
the leaves, floral bracts, and corolla, transition forms may be met with,
indicating that the two kinds have been derived phylogenetically from a
single type. The fact that both are present in different Composite, but
in different ratios, may be used to support the view that the trichome
clothing is a character which has been brought about by gradual change
and not by the sudden dropping out of one or the other kind. The
T-shaped hairs clothe the plant very completely and smoothly, the termi-
nal cells all lying very nearly parallel to each other, and to the axis, on
the various organs. The density of the covering varies, however, with the
size of the organ, as the individual hairs show no substantial amount of
response to varying external conditions.'
Before leaving this part of the subject it is necessary to point out
that in seedlings in which the stem elongates slowly, as in the field, the
primary cortical canals of the hypocotyl behave in a manner which has
not been observed in etiolated plants. The two pairs, associated with the
median leaf-tracesof the two early foliage leaves, instead of passing directly
into the petioles, anastomose and then, from the transverse lacuna thus
formed, canals pass off to enter the leaves. Other canals have been noted
to rise from the lacuna and to pass up into the epicotyl; a reanastomosis
within a short distance has been observed (plate 36, figs. 7, 8). A section
of a field seedling made through the cotyledonary node, or at any level,
if the internodes are undeveloped, will almost invariably show widely
spreading divarication of one or more of the canals (plate 36, fig. 6). In
a word, the canals constitute a branching system, each more or less in
communication with the other.
__}The mechanical conditions in axils of leaves and in the capitula cause super-
ficial changes in the shapes of the trichomes.
Anatomy and Histology. 107
THE DEFINITIVE .STEM:
PRIMARY STRUCTURE.
After the tenth internode, approximately, has been laid down, the
stem takes on its definitive structure. The number and appearance of the
various structures within the growing tip vary a good deal, according to
the rate of growth. This is largely due to the crowding together of the
nodal characters, but in part also to the size of the terminal bud, and
therefore to the number of leaves. In a thick apex more bundles of
primary elements appear at a given level (plate 31, fig. 3); also the size
and frequency of branching of the canals is greater within a given zone
(plate 36, fig. 5). For the purpose of description it will serve to present
briefly the differences observed, at various levels of a stem one year old, of
normal growth-rate in the field, as this will give an epitome of the develop-
ment of the tissues. The specimen before me is a twig which grew in 1908,
collected at the close of its elongation for that season. It is 11 cm. long,
4 mm. in diameter at the base, and 1.6 mm. just behind the tip. The
structure at the levels mentioned is as follows:
Within the last millimeter of the tip one finds the vascular tissues
undifferentiated, though the medulla and vascular zone are recognizable.
The primary cortical canals appear opposite median leaf-traces,! but
nowhere else (plate 38, fig. 1; plate 31, fig. 5). The starch sheath (endo-
dermis) is recognizable only by the starch-content, which appears only
opposite leaf-traces, while starch is absent from the endodermis else-
where. Within half a millimeter further down, at a diameter of a milli-
meter, all the 5 medullary canals appear, 17 vascular bundles are distinct,
and 16 cortical canals are present (plate 31, fig. 4). In perhaps half of the
bundles spiral vessels have developed. These are in curved plates of 1 to 3
vessels, each separated by wood-parenchyma. The epidermis is densely
clothed with T-shaped trichomes. The endodermis may be traced com-
pletely around the stele,on account ofits starch. At this level may be seen
the earliest indications of the stereome bundles in the primary leptome and
in the pith.
At to mm. from the apex (diameter 2.5 mm.) the collenchyma of 4 to
6 rows is well developed. The characteristic thickening is first seen in the
periclinal walls, and these become still more conspicuously thickened
in the later stages. The larger bundles have xylem plates 6 to 8 cells
deep radially. Interfascicular cambium is being developed. The stere-
ome is still thin-walled, but the definitive size of the cells has been reached,
and thickening has taken place at the angles. In the section before me I
count 25 primary cortical canals and tomedullary canals. The section was
evidently taken just above the plane in which the pith-canals branched, as
two of the canals are cut at the fork.
At 15 mm. (diameter 3 mm.) mechanical elements have appeared in
the hadrome, and the stereome is more advanced as to the thickening of
the walls. The collenchyma has been somewhat stretched periclinally,
the walls so placed being much thicker. The walls of both cortical and
1 Very occasionally a pair, a single one on each side of the trace, occurs.
We y a pat, §
108 Guayule.
pith cells have thickened, and in the walls of contact the reticulations, due
to the broad, ovate, closely-set pits, are very noticeable. The interspaces
are large.
At various lower levels, depending on the time of the year in which
the material is taken, will be encountered the young periderm. Ross’
speaks of this as beginning very early, and in his material as reaching close
to the apex. Ifa newly grown twig is examined toward the close of the
season it will be found that the periderm embraces only a lower zone (of a
thickness depending on the rate of growth) at the base of the stem, and its
growth involves casting off the leaves which remained on the upper por-
tion of the twig of the previous year. This uppermost zone, carrying the
overwintered leaves, undergoes some growth with considerable lengthening
of the internodes, so that the leaf-scars of the winter bud do not crowd
each other as do the bud-scale scars in plants of the temperate regions.
The periderm passes upward from this zone, and during the following dry
season slowly cuts away the leaves, until by midwinter, earlier or later
according to the character of the season, all the leaves of the previous
growing season, save the terminal ones, are cast off (plate 14, fig. B).
As the periderm extends toward the apex of the twig the epidermis is
fissured concurrently, beginning at the base.
A section near the base of the season’s growth shows the following
structure: The periderm is three to four cells deep, measuring 0.1 mm.
The xylem bundles measure about 0.5 mm. on the radius, and the pith has
a diameter of 1mm. Nearly all the bundles are supplied with both corti-
cal and medullary stereome. Tracheids are fewer in the outermost zones
-of the xylem. The primary cortical canals and pith canals have generally
enlarged, the largest measuring 0.3 to o.4 mm. tangentially, with a radial
diameter of 0.15 too.2 mm. This section has one completed series of sec-
ondary cortical canals, and a second row begun. The epidermis is slightly
fissured. This amount of growth and secondary change is by no means the
maximum possible. The thickest part of the stem of one season’s growth
of the seedling shown in plate 46, fig. A, had five series of secondary
canals, and cork o.5 mm. thick, the depth of the cortical tissues, primary
and secondary, being 2.5 mm.
A stem of two growth-periods shows the primary and one series of
secondary canals, but the two seasons’ accretions of wood are reflected in
the annular structure of the wood, as in the seedling hypocotyl before
mentioned. Here also, therefore, the relation of structure to age is less
apparent in the cortex than in the wood cylinder. The whole of the outer
leptome (that embraced between the primary and secondary series of
canals), is stereomatic; that within the secondary series still retains its
sieve character. A considerable thickness of cork has developed.
Later changes need not be followed year by year, and it will suffice
to point out the more important features summarily. The inner periderm
normally does not begin until the stem attains a diameter of over 1o mm.
(Ross, /. ¢.), and the primary cortical canals may still be found up to this
time or even very much later, e.g., in a stem 28 mm. in diameter, with cor-
tex, including bark, 5 mm. thick. The penetration of the inner periderm
1 His material appears to have been collected in December.
Anatomy and Histology. 109
is not a clean-cut process, such as we see in our common trees and shrubs,
but first appears directly opposite either a primary stereome bundle or a
primary canal, as an ingrowth, simulating invagination (plate 32, fig. 4).
The absciss layer which effects leaf-fall is similarly clumsy, so to speak.
This tissue consists of a quite irregular layer of cork-cells, continuous with
the primary phellogen. The outermost cells, those, namely, immediately
beneath the base of the leaf, first become suberized.
Until an advanced age is attained, the inner periderm does not cut
deeply. In old stems, 20 to 50 years of age, light-colored layers of cork
may be seen penetrating to half the depth of the cortical tissues, but quite
irregularly. It is of interest to note here that this cork presents a special
practical difficulty in the factory in handling the comminuted shrub after
it has passed through the pebble-mill. The bagasse is then, with the
exception of this cork, which has been broken up into flakes, separated in
water, the rubber and the cork flakes floating and the remainder sinking.
Only by means of pressure under water or prolonged soaking may the
cork be waterlogged, when it sinks, leaving the clean rubber still floating.
These layers of cork are seen in plate 2, fig. B, from a photograph of a
stem certainly forty years old.
SECONDARY STRUCTURE.
The secondary cortex is characterized by alternating concentric rows
of stereome bundles and resin-canals. Between succeeding stereome mass
and leptome parenchyma (canal-cells, consisting of endothelium and usu-
ally a single subjacent or supporting layer), there frequently intervenes
no tissue at all, and the stereome occupies the whole of the space between
adjacent resin-canals. In the inner part of the secondary cortex one finds
alternating canals and “soft’’ leptome, the composition of which raises
some points of question. The canals, as described correctly by Ross
(1908), arise as a double row of cells derived directly from the cambium
(plate 22, fig. 13). Surrounding the “‘secreting”’ cells is at least one layer
of leptome parenchyma, the usual condition in slowly growing plants. In
irrigated plants there may be two or three (or occasionally more) layers
(plate 29, fig. 1). This is followed, radially, by a mass of sieve-tissue
(plate 32, fig. 2), which may be regular in transverse outline, and com-
pletely uninterrupted by parenchyma until another canal is laid down,
or it may be narrow and more or less irregular, as in irrigated plants
(plate 25, fig. 8; plate 29, fig. 4). In any event, the sieve-tissue occupies
the radially placed space, broadly speaking, between successive canals,
and it 1s in this space that we find stereome later.
The manner in which the stereome arises is, in broad outline, as
follows: The outermost (on the radius) leptome cells undergo transverse
enlargement and become stereomatic. Successively other adjacent cells
lying farther in behave similarly. The resulting tissue, however, occu-
pies more space than did the original cells from which it arose. As the
total space which is occupied by the stereome is usually identical with the
total leptome, it follows that there must be some readjustment. This is
brought about by the discontinuous sclerosis of the leptome, so that irreg-
ularly alternating masses of this are destroyed and become compressed.
110 Guayule.
The stereome develops, therefore, within the leptome,! and in its defin-
itive form a portion of the leptome comes to occupy the volume of the
whole. The definitive stereome may be flanked more or less completely
by sclerosed leptome parenchyma, and even the adjacent cortical cells,
especially in the peduncle, may take on this character.
With reference to origin, in general terms, I am at variance with
Ross, who says on this point: “ Durch die Tatigkeit des Kambiums ent-
stehen abwechselnde Gruppen von zartwandigen Elementen und von Scle-
renchymfasern. In der jtingsten Gruppe der letzteren geht die Verdick-
ung der Zellwande erst sehr allmAlich vor sich, und in den zartwandigen
Schichten zwischen dieser und dem Kambium kommt der Secretkanal
zur Ausbildung.”’ I believe that Iam not unduly criticizing Ross’s state-
ment by saying that it is misleading. It would seem more consonant with
the facts to say that through the activity of the cambium alternating
groups of leptome parenchyma and prosenchyma arise, and that the
stereome arises within the latter. The resin-canals arise from two adja-
cent tangential layers of the thin-walled parenchyma.
The change of any particular cells into stereome is not complete
before the end of the third season’s growth, as nearly as we may judge.
This secondary occupation of the leptome by the stereome is particuarly
well shown in a preparation made of the cortex of an old stem (plate 32,
fig. 2). The sections were treated with xylol so as to extract the rubber,
leaving the tissues empty and distinct. The stereome was seen to occupy,
with few exceptions, all the space previously occupied by the sieve-tissue.
ORIGIN OF THE MEDULLARY AND CORTICAL STEREOME.
Vuillemin, 1884c, p. 223, has described stereome as arising in the
pericycle in the Composite, but he does not show its precise origin nor
that of its constituent elements; nor does his description of the leptome
(/.c., p. 99) fit the conditions found in Parthenium argentatum. Accord-
ing to Vuillemin, the sieve-tubes are of much larger transverse diameter
than the companion cells, and this is not true of our plant. There are,
however, broad elements with oblique end-walls,? intermixed with sieve-
tubes and companion-cells to form a mélange in which the sieve elements
are generally in contact with each other throughout the whole leptome
mass, and do not usually form isolated islands, as generally described for
the Composite. These elements have common origin in cambium cells;
that is to say, the broad elements and the narrow sieve-tissue elements are
of common descent. The broad cells, which later are transformed into
stereome, do not, therefore, have a distinct origin. The initial division
within the mother-cell may be periclinal or radial, separating a broad ele-
ment, destined to become stereomatic, from a similar one, which again
divides once or twice, usually twice, to form the sieve-tissue (plate 31,
fig. g). There is but little difference in the transverse diameter of these,
the companion-cells being narrowly fusiform and therefore thickest at the
middle, while the reverse, of course, is true of the sieve-cells. The broad
elements are recognizable both by their size and by their more tenuous
? Vuillemin’s description, ‘‘sur le dos des faisceaux libériens,’’ does not apply.
*The “‘libriform ” of Schwendener, 1874.
Anatomy and Histology. 111
protoplasmic content. When they become stereomatic the first step is the
great enlargement of their transverse diameters, their walls being thin
except at the angles, which are thickened after the fashion of collenchyma
(plate 31, fig. 8). During this phase of change the mutual pressure of the
‘developing stereome and the surrounding cortex results in the radial
flattening of the latter, and frequently in a crumpling of the walls in the
stereome. The limit of the stereome may readily be seen because of the
intercellular spaces between the cortical cells and those of the stereome.
Meanwhile the sieve-tubes and companion-cells become displaced and,
with sclerosis of the stereome elements, are destroyed, and may only with
difficulty be observed at all. Sclerosis of the stereome proceeds radially
from without inwardly. The compactness of the stereome, as also its
regularity and dimensions, depends upon the previous mode of growth of
the leptome as a whole, and is therefore more irregular and of uneven
texture, in irrigated plants, or, what amounts to the same thing, in rapidly
grown plants. Sclerosis also overtakes some of the adjacent leptome
parenchyma and, under certain circumstances, some of the neighboring
cortical cells, but is not preceded by their enlargement.
The stereome in the medulla (plate 31, figs. 6, 7), which has previously
been so referred to for convenience, is, like the above-described leptome-
stereome, a constituent of the mestome strand. It arises from elongated
elements clustered about the primary hadrome elements, and is the en-
doxyle of Briquet (1892), but, in the light of the occurrence of bicollateral
bundles in the Chicoriacee (Vuillemin, 1884a; van Tieghem, 1884), may be
susceptible of another interpretation, viz, that it represents the internal
leptome in these forms. This explanation is not decreased by the very
close analogy between the stereome of the leptome and of the hadrome.
In the young condition the tissue which is destined to become stereome is
recognizable (plate 31, fig. 6), in transverse section, by the absence of in-
tercellular spaces and the somewhat thickened angles, which, during the
stretching of the walls previous to sclerosis, become more apparent, as in
the case of the leptome-stereome. Interspaces occur in the adjacent pith
and in the hadrome parenchym. The tissue, taking the form of an irreg-
ular lunate arc in transverse section, is, therefore, while in contact with
the hadrome, not to be referred to this without careful consideration.
The progress of change into stereome is identical in all respects with the
leptome-stereome, and calls for no particular description; this refers also
to the mutual displacement of tissues (plate 31, fig. 7). The analogy to
the leptome-stereome is strengthened by the circumstance that longitu-
dinal divisions may take place in the earliest formed elements, before the
final complement of stereome cells is arrived at, though it must be said
that these divisions are not of sufficiently frequent occurrence toenableone
to see more than a very few at atime. The form of the elements further
likens them to the analogous ones in the leptome, being elongated and
having slightly inclined end-walls. I am therefore inclined to regard the
medullary stereome as a tissue per se with respect to the hadrome, and as
having much in common with the stereome of the leptome, so that it
would seem to be properly regarded as representing the internal leptome
in genera of the Chicoreacee
112 Guayule.
Precisely these relations occur, to all appearances, in certain of the
Boraginacee, e.g., Symphytum tuberosum, Nonnea alba, Omphalodia lint-
folia, etc. (Jodin, 1902). Concerning the leptome, Jodin says, after speak-
ing of the disappearance by crushing of the sieve and companion elements
Z
(‘les primaires tubes criblés’’):
En méme temps que s’accroissent les éléments libériens primairés, on peut
assister, dans certains genres A un €paississement notable de leurs parois * * * *
Dans d’autres cas, cet épaississement est trés faible ou méme n’a pas liew * * *
(EGz sp sgos-)
But no such thickening takes place in the secondary leptome. Appar-
ently the thickening of which Jodin speaks goes no further. He does not
trace the precise origin of the cells with thickened walls.
As to the medullary stereome, he says little, but his figures show very
clearly the earlier, prestereomatic condition which I have shown in my
own figure (plate 31, fig. 6). To quote again:
Nous aurons peu de choses a dire de la moelle; nous avons eu occasion de
parler, A propos des faisceaux du bois, de la zone périmédullaire, et des rayons
médullaires. La région médullaire proprement dite se distingue par la taille de
ses cellules qui sont arrondies en coupe transversale, et qui laissent entre elles de
nombreux méats triangulaires. (l.c., p. 322.)
This author, it is seen, points out the same distinctions between the
perimedullary zone and pith which I have already made. From this com-
parison between the guayule and the borages it seems clear that we are
dealing with the same behavior, with the very interesting distinction that
in the guayule the histological differentiation of the fibers proceeds to
completion, while in the plants studied by Jodin they are arrested in their
course of development. This appears to be connected with the herba-
ceous character of the stems in the Boraginacee.
In this connection, Schwendener’s observations on certain Composi-
te are of particular interest:
Im Phloem der grésseren Aster und Solidago Formen, * * * kommen inner-
halb der starken primaren Bastbiindel kleine secundare Gruppen mechanischer
Zellen zur Entwickelung, welche zum Theil mit den kiirzesten Libriformzellen,
die iiberhaupt vorkommen, tibereinstimmen, und jedenfalls durchgehend vom
typischen Bast verschieden sind. Die Lange diesen Zellen variert zwischen 150—
300 Mik.; die Ktirzesten erreichen oft nur bis 80 Mik. Dazu kommt, dass die
nebeneinander liegenden schiefen Querwadnde ahnliche Zick-zacklinien bilden, wie
sie sonst nur in kurzzelligen Libriform vorzukommen pflegen. Bei Aster bilden sie
im Querschnitt netzformige anastomosirende Bilden, zwischen denen ein parenchy-
matisches Cambiform stellenweise mit deutlichen Siebrohren, eingebettet liegt.
This distinction made by Schwendener between the sclerosed element
of the “phloem” and typical bast applies throughout to Parthenium ar-
gentatum. This plant, however, differs in the distribution of the sclerosed
elements, forming as they do dense masses occupying the space previously
occupied by the whole of the leptome and its associated libriform.
Schwendener, however, appears to assume the independent origin
of the libriform cells in the leptome, and it is on this point that I advance
the view that they have a common origin with the sieve and companion
1 Schwendener, 1874, p. 152. I have not had access to this paper.
Anatomy and Histology. 113
cells. After arriving at this conclusion, I found that Servettaz (1909, p.
232) had already done so with respect to certain of the Eleagnacee. The
close resemblance in the behavior of the medullary mass of stereome to
that of the leptome forces criticism of this view to the effect that the
analogy which I have drawn is based on the origin of the stereome in
the hadrome and leptome from libriform of an identical mode of origin
on either side of the cambium. This view, while admittedly possible, does
not agree with my observations, and it is hoped that further research will
bring evidence to light which will show which view is correct.
The secondary resin-canals, when fully formed, are composed of an
endothelium backed usually by one row of leptome parenchyma (plate 29,
figs.1,2). Intransverse outline, after full development, they are rounded,
but gradually become compressed radially as they pass outward toward
the bark. The youngest ones measure upward of o.2 mm. in tangential
diameter, and grow in size till, at the outer part of the living cortex, they
may measure, in a cortex 5 mm. thick, over a millimeter tangentially,
and with a width a third of this. The secreting cells undergo more or less
periclinal divisions (with reference to the axis of the canal), producing
sometimes two to three layers of cells of endothelial origin. The resin-
canals at length frequently become partly or completely closed by an
ingrowth of tissue (Lloyd, 1908b) of the same character as the cortex and
forming an interesting analogy to tracheal plugs (tyloses). These I call
pseudotyloses (plate 32). The cells of the pseudotyloses at length become
filled with rubber and continue in a living condition somewhat longer
than the surrounding cortical tissue, retaining their normal appearance
when the cortical cells toward the outside of the stem have passed over
into suber. These parenchymatous plugs are not confined to the very old
tissues, but may be found also in young stems and roots,’ though less
frequently. Occasionally the medullary canals, in old plants at any rate,
become partially plugged in the same manner (plate 32, fig.3). In addition
to these outgrowths, resembling roughly a bunch of grapes, one frequently
finds trichome-like structures, sometimes projecting from the walls and
also from the plug-tissue (plate 32, figs. 1,6). Somewhat similar appear-
ances have been observed by Col, and to this I shall call attention again.
In this connection, however, I feel inclined not to agree with this author
in his criticism of Vuillemin, who recorded observing structures which he
called “ poils glanduleux”’ in the canals in old rhizomes of Arnica montana
(Col, 1903, p. 166). I suspect that these “ poils glanduleux”’ are the same
structures as those which I have called pseudotyloses.
The pith undergoes a considerable amount of secondary enlarge-
ment, so that in a stem 2.5 cm. in diameter, in which it may still be found
in a living condition, its diameter is between 3 and 4 mm. and is irregular
in outline. The medullary stereome does not receive any secondary accre-
tion, but the growth of the inner part of the parenchyma rays concomitant
with that of the pith, between the edges of the xylem wedges and the
flanking stereome, results in the periclinal separation of these. Sometimes
one may find that cells near the periphery of the pith have undergone a
1T have observed them in the primary canals (plate 32, fig. 7).
8
114 Guayule.
rather regularly repeated periclinal division, and the tissue, therefore, has
much the appearance of a cambium. It may also happen that repeated
divisions occur in a zone about one of the medullary canals. The cause of
this is not clear, though it is possible that this also is a mode of growth of
the pith (plate 42, fig. 5). It does not appear to be the same as the forma-
tion of cork, such as I have observed to occur following injury to the pith
or adjacent tissues.
In field plants normally neither pith nor parenchyma rays (save a
very few cells) ever become lignified.
The wood in large stems shows the usual distinction of alburnum and
duramen. The latter is reddish-brown in color, and all the tracheids are
plugged by ‘“‘“Gummipropfen.’’' Temme (1885) and Ross (1908) note
their positive reaction to phloroglucin, which I have verified. They are
very sharply confined to the duramen in uninjured stems. In one, 2.5 cm.
in diameter, in which the plugs are beginning to be formed with irregular-
ity, their genesis may be followed. They first appear as a thin, partial or
complete lining, increasing irregularly and gradually filling the lumen.
Their conformation suggests the behavior of a dense fluid. Their positive
reaction to aniline blue, which is very marked, may indicate that they are
at first similar to callus, but, as phloroglucin shows, they later become
lignified. In the old wood the plugs appear homogeneous, but they stain
unevenly with, e.g., Bismarck brown. Here and there one may note a
stratification in planes parallel to the surface of the lumen. That resins
are absent from these structures is shown by their total failure to react to
alkanet. Molisch (Zimmerman-Humphrey, 1893) showed that gum-plugs
behave, with certain reagents, like lignified membranes, but a total par-
allelism is denied by the above reactions. Lignification in any event
would appear to be a secondary feature. Tschirch (1906, p. 1180) identi-
fies the substance as “‘ bassorin.”’
ANNULAR STRUCTURE.
The mature wood shows to the naked eye an annular structure which
is frequently regarded as annual-ring structure. In an old stem what is
seen in part is a banded appearance due to differences in color intensity
(plate 2, fig. B), having no relation at all to a true annular structure, which
is readily seen under magnification. This is shown in the two figures, one
of which (plate 33, fig. 1) was drawn to scale from the inner alburnum of
a very old stem, and the other (plate 33, fig. 2) from one a centimeter in
diameter, showing ten rings. It is not at all unlikely that these rings
represent ten years’ growth, but this would not justify the conclusion that
the rings are always correlated with age in years. It must not infrequently
be the case that more than two accretions of growth occur in response to
the distribution, in time, of the rainfall, and these rings, therefore, repre-
sent periods of growth following rain. That these growth-periods for field
plants usually coincide with the summer seasons follows from the general
1“Wound-gum” (Temme, 1885) seems hardly a suitable term, since the
phenomenon is perfectly normal, though, as will appear, the earlier secretion is
provoked by natural and by artificial wounding. A direct translation of Gummt-
propfen would be preferable.
Anatomy and Histology. 115
character of the precipitation, as elsewhere described. The evidence gives
strong support to the view expressed by Holtermann (1907) that the ring-
structure of the wood is correlated with cessation and resumption of tran-
spiration. While it is not clear why an annular structure within the annual
ring is present in the wood of irrigated plants, it is quite possible that
it is due to stimulation by successive irrigations. These considerations
show that it is practically very difficult to determine the age of a plant
by counting the rings, and this is rendered still more so by their short
radial measurements. In the case before us (plate 33, fig. 2) 10 rings
are counted on a radius of 2 mm., so that the rings, taken altogether,
have an average thickness of 0.2 mm. Excluding the innermost (the first
season’s growth) and the deepest, the rings vary from 0.06 to 0.3 mm.
approximately. This, coupled with their frequently great irregularity and
indistinctness (Ross, 1908), makes them difficult of recognition.
The suggestion has been indicated inferentially in this connection by
Ross that the age of a stem is to be inferred from the number of secondary
canals and stratifications of the secondary cortex. A stem examined by
him, 1g mm. in diameter, showed eight zones of canals and of alternating
phloem layers, and he agrees with Endlich that the stem was about ten
years old. I feel quite sure, however, that this inference is far from jus-
tified. For example, the stem from which plate 33, fig. 2, was taken
was certainly over four years of age, and as certainly eight to ten. There
were only four rows of secondary canals. In astem with a radius of 1 cm.
I count at least ten cortical zones, while there are but five in another cor-
tex of the same thickness. These, together with the further fact that
under irrigation a seedling in five months developed five rows of second-
ary canals, show that the number of canals depends upon the rate of
growth and not upon the number of seasons, and in field plants the num-
ber of rows of canals is, roughly, a third to a half less than the age of the
stem in years. A cortex 5 mm. thick, exclusive of the cork, taken from a
very old plant, about forty years of age, shows about twenty canal zones.
Some had of course been cut out by periderm, but scarcely as many as
twenty.
TABLE 40.—Determinations by weight of ratios of bark to wood in field plants
(Whittelsey, 1909).
Ratio Ratio |
Plant. | Parts of the plant examined.| of bark Plant. | Parts of the plant examined.| of bark |
to wood. to wood.
pee | 1.29 |
I_ | Roots and thicker stems 1.36) _.| 1.63
0.79 I .30 |
II | Branches and twigs... ./} 1.7 |
Il OGG? och eer 0.85 tO Va
GUSTS. 24 Sa roe ste, ack t= EOS TiAl,
Es |
116 Guayule.
THE EFFECT OF ABUNDANT WATER UPON ANATOMICAL
STRUCTURE.'
The effect of irrigation upon the structure of the mature plant is very
marked. Thisis especially noticeable with respect to the relative volumes
of the wood cylinder (including pith and medullary rays) and the “bark’’
(cortex and cork). As this is a question of prime importance economic-
ally, it will be treated first.
By means of weighing, Whittelsey (1909) determined that, in vari-
ous portions of the plant, the trunks are made up of 44 to 65 per cent bark
(cortex and cork), the amount of bark being relatively larger in the smaller
twigs. The material was quite dry. (Table 40, page 115.)
In Whittelsey’s determinations, the pieces examined were first steamed
to render it possible to separate the wood from the cortex. A slight
Wet
Dry
Fic. 16.—Relative dimensions of wood cylinder and cortex, wet and dry, in twigs of field and
irrigated plants. X20.
error is introduced by this method, as some of the resin exudes from the
cut ends of the cortex and infiltrates into the wood. This error is appar-
ent in table 41, in which the ratios of steamed material are smaller than in
TaBLE 41.—Ratio of bark to wood by weight for field and irrigated shrub, as deter-
mined after (a) steaming, (b) moist chamber.
Two pieces each of field and irrigated plants (Cedros, April 1909): (a) Field
plant pieces 3.9 to 4.8 mm. diameter; (b) irrigated plant pieces, 3.4 to 4 mm. diam-
eter. Each piece of (a) segmented into fourths and alternate fourths placed in
each of two lots; (b) segmented into fourteenths, similarly placed in each of two
lots. One lot (I) of (a) and of (b) steamed, de-barked, dried in oven, and weighed.
The other lot (II) of each placed in a moist chamber till fit for separating wood and
bark; then dried in oven and weighed.
Class of plant. | Treatment. Mish of | NYgighe of | Ratio.
gram. | gram.
(aj) kikield (Cedros)s-5s5e5c HE aAMeG vc. ack: 0.1604 | 0.2817 | 1.75+
(a) II. Field (Cedros)........ | Moist chamber...) 0.1663 | 0.2968 | 1.784
(O)) nies (Cednos)s see Steamed. J. .... Glatk 90.458 || ror
(6) LIS Mascioai(Cedros)sieewe- Moist chamber.. .| 0.324 0.3608 | r.11
‘The substance of what follows under this caption was presented in a paper
entitled ““The Responses of the Guayule, Parthenium argentatum Gray, to Irriga-
tion,’’ before the Botanical Society of America, at its Boston meeting, December
1909. (Lloyd, rgrob.)
Anatomy and Histology. 117
control material softened in a moist chamber. But the small error, less
than 1 per cent, is of significance only when such data are used in large
calculations.
From the present point of view a volumetric method is of more value
than weighing, since the ratios derived by the latter are disturbed by vari-
ation in the specific gravity; but as a comparison
—_—
= a= ee of the ratios derived by both methods is of use in
cot d i
— practice, they have both been introduced. For
we similar reasons it is im-
pry _ cork _ portant to know the
ratios derived from dry
material, and for this
purpose the method of
displacement of alcohol
has been used.
RELATIVE VOLUMES
OF CORTEX AND WOOD.
The material from
which the datatabulated
in table 42 were obtained
was collected at Cedros.
The irrigated material
was taken in Sextember 1908
from stems (fig. 16) of two sea-
sons’ growth. The difference in
thickness of wet and dry cortex
is very slight, and is not given.
It is seen that the volume
ratio of bark to wood (when dry)
in the irrigated plant is near to
unity in the smaller twigs to 0.27
Fic. 17.—Relative thickness of cortex in stems of irri- in the larger, uP toa diameter
gated and field plants. the wood cylinders being of I3.5 mm., beyond which no
of equal diameter when dry. : ;
material was available. In field
plants the ratio for the smaller twigs approaches 2.5, being reduced to
1.7 for stems 13 mm. in diameter, and still further, namely, to unity, for
stems exceeding this diameter (20 mm. and more). From the economic
point of view this material reduction of cortical tissues in irrigated plants
is an important consideration, sincé it is these tissues which contain the
rubber.
The ratios for the wet tissues indicate the large water-holding capac-
ity of the irrigated cortex, especially as compared with the field material.
These differences in volume are quite obvious in the radial measurements
of the wood and bark. In tables 43 and 44 some accurately made
measurements are given. The ratios are illustrated in figs. 15, 16. For
the better direct comparison of field and irrigated plants dry twigs of the
same initial total diameter were chosen and were measured both dry and
after being soaked in water. The initial size is shown in the diagrams
‘Escalon (field)
118 Guayule.
by a half-circle. To the right of the vertical diameter is shown the irri-
gated plant; to the left of it the field plant. The upper quadrants show
these when wet; the lower, when dry. To be noted are the greater capacity
of the wood cylinder in field plants for swelling, due to the larger volume
of the parenchyma rays; and the smaller capacity of the cortex tissues for
swelling, due to the larger rubber-content. The greater volume of cork
TABLE 42.—Volume of wood and cortex (Cedros, Sept. 1908).
| Irrigated plants. | Field plants.
Diameter Thick- | Ratio of cortex || Diameter | Thick- Ratio of cortex
of wood ness of to wood by || of wood ness of to wood by
cylinder. cortex. | volume. cylinder. cortex. volume.
|
| |
Dry. | Wet. Wet. Dry. | Wet. Dry. Wet. Wet. Dry. Wet.
| = | |
| mm. mm, mm. mm. mm. mm.
2.0 2.0 ©.7 \On193 EOS eH] is Tey) 0.9 2ra8 25
ea 2.4 Ose) || atx Less, | 27 | Beg! ys res 2.0 2.65
ye ee | E,o O94 1.28) |) 32% An8 a7 TONS 2k
ARO tt Ans Zt On, GOR |. wsasy |) Aa x AnT sles 12130 TG 2.25
ee Pcl pee a m4. On43) 1) fOr 29 GeO a a2eS Taf Biot
iitjers | ir avo 2.0 0.33 GO” iO r2b On O8 | oO) el emer 2.08
Tag as 2.0 on27) 0.55 || AE, Opn 53 5 ae it, st
ce ae Vabgee ee Se aes|| | * Creer 22.0 | 5.0 ren rei
es ae a a: | nee? 1 ++ | 22.7 | 4.9 T.05
and of the cortical intercellular spaces in irrigated plants must also be
considered. As these tissues are included in the tables under the term
“bark,” it is obvious that an error is introduced which is larger for the irri-
gated plants. Hence the ratios ought to be, for these, relatively smaller.
Table 44 shows the same relations for branches of larger size, in which
the ratios of bark to wood are smaller, but relatively more so in irrigated
plants. The figures are of special interest, as they include the ratio seen
TABLE 43.—Transverse dimensions of terminal twigs of irrigated and field plants of
the same initial size, before and after drying (fig. 15).
ee ' : Ratio of field
Total Diameter Thickness of Thickness of ois
diameter. of odd: eae Moe 3 - Bore sl
Dry. | Wet. | Dry. | Wet.| Dry. Wet. Dry. Wet. Dry. Wet.
( ) I zs t d re mm, | Te | mm, mm, mm, mm, Qe |
a) Irrigated. .| 2. BAG" SGI) aN ICR SYS OA Chis || Osi
(b) Field ..... 2.6 | 3.15 | 1.5 |1.7 |0.55 | 0.725 | 0.10 |0.10-0.15 | f 77? e985
(c) Irrigated. .| 3.9 | 4.85 | 3-0 | 3-15|0-45 | 0.925 | 0.15 | 0.19 bey al Beg
(a) Bield 2. =. Bey lactate ies) ace |e | EO @:10)|6.1i—O.05: |} ~ 7
in a plant from the Hacienda de San Isidro, near Escalon, Chihuahua,
where guayule is said to grow rapidly. While the rate of growth is not as
great as supposed, nevertheless it is sufficiently so to be reflected in the
structure of the stem, which is intermediate in character between Cedros
field and irrigated plants. In the three cases the initial wood cylinder
diameter (20 mm.) was the same—in this way the largest available sizes
could be compared. The thickness of the cork is, when wet, nearly the
Anatomy and Histology. 119
same in all, though its irregularity makes accurate measurement impos-
sible. When dry it is thickest in the Cedros field plant, thinnest in the
Chihuahuan plant, and intermediate (though much more irregular in
thickness) in the Cedros irrigated plant. The differences in the cortex are
TABLE 44.—Comparative radial measurements, in millimeters, of medium-sized stems
of guayule, wet and dry. Wood (dry) cylinder 20 mm. diam. in all. August
29, 1909 (fig. 16).
Dry. | Wet. Ratio of
. a eo l == 7 - cortex
Kind of plant. | Bark | Cortex | | Bark | Cortex | © wood,
|Wood. | (cortex | (without | Wood. (cortex | (without | Ralach
| +cork). | cork). | +cork).| cork). et
3 = | mm, mm, mm. mm, mm, mm,
Cedros irrigated, 3 |
years Old sii. - 2 | ro at eee TOR 2SN Meas 2.0 0.28
San Isidro (near | |
Escalon) field...| 10 BAG BEB Fe li VE O'- 25a lia See 4 2.95 0.53
Cedros field....... be) Ae | 23 75 10.37 | 5.25 4.8 0.89
The irregularity in the thickness of the cork makes it difficult to measure it
properly. It is, at all events, less than 1 mm., and relatively thinner in field plants.
apparent; the index of imbibition of the wood cylinder, while still greater
in the Cedros field plants, is relatively much smaller than in smaller stems,
because of the compression of the medullary rays.
Tables 45 and 46 are based upon comparative measurements of Cedros
field and irrigated plants. The latter material, however, was collected in
TABLE 45.—Relative amount of bark and wood in guayule, by volume (dry). Irrigated
plants (Cedros, Apr. 1909).
Total Diameter Thickne Ratio of
No. ae acter, of wood. oe fee Veluae Ur Bask
mm, mm, mm,
I I5.0 E30 bane) O32
2 Q.12 ees2 0.9 0.48
3 See On7 0.8 °.50
4 5.8 4.25 ee Wy | °.79
5 | 5.6 4.35 0.62 0.70
6 Kee 8275 0.735 I.00
7 315 256 0.45 I.00
8 2.4 in ay Ons 1.46
9 2.55 1.4 Ons7 Delt
Io 1.52 0.95 0.29 1.67
| II or 18.6 Ee 55 O35
12 Eh A 10.9 Onis 0.40
Nos. 1 to ro, stems from single plant taken Apr. 1909. No. 11, stem from
different plant taken Sept. 1908. This is the trunk (main) brought from Saltillo.
No. 12, root from plant taken Sept. 1908. No. 8, base of piece through crowded
nodes, bottom of 1909 growth, hence bark a little thicker here.
Apriltgog9. At this time growth had only just recommenced, from which it
is evident that the amount of water received between September 1908
(at which time I left Cedros) and the time of my later visit was very
small—and the information obtained showed this to have been the case.
The chief value of these tables, besides indicating somewhat more
fully the points already made, lies in the evidence they bear that the ratio
120 Guayule.
of bark to wood has increased during the period between the dates given
above, as shown in brief in table 47.
As will be seen, this change in volume in the irrigated cortex is to be
referred chiefly to an increase in the rubber-content.
TABLE 46.—Relative amount of bark and wood in guayule, by volume (dry). Field
plants (Sept. 1908).
ees t laisameter | Thick. | Ratio of | T Dinmeter Thick- | Ratio of
No. | giameter. | of wood, | ess of val Se ae diameter. | of wood, 7eSS,0f Vol. of bark
mm. | mm, | mm, | mm, mm. | mm.
ele ZOrety Oya re a5 WwW
Hv
PRIMARY CANALS IN THE ROOT AND HYPOCOTYL.
These have their origin in the endodermis and are included within
it, as shown for many Composite by Vuillemin, van Tieghem, Col (/.c.),and
Holm (1908).
To be noted is a formation of a band of Caspary in the new walls aris-
ing in the cells destined to become a part of the canal. In the root there
are two groups, one group of two to four (or occasionally six) canals op-
posite each primary phloem bundle (plate 36, fig. 1). While this grouping
is generally true for the Tubuliflore, the number of canals varies, e.g., in
1 For a summary of the knowledge of the resin or oil canals in the Composite
up to 1903, see Col (1903). An excellent historical sketch of the development of
our knowledge of organs of secretion of oil, resin, etc., is given by Tschirch (1906)
at p. 1095.
?In the usual sense as employed by, e.g., Vuillemin (1884b), and by Calvert
and Boodle (1887).
$ Vuillemin (1884b) properly pointed out the independence of the canals of
the hypocotyl and epicotyl. He says: ‘‘les systémes sécréteurs des deux membres
ou des régions differentes de méme membre sont toujours distinctes.”’
4 As to origin.
165
166 Guayule.
Silybum. Col states that there are six in each group, and it appears from
his account that the number of those which pass into the hypocotyl is
scarcely reduced.' In Parthenium argentatum, however, the number of
primary canals is usually not more than four; hence it appears that in the
transition zone the number of canals may be doubled. The four pri-
mary canals of the hypocotyl pass into the petioles of the cotyledons in
pairs, there to end blindly (plate 31, figs.1 and 2). They do not reach as
far as the blade.
The absence of canals in the blade of the cotyledons is to be noted.
According to Vuillemin, the more numerous canals in the hypocotyl of
Calendula officinalis pass (in part?) into the cotyledons, on which point
Col takes issue. Col’s figure of the seedling of this species shows groups
of canals opposite four epicotyledonary bundles, and these he identifies
with the hypocotyledonary canals, and shows none in association with
the paired median-trace bundles of the cotyledons. The position in which
Col’s drawing shows the canals suggests that they may be the lower ends
of the epicotyledonary canals. In many cases, indeed, the true hypoco-
tyledonary canals may not follow the primary median bundles even into
the petioles on the cotyledons, while in other cases they may. They may,
therefore, end blindly in the hypocotyl, by a morphological recedence
which Col has cleverly traced for the plant as a whole by his extended
comparative study of numerous Composite. In Parthenium argentatum
there are no other canals in the cotyledons (plate 34, figs. 4 to 6).
PRIMARY CORTICAL CANALS.
IN SECONDARY ROOTS.
Primary cortical canals in secondary roots and in those of higher
orders arise de novo from the endodermis of the new member. This is
brought about by the morphological independence of the endodermis in
the roots of different order. Secondary roots are not infrequently triarch
(plate 36, fig. 4), and have then three groups of canals, two to four in
each group. In roots, either primary or of a higher order, which grow
chiefly in length, the canals attain relatively large transverse dimensions,
and, with a lacunation of the septe between them, there arise columns of
cells connecting the tangential walls (plate 36, fig. 3). The interpreta-
tion has been properly applied by Col (/.c., p. 166) to similar appearances
in Solidago. Col’s observations do not, however, negative Vuillemin’s
previous conclusions, ‘‘dans les vieux rhizomes d’Arnica montana, etc.,”’
as I point out elsewhere.
IN THE EPICOTYL AND DEFINITIVE STEM.
As one ascends the axis the endodermis becomes, as is usually the
case, a less definite structure. For this reason it becomes increasingly
difficult to determine with precision the exact origin of the primary corti-
* Vuillemin (1884a) notes in Silybum a reduction in the number of endodermal
root-canals by ending blindly, so that a reduced number pass through the hypo-
cotyl into the cotyledons. The question naturally arises whether the reduction
in number is not produced by coalescence, as in guayule.
)
The Resin-Canals in the Guayule. 167
cal canals. At the level at which the earliest canals appear, namely, im-
mediately above the level of the cotyledonary node, the difficulty is not
as great as higher up. Here the endodermis is evidently involved, and it
seems conclusive that the whole of the canal structure is derived from it,
though the cell lineage is not as evident even in a young condition as it is
at higher levels in Parthentum incanum. This at any rate accords with
previous observations,' and is without any doubt the case in those parts
of the stem where the endodermis is regular enough to display its morpho-
logical relations. I therefore conclude that, were it possible to follow the
development of the structure, it would be found, even in the higher parts
of the stem in Parthenium argentatum, where the endodermis is quite ill-
defined, to have originated in this.
The course of development is as follows: A tangential division takes
place in one, or it may be two or three neighboring endodermal cells. In
the cell destined to give rise to the canal a radial” division crosses the first
wall so as to form four cells, realizing the ‘‘ division crucial” of van Tieg-
hem. Periclinal divisions, however, take place, cutting off special secre-
tory cells, four in number, from a tier of supporting cells, while these suffer
a still further subdivision. Two pairs arising from the inner two cells of
the original four are cut off, and are, so to speak, discarded from the canal
structure, as occurs also in the primary root-canals. Only the outer cells
of the outer original two become divided, so that fourteen cells in all arise,
of which four are the original secretory cells, six are the supporting
cells, and four, or perhaps six, excluded—tthis in the mariola, Parthenium
incanum (plate 37, fig. 8).
The canals of guayule (plate 37, figs. 1-5) bear sufficient resemblance
to those of the mariola, so that it would be unsafe to deny their entirely
endodermal origin. Secondary changes, by which the number of secre-
tory cells as well as that of the supporting cells is multiplied, need not
be described, as they consist only of repeated radial divisions and some-
times of tangential ones in the secreting cells.
These canals suffer more or less displacement (plate 37, fig. 3) accord-
ing to circumstances, often sufficient to mask their origin. For this reason
they have been alluded to as cortical by Ross (1908) and by Fron and
Francois (1901), without raising the question as to their origin. This is,
perhaps, the reason that, although Col (1903) asserts the endodermal
origin of the cortical canals in the Tubuliflore, his drawings sometimes
fail to show clearly this derivation, as, e.g., in Aster @stivallts.
In Anthemis mixta and Lasthenia glabrata, however, the origin is
clearly shown, and it seems that the canals are less elaborately organized
than in Parthenium and suffer less displacement. My own effort has been
to show conclusively the origin of these canals, with the result that the
work, in part of Vuillemin, of van Tieghem, and of Col, is supported.
Van Tieghem (1884) insists correctly upon the endodermal origin of the
rimary canals, but I am unable to recognize the distinction between canals
‘bordés’”’ and ‘“‘non-bordés,’’ though, correlated with the more definite character
of the endodermis in the roots, the canals are here more regular and somewhat
simpler in their structure (but certainly not ‘‘non-bordés’’) than in the stem.
? With respect to the stem.
168 Guayule.
TOPOGRAPHICAL RELATION OF CORTICAL CANALS.
The canals of endodermal origin, instead of taking a cortical position,
may, in various plants, take a position within the pericycle alternating
with the bundles, or opposite the bundles between the leptome and the
endodermis. Holm finds such canals in Ambrosia artemisiefolia, though
such was supposed to be the case for A. trifida only (Vuillemin, van Tieg-
hem). In Eupatorium (Holm, 1908), also, canals occur “‘ outside the lep-
tome.’’ The displacement of the canals and accompanying cells of the
endodermis to a position nearer the axis appears to have led Vuillemin to
draw the conclusion that the endodermis of the stem-is superposed on
that of the hypocotyl, an inference which, as Dangeard (1889, p. 122) has
said, needs confirmation. Vuillemin’s figure (/.c., p. 191) is susceptible of
a different interpretation.
In the young epicotyledonary axis in Parthenium incanum, the canals
of the cortex are more usually arranged in pairs, flanking the median leaf-
traces. This is the permanent arrangement, as, e.g., also in Zinnia (Vuil-
lemin) and in Olearia haasiwi (Col, 1903). It comes about, therefore, that
in the definitive stem of this plant the cortical canals are alternate in posi-
tion with the bundles. Inthe guayule they are usually placed on the same
radius with the bundles, and stand therefore opposite the leptome. Both
of these arrangements occur in various Composite.!
The transition from an alternating position of the canals with respect
to the bundles in the epicotyl to the radially opposite position presents
an ontogenetic summary of these two conditions characterizing various
Composite in which one or the other arrangement occurs. It may be
added, however, that the position opposite the bundle in guayule is not
invariable; exceptionally, canals occur opposite medullary rays. This is
true of both primary and secondary cortical canals, though Ross states
the contrary.
ANASTOMOSIS.
Anastomosis and branching frequently occur between the canals of
the primary cortical system. The four earliest-formed epicotyledonary
canals, which arise in pairs associated with the first and second primordial
- leaf-traces, are connected, each with the other canal of each pair, by a trans-
verse meatus, which lies above the level at which the lateral cotyledonary
traces pass out from the axis. This transverse meatus is a prominent fea-
ture of the epicotyl, and is frequently the starting-point of several, usually
four, canals. Anastomoses in the definitive stem are usually to be found
at the nodes, and in stems with very short internodes they are frequently
quite numerous. For this reason, in part, the number of primary cortical
canals seen in a transverse section varies, as stated by Ross (1908). As
the stem thickens (aside from secondary changes) the number of canals
increases, so that from 5 to 20,approximately, may be seen (plate 36, fig. 5).
1 There are very few cortical canals in Parthenium lyratum and in P. hyster-
ophorus, and they occur on one or both sides of a bundle, but not opposite to it.
Neither do they stand opposite a medullary ray, strictly speaking, though this
appears to be the case in P. incanum. In P. arctium Bartlett they are numerous
and alternate with the bundles.
The Resin-Canals in the Guayule. 169
MEDULLARY CANALS.
IN THE EPICOTYL.
All medullary canals are protogenic. Secondary ones do not occur.
The typical number of canals is not established for ten or more internodes,
this probably being variable. In field seedlings, or ones of slow growth,
the distance from the cotyledonary to the tenth node is very short, and
the particular behavior of the canals is difficult to determine. Etiolated
seedlings, therefore, throw more light on the matter, though it can not be
asserted that the behavior in such is always normal, e.g., when the canals
end blindly above, as they have been observed to do, instead of continuing
to the apex of the stem. These short canals may, perhaps, be regarded as
“poches sécréteurs’’—the pockets in which Col sees reduced canals. The
following notes, based upon a series of sections made of a seedling about 10
cm. tall, with 16 nodes, show that the definitive condition is established
only at length, even the sixteenth node being sometimes reached before
the full complement of canals occurs. .
No canals below the fifth node.
At fifth node, one canal passing into bud.
Fifth internode, lower part, no canals; upper part, four canals.
Sixth node, one of these into bud. One branches, making four enter-
ing lower part of sixth internode.
Upper part of sixth internode, two canals; higher up, three, one send-
ing a branch to bud of the seventh node.
Seventh node. At this level two more canals, making five to enter
the
Seventh internode, in which one ends, leaving four in middle part.
Eighth node, four canals, of which one branches into bud.
Eighth internode, two canals in middle part. One branches, making
three to the
Ninth node, at which the bud receives a branch.
Tenth node, four, one branching to bud. All but one end blindly, so
that the
Tenth internode receives only one canal. Two more arise, making
three for the
Eleventh node. One passes without branching into the bud, leaving
two to enter the
Eleventh internode. One of these ends, so that one canal reaches the
Twelfth node, at which one more arises by branching, and enters the
bud.
Twelfth internode receives one, which ends blindly on reaching the
Thirteenth node, where a new one arises and passes into the bud.
Thirteenth internode has no canals in the lower part.
Fourteenth node, one canal arises and passes into the bud.
Fifteenth and sixteenth internodes, two canals in each.
Despite the irregularity in numbers, and also in position, it is clear
that the canals in the pith have peculiar relations with the nodes. When
one arises it does so in connection with the development of an axillary bud,
and either enters it or sends a branch to it. This is to be inferred also
170 Guayule.
from the regular occurrence of live canals, the primary number in the
growing stem apex. In seedlings with short internodes the canals appear,
of course, nearer the hypocotyl. Ina field seedling 3 cm. tall, with two
dozen or more nodes, I found one canal at 5 mm. above the hypocotyl.
The next section cut had one. Similarly in an irrigated seedling with
short internodes.
The absence of pith-canals in the epicotyl suggests a primitive al-
liance with those tubuliflorous forms in which canals are entirely absent
from the pith.
IN THE DEFINITIVE STEM.
At the growing apex within o.5 mm. one finds a strictly primary ar-
rangement of these canals. There are five, one opposite each orthosti-
chy.! Inaslowly growing stem, however, in which the nodes are crowded
upon each other, through frequent branching and anastomosis, the num-
ber seen will vary usually between three and six. The union and separa-
tion of the canals is associated with the formation of large lacune giving
off large passages of irregular shape, but on the whole running longitudi-
nally. Ina single section, therefore, one may count as many as a dozen
canals, and nearby as few as three or four. In rapidly growing shoots the
anastomoses and branches are not so apparent, though they occur here
also. From the canal nearest to it each bud receives normally a single
branch, which, itself branching after entering the bud, increases till the
complement is reached. Pith-canals do not enter the leaf.
TOPOGRAPHIC RELATIONS OF MEDULLARY CANALS.
Although the primary number of pith-canals is more or less masked
by branching and anastomosis, as already mentioned, a study of the on-
togeny of the stem can not fail to show that five is the primary number
(plate 38, fig. 1),and further, that they arise in the same orderas the leaves
and, therefore, buds. These relations are seen best in growing tips of stems
of not too slow growth, or in seedlings, just above the levels at which the
pith-canals first come in. It is also evident from the positions taken by
the solitary canals which appear in the epicotyl before the full complement
is established.
The very frequent anastomosis and divarication, coupled with the
transverse expansion of the canals, give rise to a great many columnar,
trichome-like structures; already alluded to. They lie approximately
in radial planes, and can be explained only as imperforate longitudinal
septe (plate 32, figs. 1, 6).
In older stems the breaking down of the pith results in the opening of
the resin-canals, except when plugged by pseudotyloses. There results a
downward filtration of resin which finds its way into the central zones of
the old wood. This often becomes richly impregnated with resin, though
primarily it contains none at all. In this way the resin-content of old
wood, shown by chemical methods, is to be accounted for (Lloyd, 1909).
1In the pith of Cynara carduncula (Col, 1903) 5 to 10 canals occur; in
Parthenitum hysterophorus I count about 20; in P. lyratum about 12.
The Resin-Canals in the Guayule. 171
THE CANALS. IN THE LEAF.
Since the canals in the leaves are related only to the primary cortical
system, this relation will now be taken up.
EARLY FOLIAGE LEAVES.
The above-mentioned pair of primary cortical canals which enter the
petioles of the earlier leaves end blindly at different levels in the petioles
and in the leaf-blade' (plate 38, figs. 3 to 9). The marginal leaf-traces
enter the petiole unaccompanied by canals, but arise de novo in the petiole
dorsal to the lateral traces. These they follow into the leaf-blade, and
branch, constituting a latero-dorsal system. The dorsal system may be
entirely absent from the blade (plate 38, fig. 5). There is also a ventral
system composed of three canals, one opposite each of three prominent
bundles, namely, the median and two lateral. These arise independently
and de novo, that opposite the median trace in the petiole, and those oppo-
site the lateral ones, in the blade. They originate analogously with the
pith-canals, independently of the endodermis’ (plate 37, figs. 6, 7).
THE LATER: LEAVES.
The later leaves, in which their definitive character is assumed, re-
ceive usually three to five (occasionally six or seven) cortical canals from
the stem, one with the median and two with each of the stronger lateral
traces (plate 38, figs. 2, 10 to 18). These canals, which enter the blade,
follow the traces which constitute its prominent veins. The lateral
canals may branch, usually not more than once. Thus the dorsal system
of canals has, at most, usually not more than five ducts. The median canal
follows the midrib to the apex of the leaf. The lateral ends some distance
from the apex. The ventral system arises de novo in the petiole as three
to five independent ducts (plate 38, figs. 10 to 13), the median arising first.
The lateral canals follow the main limbs of the lateral traces and give off
branches to veins of a higher order, until, in a transverse section, there
may be five or more on each half of the blade. It is thus seen that the
ventral system is peculiar to the leaf and is more extensive than the dorsal
system. The canals anastomose in the upper part of the blade and follow
the veins.
PRIMARY CANALS IN BRANCHES.
The primary system of cortical canals in a branch is derived from two
canals on either sideof the appropriate leaf-trace. Atthe level at which the
bud appears, the adjacent canals in the chief stem enlarge radially and send
‘The behavior described is not invariable. One case was found in which
only one branch of the canal anastomosis entered the first leaf, while the second
leaf was normal, having two canals. The third foliage leaf in this plant also had
but one canal. One instance of a leaf at about the twentieth node had two canals.
This condition offers an analogy to that in the cotyledons, which may be held,
though only tentatively, as speaking for the more primitive character of the
double arrangement.
* It is worth noting here that there is a single ventral canal opposite the mid-
vein in the cotyledon of the common stinflower, Helianthus annuus.
172 Guayule.
off a number of branches which distribute themselves in the cortex of the
bud. As already said, generally a single branch from the pith-canal oppo-
site the bud enters and branches to produce the complement of canals
(plate 39, figs: sand 7).
SECONDARY CANALS IN ROOT, HYPOCOTYL, AND STEM.
These arise, as described by Ross, from special leptome parenchyma!
derived directly from the cambium, and quite in the same way in all parts
of the plant. They are at first flattened radially, opening out later to be-
come rounded or even circular in transverse section, and finally becoming
again flattened and secondarily distended, in company with the growing
(secondary) cortex (plate 22, fig. 13). These canals constitute concentric
branching and anastomosing systems, each succeeding zone being a sys-
tem separate from all the others. Their appearance in tangential sections
(plate 39, fig. 8) recalls the figure published by Tschirsch (1906, p. 1193)
of the canals in wound-tissue in Larix.
CANALS(N: THE .PEDUNCEE,
It has already been pointed out that the inflorescence is terminal;
the peduncle is therefore the morphological chief shoot. I have shown
that when an axillary bud develops it usually receives one canal from the
pith (plate 39, fig. 1). The last bud formed on the chief shoot which ends
in a peduncle, however, receives all of the canals from the pith, these being
diverted en masse. The peduncle, therefore, contains no medullary canals
(plate 39, figs. 2, 3). Primary cortical canals alone occur, there being but
very little secondary thickening.
Exceedingly interesting relations in this regard are displayed by
rapidly grown plants (plate 39, figs. 4to 7). In another chapter two types
of guayule have been described, in one of which the sharp delimitation
between peduncle and foliage stem is not present. When guayule is irri-
gated there frequently results, associated with rapid growth, a tendency
of the relatively chief shoot to run out into inflorescence,” when otherwise
there would be a sharp transition from stem to peduncle, and the upper
axillary bud would develop strongly. When the morphological transition
is gradual, there is also a correlated anatomical transition, which the long
internodes make it possible to analyze. In a specimen examined, as in
the normal condition, the peduncle has no pith-canals, but the first inter-
node below this has, instead of five, only two, which pass into the upper-
most axillary bud.
The sector of the stem under the peduncle contains much more stere-
ome, and the two canals are confined to the sector beneath the axillary
bud, while from the basal part of the internode they are absent! Their
orientation above is such as to bring them opposite the first and second
leaves of the axillary bud; they are, therefore, the canals which give
branches to the first two axillary buds of the branch.
The axillary bud of the second node below the peduncle receives from
the stem one canal only of four which are to be found in the internode be-
‘ Secondary leptome-canals have been described in Centrophyllum lanatum
(Gol. uke):
* Simulating the normal shoot in P. incanum (mariola).
Rubber in canal cells, nearby cortex and inner
ray cells. Root 1.2 mm. diam.
Older root. More rubber in rays.
Root 2 mm. diam.
Parenchyma ray from fig. 2.
Upper part of hypocotyl, same age as fig. |.
ww
6. Longitudinal section through old wood.
7. Longitudinal section through mature leptome
parenchyma, with a few parenchyma ray cells.
8. Leptome; elongated elements.
9, Companion cells and sieve tubes.
in younger leptome on the left.
No rubber
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Cortex, stem of field plant with maximum rubber content.
Cortex of a 20-year-old stem.
Root; rapidly grown seedling, two months old. Rubber in granules.
Rubber in process of accumulation in an irrigated plant.
Primary resin canal, root | mm. diam.
The Resin-Canals in the Guayule. 173
low the second node. The other three end blindly before they reach the
node, so that the following internode has none, as above said. It is evi-
dent that we find here a sort of morphological indecision, as if the stem
_were trying to retain its stem character, and still being gradually over-
come by the tendency toward changing into a peduncle. The same prepa-
rations show also the formation of chlorenchyma strips in the cortex of
the peduncle sector, nearly down to the base of the first internode below
the peduncle.
The axillary bud of the third node below the peduncle receives a
single branch from one of five canals, the normal number, present in the
internode below. Here, therefore, the complete stem structure is first
met in our descent from the peduncle. It would be interesting to specu-
late on the internal causes which result in diverting the canals, en masse,
from the chief shoot into an axillary bud.
THE CANALS IN RETONOS.
New shoots which take their origin from roots have this peculiarity
in common with the epicotyl, that they do not possess medullary canals
till several internodes have developed. They are further peculiar in lack-
ing primary cortical canals near their bases. A retofio 23 mm. long was
examined and measured. A section near the root at the level of emerg-
ence showed neither pith norcorticalcanals. At 5mm. aboveit five cortical
canals were found. At 10 mm. there were three medullary canals, and at
15 mm. five of these, so that at the level of 15 mm. the definitive structure
had been attained.
In another specimen 25 mm. long, collected September 8, 1908, only
one medullary was found at the level of 18 mm., and fourat 21mm. In
still another, one canal was found at 15 mm.
An examination of a full series of transverse sections through suc-
ceeding nodes and internodes discovers an important relation of the pith-
canals to branches, in general harmony with the facts cited immediately
above. The material thus studied was a retofio several centimeters long
which developed in 1908. The first leaf and its axillary bud were devel-
oped at the height of 2omm. The internode between the mother-root and
this node had no pith-canal. At the first node a single canal appeared just
below the level of the bud, and entered this. The succeeding two inter-
nodes (second and third) were also devoid of canals, though at each of the
corresponding nodes a single canal originated in the pith and passed into
the axillary bud. At the third node, however, the canal branched, one
limb passing up into the fourth internode, in the upper part of which two
other canals appeared. One of these three sent a branch to the bud of the
fourth node, and one ended blindly, leaving two passing into the fifth
internode. At the fifth node one of these sent two branches into the bud,
two canals passing into the sixth internode. At the sixth node both of
these branched, one branch going into the bud and three upward into the
seventh internode. At the seventh node all three branched, one of these
going into the bud, leaving the full complement of five canals for the suc-
ceeding internode, theeighth. The youngest canal always stands opposite
the youngest bud.
174 Guayule.
THE CONTENTS OF THE CANALS ; .THEIR: ORIGIN.
The very small size of the primary canals in the root and hypocotyl
makes it very difficult to determine the nature of their contents. The
canals elsewhere are known to contain resin which, upon wounding, exudes
as tears, which fall to the ground and harden slowly as pale yellow, limpid
masses. The origin of this secretion is of special interest here. There is
no doubt that the resin is confined to the canals, and there is no evidence
that the resin occurs in the protoplasm of the wall-cells of the canal,
which have been spoken of as secretory. Treatment with alcohol or with
acetone leaves the cell-contents quite unchanged to all appearance, though
subsequent staining with alkanet discloses, when this is originally the case,
a substance which may be dissolved out by means of xylol or other appro-
priate solvent, namely, rubber. My own observations, therefore, give
support to the general view, advanced by Tschirch, that the resin is to
be accounted for by chemical activity in the outer part of the cell-walls
facing the meatus. It is nota direct result of protoplasmic activity, but of
enzymatic activity in the cell-wallitself.1_ It is worthy of remark that the
wall (secretory) cells of the resin-canals have the two-fold function of secret-
ing rubber (in common with the ground-tissue) within the protoplasm and
resin without.
I have, however, attained no success in demonstrating a mucilaginous
or gummy lining to the meatus, such as is described by Tschirch (1906,
p. 1119) in many plants, to which he ascribes the origin of resin formation.
But Tschirch himself confesses to a similar difficulty in studying, among
others, the Composite.
The distribution of starch in the cortex and its apparent connection
with the secretion of resin have been elsewhere noted. The presence of
tannin in the conjunctiva of the young stem, especially associated with the
chloroplasts, is to be noted, and recalls Tschirch’s hypothesis of the origin
of resin from tannin. The number of Composite which contain tannin
is small, relatively to the size of the group, judging from the list given
by Dekker (1906).
THE ROLE OF RESIN.
It has often been pointed out’ that resins and ethereal oils stand in
relation to climatic conditions, especially those of the desert. The frequent
occurrence of resin in desert plants is a matter of general observation, but
its function is still a matter of speculation. Tschirch rightly lays stress
upon the occurrence of secretion-containing structures near the apex of
the young parts as of significance, and this has been pointed out for
the guayule. The evidence regarding the relation of resin to rubber
leads us nowhere, and no evidence is yet forthcoming as to the real role
of resin.
1 Tschirch, A. Die Chemie und Biologie der pflanzlichen Sekrete. Leipzig, 1908.
2e.g., Tschirch, 1908, pp. 8-9.
The Resin-Canals in the Guayule. 175
RESIN-CONTENT OF GUAYULE BY ANALYSIS.
The percentage of resin in the branches and twigs of field plants,
according to figures obtained by Whittelsey (in manuscript), is between
about 10 per cent for the smaller and about 17 per cent for the larger
branches. The amount probably varies according to the structure, and
this with the rate of growth of the parts. For irrigated plants the follow-
ing figures were obtained. The material was the same as that referred to
in table 53.
TABLE 51.
Percentage
Parts. of resin.
ep St ran artee a Se aS. kk MER GIRS. 2.46
ia Word eb mgo7) Sto Wt s..25, \cbe(s a0"% wased gysle Wo oP ais oe 1.36
WORE Co thexao fat MiSmi eset eer ieka cs « as dopaneus oeeewe ao c, snhigsiw + 4.06
MT VGroyEWScOl TGOS WItACh eee. wn se joa oe ee wns 7.56
IV. New growth of 1909 with leaves.................. 26710
VES s os std ee ee ae oe ETS Bee 10.80
Aside from possible errors, it seems that, bulk for bulk, the irrigated
plant contains less resin than the field plant. This is due in part to the
larger relative volume of the wood cylinder. The reduction of the amount
in older growths is due also in part to the radial compression of the resin-
canals in irrigated plants, whereby their capacity is much reduced. The
force of this explanation of the figures appears when we compare the per-
centage of resin in II] above. When we introduce the rate of growth as
a factor we must conclude that the total secretive activity is not reduced
under irrigation, nor is the secretive activity of the resin-secreting cells
lowered. The result, however, is had that in a given volume of cortex
there is less resin in irrigated plants. In the pith, however, this does not
hold, since the relative volume of the resin-canals under irrigation is as
great or greater than in field plants. The reduced amount of resin of the
cortex, volume for volume, appears, therefore, to bea secondary matteronly,
and bears, so far as we can see, no explanation in terms of adaptation.
CHAPTER VII.
THE ORIGIN AND OCCURRENCE OF RUBBER.
Well-nigh nothing is known about the cytology of rubber-secreting
cells. The great initial difficulties in the investigation have arisen from
the fact that in most rubber-producing plants this material occurs in
latex. In the guayule, as in a few other known plants, the rubber is laid
down within certain cells,in a manner analogous to the formation of starch.
Although the study of the early cytological activities which lead to the
accumulation of rubber still presents great difficulties, since some of the
agents used dissolve out the rubber, nevertheless it has been possible to
determine the relation of growth and of some of the more important ex-
ternal conditions to rubber secretion. These results are important eco-
nomically, since we are able to determine the time at which the maximum,
or near the maximum, amount of rubber occurs, and during what period
rubber is absent from the new tissues, and thus establish rules of pro-
cedure in the harvesting of the shrub.
METHODS.
The solubility of rubber in xylol and the like prevents the use of
paraffin. The preparations must therefore be studied in such a fashion
that the rubber is intact. When present in large quantities it is easily
recognized, after one has become acquainted with its appearance.” When
in small quantities, however, it may easily be mistaken for droplets of oil
or resin, or for protoplasmic or other granulations, and inasmuch as oils
and resins as well as rubber are stained by alkanet, these substances, if
present, must be removed by suitable solvents which will leave the rubber
unaffected. For this purpose I have treated sections with high-grade and
absolute alcohols, acetone, and potassium hydrate, applying alkanet both
before and after. There remains the possibility that the substances which
remain and which react to alkanet are not always rubber in its final form,
but there can be little doubt that the materials which are referred to below
are either rubber or are substances in the course of change into rubber.
The evidence seems to indicate, however, that it is rubber which we are
dealing with.
In seeking to determine with accuracy the facts of the distribution
of rubber in the tissues, the accident of displacement of rubber in the act
of sectioning must be properly guarded against. When rubber is present,
1 The substance of this chapter was presented in a paper entitled ‘The
responses of the guayule, Parthenium argentatum Gray, to irrigation,’”’ before the
Botanical Society of America, Boston, December 1909.
? When in readily appreciable quantities, resin and rubber in the guayule may
readily be distinguished by alkanet. Resin takes ona brilliant scarlet, while rubber
has a purplish tinge, and is, to the naked eye, blood-red.
176
|. Apex of terminal twig of 1908, field
plant, July 22.
2. Near base of same.
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. An old leaf trace.
. Outer cortex of a field stem.
. Outer edge of cortex and inner zone of
cork derived from collenchyma.
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The Origin and Occurrence of Rubber. 177
the contents of the cells ruptured during the sweep of the knife agglomer-
ate and stick in irregular masses to the section. It is also to be suspected
that particles of rubber displaced from one cell may remain attached to
_ other cells in such a manner as to simulate an original position in them.
This danger is greater where the particles are small, since with smaller
size the chance against agglomeration is greater. With some experience,
however, this general difficulty is reduced, so that, with proper observa-
tion, mistakes are easily avoided. The guiding principle of observation is
simply to confine study to uninjured cells.
GENERAL* DISTRIBUTION: OF RUBBER IN: THE PLANT.
It has been known for some years' that the rubber in guayule occurs
in the parenchyma “‘cells’”’ of the stem and root; that is, in the pith, paren-
chyma rays, and cortex (the conjuctiva,in a word). These facts, though
known toa few, were first clearly stated by Ross in 1908, according to whom
rubber occurs in almost all the cells of the ground-tissue in root and stem;
that is, in those of the pith, parenchyma-rays, primary cortex, and also in
the wood-parenchyma. The leaves, he adds, contain little or none. While
I have been able to confirm these conclusions in general, several additional
details have come to light.
Rubber occurs invariably in all the cells of the resin-canals (plate 41,
figs. 4-6). While Iam unable to state positively that it occurs here earlier
than elsewhere, it certainly is secreted most rapidly. There is, however,
evidence that the former statement is true.
In the primary hadrome parenchyma rubber does not occur early.
In the preparations from which the photographs on plate 42 (figs. 1 and 2)
were taken, there was no trace of rubber. There can be no doubt, how-
ever, that rubber is secreted by some or all of these cells later on, as they
are replete after some secondary thickening has occurred in the leaf-trace,
which, of course, Suffers no secondary change? (plate 42, fig. 6). The cells
of uniseriate parenchyma rays, in consonance with other parenchyma-ray
cells, contain rubber.
In the primary leptome, under the conditions noted for the primary
hadrome in the preceding paragraph, rubber occurs at least in the fiber-
cells and in the parenchyma (plate 42, fig. 6). This takes place after the
abscission of the corresponding leaf. In secondary leptome I have seen
small amounts in mature fiber-cells, just before sclerosis sets in. Occur-
rence of rubber in these elements appears, therefore, to be a function of
age. It is secreted normally in leptome-parenchyma, and at the same
time is in adjacent parenchyma-ray cells (plate 42, fig. 4).
In the secondary leptome rubber may also be seen in all the elements
of the sieve-tissue (plate 4o, figs. 8,9). It is true that the very narrow
‘ Fron and Frangois, rgot.
?The parenchyma of the secondary hadrome is rather scanty and of small
elements. They do not secrete rubber as early as the medullary-ray cells in the
same zone, but ultimately do so. The rubber may best be seen in longitudinal
sections, treated with boiling ro per cent caustic potash and stained with alkanet.
The rubber then appears as small series of globules. The wood may also be macerated
by means of Schultz's medium, and later stained.
12
178 Guayule.
elements contain very little, but this amount may be clearly demonstrated
in longitudinal sections treated as above described.
In the cork-cells rubber occurs in a secondary condition as small
droplets, derived by the breaking up (possibly an emulsification) of the
compact masses in theouter cortex. These droplets are larger in cork-cells
on either side of collenchymatic zones which are remnants of the periclinal
walls of collenchyma (plate 42, fig. 8).
Rubber is secreted in the parenchyma of the es a (plate 42,
fig. 3), quite as in the adjacent cells.
In the leaf the amount of rubber, though always small, is in propor-
tion to its age. In the oldest leaf I have observed, rubber occurs in drop-
lets in the outermost palisade-cells of both surfaces, and less conspicuously
in the subjacent, but usually in no other chlorenchyma cells (plate 43, fig.
5). It may occur only in the ventral palisade in younger leaves. In very
minute droplets it is to be found also in the collenchyma and endodermis
of the midvein and in nearly all of the non-chlorophyllous cells in the
region about it, and in the leptome, both in the companion and sieve cells.
Curiously enough, it is not to be found in the secreting-cells of the resin-
canals, though, on the other hand, it isin small but conspicuous quantities
in the subjacent cells. A minute amount occurs in the epidermis, espe-
cially near the midvein, andin the non-chlorophyllous cells near the smaller
veins. The maximum quantity, negligible from the economic point of
view, occurs in the oldest leaves which have passed through a drought
period. The material which gave these results was collected in the spring
of 1909 before the summer rain of that year.
In material collected from irrigated plants at Cedros in April 1909
rubber may be detected in exceedingly minute quantities in the basal part
of the leaf only. A single minute droplet—not more than one-fourth the
diameter of those seen in the field plant—may be seen in each outermost
palisade-cell of the upper (ventral) surface. They are a trifle larger near
the midvein. In the non-chlorophyllous tissue near this the rubber may
also be detected in still more minute quantities.
Since within the periphery of the wood cylinder only the conjunctiva
andasmallamount of wood-parenchyma contain rubber, and since in older
wood the medullary rays (in part) and the pith and its canals are dead
and disintegrated, the wood cylinder contains less rubber than the cortical
tissues, but it is also less resinous inits primary condition. Recent work
by Whittelsey (1909), however, indicates that in stems of an advanced
age, at any rate, the amount of true rubber is practically reduced to mzl,
though in the young twigs the proportion of rubber within the periphery
of the wood cylinder is large. We must conclude, therefore, that the
rubber in the older wood undergoes chemical change, and is broken down
into related materials. There is no doubt that some such change takes
place also in the secondary cortical tissues cut out by the inner periderm,
and this, as I have shown, is a considerable part of the volume of the
“bark” in older stems.
The Origin and Occurrence of Rubber. 179
APPEARANCE OF RUBBER IN RICHLY EOADED: TISSUES.
A section taken through any young stem of a field plant after some
_ period of drought will give a typical appearance (plate 42, fig. 7). All the
cells of the conjunctiva appear to be filled with a gray substance. A good
deal of it will have been swept out of ruptured cells by the knife-edge and
agglomerated, the resulting masses having irregularly rounded outlines,
with strands stretching here and there, still attached to the tissue. These
masses, seen obscuring the pith to some extent in plate 43, fig. 2, and the
dense cell-contents stain deeply with alkanet, the stain being more bril-
liant if the sections have been previously boiled in a ro per cent solution
of caustic potash. If the sections have not been acted on by alcohol or
potash drops of yellow resin will be seen in the resin-canals, but nowhere
else, except accidentally.
Closer examination of the rubber within the cells shows that the mass
is not homogeneous, and does not entirely fill the cavity. It may form a
heavy layer about the wall, leaving a more or less irregular space within,
or, if apparently filling the entire cell, it will contain numerous spherical
spaces (plate 41, figs. 1,2). Sections which have lain in glycerin may show
the masses tobe contracted, owing toa plasmolytic action upon them, from
which it is to be inferred that they have a considerable water-content,
held within the vacuoles, in part at least (plate 41, fig. 3). The rubber may
also accumulate as a round drop within the vacuole of the cell (plate 42,
fig. 7), its size depending upon the age of the cell. Plasmolysis shows
further that all the parenchyma cells are not equally densely filled, though
of the same age. This is often conspicuously the case when the cells of
the cortex and those of the adjacent parenchyma rays are compared. In
the cortical cells the rubber forms a dense rounded drop (plate 42, fig. 7),
while the cytoplasm may be seen between it and the cell wall. In paren-
chyma-ray cells the rubber mass is frequently irregular, full of irregular
vacuoles, and the cytoplasm appears usually to have shrunk away with
it. In the parenchyma-ray cells in some preparations it is quite as regular
as in the adjacent cortical cells, but appears to be more dense, owing to a
very much larger number of minute spaces. This difference,in one form or
the other, is quite constant, and seems to indicate that the rubber-content
of the cortical cells is higher than that of the adjacent parenchyma-ray
cells.
Cortex which has been cut out en masse by inner periderm also con-
tains rubber. In the cells of this tissue it has a still different appearance,
being segregated into droplets of various sizes, in a fashion to suggest the
analogous appearance.of dead protoplasm. In newly formed cork-cells
proper, just outside of the periderm, a different behavior is seen.
BEHAVIOR OF PERIDERMAL DIVISIONS TOWARD RUBBER.
Since the secondary cortical cells in field plants contain a large
amount of rubber in the condition described, the fact that the cork-cells
immediately outside of the actively dividing suberogenous cells may contain
no rubber at all, or only occasionally a small amount, calls for explanation.
The suberized walls of the cork take up alkanet readily, so that, after
180 Guayule.
treatment with that reagent, the contrast between the rubber-containing
cells of the cortex and the empty nearby cork-cells is very clear and striking.
Inasmuch as the peridermal divisions, though several times repeated inthe
same mother-cell, finally involve a considerable depth of tissue, and as
the rubber can not travel from cell to cell as such, we must conclude either
that the rubber is translocated, which is unlikely, or that it disintegrates.
In support of the latter conclusion we note the following ocular evidence:
t. When the first cork-cambium division takes place the partition
passes through the rubber-content, whereby the two daughter-cells each
receive a share (plate 31, fig. 14). From the outer cell, which becomes
suberized, the rubber disappears.
2. This disappearance is gradual. The rubber may first break up
into droplets, which become fewer in number till, in the second series
of cork-cells, scarcely any evidence of its former presence remains, or it
may become shrunken in appearance. During this time the rubber, if it
still is such, reacts less characteristically to alkanet, and takes on a dirty
bluish tint. In one young root, however, I observed droplets of rubber
giving the characteristic stain, out several cells distant in the cork. The
explanation may be that after the death of the protoplasm the oxidizing
enzymes present hasten the disintegration. This may be less rapid in the
root, though it is difficult to say why. The mere contact with the air
would seem an insufficient explanation, since disintegration of the rubber
in cortex cut out bodily by inner periderm is very slow.
THE. DEVELOPMENT OF RUBBER JIN Tile (CELL.
All that we are able to do microscopically in regard to the method of
origin of rubber in the cell is to detect its first appearance and the subse-
quent accumulation, and we are therefore precisely in the position of
the poet who said of a matter usually regarded as far removed from the
realm of science,
‘Sie kommt, und sie ist da.”’
We are unable to say at this point whether the origin is associated with
special organs as plastids or not, though my observations up to the present
indicate that there are no such organs.
The relation of nuclear activity in general to secretion is well known.
The rubber in the palisade-cells of the leaf appears first in all cases in
contact with the nuclear membrane, and for this reason does not take the
form of spherical but of concavo-convex droplets, seen in plate 43, fig. 5.
Elsewhere the earliest appearance is as very minute, well-nigh invisible
droplets (plate 41, fig. 4), scattered in the protoplasm. They grow in
size and increase in numbers until the protoplasm is loaded sufficiently
to render it exceedingly frothy in appearance (plate 41, fig. 5). These
droplets may travel toward the interior of the cell and be extruded into
the vacuole, where they run together to form a larger droplet or a more
or less irregular mass. This is not homogeneous, as might be supposed,
but is vacuolated, sometimes so much so that it is quite alveolar in struc-
ture (plate 41, fig. 1), sometimes less so, the vacuoles being widely scat-
The Origin and Occurrence of Rubber. 181
tered. That these vacuoles contain various substances in solution in the
inclosed water can not be doubted, and it seems likely that among these
are enzymes’ which may act upon the rubber after extraction by the
mechanical processes in vogue. It also seems likely that the protoplasm
of the cells becomes intermingled with the rubber during extraction, ren-
dering it more or less albuminous and liable to give off the products of the
decay.
CENTERSMOF SECRETION.
THE ROOT.
With certain exceptions, the secretion of rubber both in the stem
and the root, including the hypocotyl, appears to proceed from definite
centers. This is exemplified with especial clearness in the root, where, in
the cortex, the secreting-cells of the resin-canals” are the first to show the
presence of granules of rubber (plate 41, fig. 6). It is argued that secre-
tion actually begins earlier in these cells because the surrounding cortical
cells, primary-on the outside, secondary on the inside, contain, at an early
stage of secretion, less and less rubber, as one proceeds farther from the
canals. The figures of plate 40 illustrate this advance in secretion, the
beginning of which is seen in a young stage in the development of the
root (plate 23, figs. 3, 7; plate 4o, fig. 1). If the rate of growth has not
been too rapid, so that a part of the primary cortex has had the necessary
time to secrete rubber before being cast off, the activity of secretion is
seen to be taken up successively by the cells further removed, until the
whole tissue becomes loaded (plate 40, figs. 2,3). The greater amount of
rubber, however, is evidently held by the cells nearer the resin-canals.
In the hypocotyl the same physiological relations hold.
The secretive activity of the secondary cortex is taken up, aside from
those cells in the neighborbood of the canals, by successive layers of cells,
beginning on the outside. With the appearance of the secondary resin-
canals, however, a superior activity in rubber secretion in their secreting-
cells is to be early noted.
On the other hand, simultaneously with the appearance of rubber in
the primary canal-cells, it appears also in the innermost cells of the paren-
chyma rays, the function of secretion being taken up successively by the
next outer cells,andsoon. Thisis apparent in the figures (plate 40, figs. 1
to 4). Ifa period of rapid growth follows one of stasis, the newly formed
parenchyma-ray tissues will show an entire absence of rubber (plate 40,
fig. 3). When secretion again begins, it starts simultaneously in the
outermost and innermost cells of the parenchyma ray.
THE HYPOCOTYL.
In the hypocotyl a similar condition prevails, though here, as in the
definitive stem, there isa pith. That is, the innermost parenchyma-ray
cells assume secretive ability earlier than the pith-cells, which is not true
for the definitive stem (plate 40, fig. 5).
1 The presence of oxidases in extracted rubber, both in latex rubbers (Spence,
1909) and in guayule rubber, is known.
?In view of the emphasis which has been placed by many writers on the endo-
dermis as seat of high physiological activity, the beginning of the secretion of rub-
ber in the resin-canal cells, which are constituents of the endodermis, is of very great
interest.
182 Guayule.
THE STEM.
In the stem, the first evidences of rubber are to be observed in the
secreting-cells of the cortical and medullary canals simultaneously. The
dark appearance of these cells in figure 1, plate 42, is due, in part, to their
larger rubber-content, but in part to the denser protoplasm. The condi-
tion to be seen in these cells is represented by the camera drawing in plate
31, figs. 1o and 12. The section was taken toward the apex of a newly
grown twig of a field plant, collected on July 22, 1908, and was then about
six weeks old. In all the cells of the conjunctiva very minute granules of
rubber could be seen, but not more in the cells near the canals than else-
where.- In the stem, therefore, secretion appears to begin first simultane-
ously in the canal-cells of the pith and cortex, and then in the conjunctiva.
It is, however, quite readily determined that the physiological activity of
the pith is greater than that of the cortex. In fig. 2, plate 42, 1s shown
a section taken from the twig just mentioned, but near the base of the
new growth. One or two peridermal divisions have ensued, while other
secondary changes may be noted. The rubber-content of the pith-cells is
obviously greater than that of the cortex in this section. Further, I have
noted in irrigated plants that the amount of rubber is greater in the outer
than in the inner cortical cells (plate 43, fig. 1). It seems, therefore, that
the deportment of both root and stem is essentially the same and that the
hypocotyl, though possessing a pith, behaves as the root.
During secondary thickening, as in the root, the secondary cortical
canals exhibit early activity in rubber secretion, while this is taken up by
the oldest parenchyma-ray cells first, simultaneously, therefore, at the
inner and outer edges.
THE LEAF.
In the leaf the earliest appearance of rubber is in the outer palisade
in the ventral moiety. I found rubber in these cells only in old leaves of
irrigated plants. The analogy with the condition described for the stem,,
in which superior activity is shown by the pith, is clear. But the failure
of the leaf-canal cells to show greater activity than the neighboring con-
junctiva detracts from the force of the comparison. The leaf observed
by me to be most richly supplied with rubber contained a single drop-
let, with a diameter about half the transverse diameter of the cells, in
each palisade-cell toward the median vein. The amount of the rubber
became less and less toward the margin. This was true also of the outer
palisade of the dorsal (lower) surface, and in a less degree of the inner
palisade.
Minute granules occurred also in all the non-chlorophyllous cells,
mechanical and conjunctive, forming the midrib, excepting the vascular
and sieve elements. It would seem, therefore, that, roughly speaking, the
midvein is the center of rubber secretion, which proceeds through the
lamina toward the margins; further, that activity is shown first by the
outer palisade-cells, then by the inner, and first by the ventral and later
by the dorsal. In this regard, as already said, the analogy to the stem is
clear.
The Origin and Occurrence of Rubber. 183
RATE OF RUBBER SECRETION RELATIVE TO GROWTH.
The material which I have studied in order to determine the relation
of growth to the rate of rubber secretion was collected during and follow-
ing the growing-season of 1908, which began about June 1. Growth is
rapid for the first part of the season, during which several centimeters of
stem-length are attained and one to three flower-stalks are developed. A
period follows in which there is little lengthening, and more or less second-
ary thickening occurs, according to the length of the period during which
growth of any kind may take place. During the first part there is no evi-
dence of secretion of rubber in the new parts; during the second, which
began in 1908 in late July or August, there is a slight evidence of secretive
activity as regards rubber, though the secretion of resin is synchronous
with growth. The relation may best be expressed by saying that the secre-
tion of rubber is a secondary physiological process, its rate of appear-
ance being inversely to the rate of growth. The rate relation is brought
out best by plants grown under experimental conditions, in which the
more rapid growth is accompanied by a less rapid secretion of rubber. No
exact quantitative statement can be made, since the conditions under
which experimental plants have been grown have not been fully con-
trolled. In studying material, I have tabulated numerous observations
in field and irrigated seedlings, of various ages and at different periods of
the year, and compared the rubber-content of the cells in all the tissues
with that in irrigated seedlings. The same has been done for mature field
and irrigated plants. For this purpose the material which has frequently
been alluded to was at hand, viz, the branches and stocks of irrigated
plants at Cedros (plate 4, fig. B) and at Caopas (plate 46, fig. B), both
immediately at the close of growth-periods and after a period of drought.
The attempt was made to grade the preparations on the rubber-content
of the cells, and while this method of procedure has little to recommend
it for more than approximate accuracy, it enables us to draw reasonable
conclusions as to the rate of progress of secretion. My observations have
been digested in the following notes, which will serve to present sufficient
concrete evidence to support my conclusion.
1. At the close of the dry season (May 1908) all the cells of rubber-
bearing tissues produced by growth during 1907, both in new shoots and
in new tissues in older shoots in field plants, contained rubber in maximum
quantities (plate 42, fig. 7).
2. The same may be said, generally, for the field seedlings. There is,
however, evidence that in the cells of the pith near the top of the seedling
the maximum content of rubber is not reached. Seedlings (plate 17, fig.
A) of rapid growth in 1908 had not reached the maximum content (as
shown both microscopically and by the analysis on p. 187) in April 1909.
In the cells of the root it was more densely agglomerated than in the stem.
Here the rubber had the same appearance as in irrigated plants. It was
only partly agglomerated, and only partially filled the cells. It is quite
probable that this condition occurs occasionally in mature plants in drier
habitats after exceptional rainfall and regularly in moister conditions.
3. A medium-sized twig, grown in 1908, beginning about June 1,
measuring 3.3 mm. in diameter at the base and 1.2 mm. at the tip, was
184 Guayule.
examined Aug. 14. At the base the rubber in the pith was finely granu-
lar, showing in addition a tendency to agglomeration (plate 42, fig. 2); in
the extreme inner and outer cells of the parenchyma rays the rubber was
very finely granular, while in the cells lying on either side of the cambium
there was none or extremely little; in the primary cortex it was finely
granular, but was in somewhat larger granules in the secondary cortex;
large granules occurred in the younger resin-canal cells (in the secondary
cortex) and agglomerated masses in the older canal cells (in primary
cortex and pith). Near the apex of the stem the rubber was found only
in extremely minute granules everywhere (plate 42, fig. 1) excepting in the
resin-canal cells, where they were somewhat larger, but still small (plate 31,
figs. 10 to 12).
4. A similar twig, examined Sept. 8, showed that the condition seen
at the base in the twig described immediately above had advanced toward
the apex about one-third the length of the twig. At the base the rubber
had increased till it had become coarsely granular, except in the paren-
chyma-ray cells nearer the cambium, in which it was still finely granular.
Five mm. from the apex there was still scarcely sufficient rubber to be
observable, except in the resin-canal cells.
I was unable to obtain material during the succeeding few months,
so was prevented from following the march of secretion after September 8.
It is, however, clear that the rate of secretion is so slow, as compared with
the rate of growth, that for at least four months after the beginning of the
rainy season the new parts contain only very small quantities of rubber.
From this time on the secretion of rubber probably proceeds more rapidly,
but it is still to be determined when the maximum is reached. This is a
point of very great importance.
5. Turning to irrigated plants, I need cite the evidence from only
three examinations:
(a) A branch (plate 21, figs. A, B) of a Cedros plant (plates 4and 17, fig.
B) which began to grow in 1907 and was examined in August 1908. In
examining the 1907 growth no rubber was detected in the pith, probably
because the small amounts secreted in 1907 had disintegrated; the older
cells (of 1907) in the parenchyma rays contained rubber in fine granules
near the cortex; in the cortex and resin-canal cells there were coarse
granules with more or less agglomeration. The new tissues of 1908 con-
tained only very minute granules. In the 1908 growth, near the base, the
rubber was visible in very fine granules, save in the primary cortex, where
there was none; in the resin-canal cells coarse granules, these still larger
in the pith-canals; midway between the base and apex there were very
fine granules of rubber in the pith and parenchyma rays; the resin-canal
cells had coarse or agglomerated granules; fine granules were visible in the
secondary cortex, but none in the primary. Four centimeters from the
apex, where the stem was still herbaceous, minute granules of rubber
had appeared only in the pith and inner parenchyma-ray cells nearbv;
it Was present in coarse granules in the resin-canal cells of the pith, and
in fine granules in those of the cortex; the cortex itself contained none
(plate 43, fig. 1).
(b) A branch from a single Cedros plant collected in April 1909 (plate
17, fig. B), after a prolonged drought extending with practically no inter-
The Origin and Occurrence of Rubber. 185
ruption from August 1908. Rubber was found in dense rounded agglom-
erations throughout, but evidently not reaching a maximum content
(plate 43, fig. 2).
(c) A branch from a plant grown at Caopas, from stocks transplanted
by Don Teofilo Delgadillo about January 1908 and taken in October 1909.
These had less irrigation than the above-mentioned Cedros plants. 1908
growth: the rubber was densely agglomerated in the whole of the con-
junctiva (plate 43, figs. 3, 4),in amounts exceeding that in Cedros material
(plate 43, fig. 2); the 1909 growth contained rubber in coarse granules
more or less agglomerated throughout.
6. Irrigated seedlings of all ages up to five months were examined.
Very young individuals were seen which contained no rubber at all. A
five-months-old seedling (plate 20, fig. B) contained rubber in coarse
granules throughout the conjunctiva, being in sufficient quantity in the
secondary cortex to become agglomerated.
The method which was used in obtaining the foregoing data, despite
its limitations, could doubtless be used by the grower of guayule, enabling
him to follow the behavior of the plants under his charge. The evaluation
of the evidence is Somewhat difficult, but it could be mastered, as may
be seen, I think, on examining plates 40 to 43. The final control must,
however, be had by chemical analysis. Tables 52 to 54, which follow,
contain a few results which comport with the evidence preceding.
RUBBER-CONTENT BY CHEMICAL METHODS.
The analysis of the guayule plant in order to determine its rubber and
resin content presented unexpected difficulties, but the results attained,
after these had been met, are undoubtedly more reliable than earlier
analyses. I therefore adopt them as exposed in table 52 (Whittelsey,
F9OO; PP: 3,5).
TABLE 52.—FPercentage of rubber in various parts of guayule shrub. Field plants.
|
Parts. Rubber. |
per cent. |
Alaconol< | g¥2h qlee cae eee ene eee ee
Rootmsatkns. c1 ttl oc. les. + Toes) |
Branches and leaves....... On7
heel GIS IOC. 88 ance’ s mae ere «2 0.0
TRGO te WOOG Gr cjocc 8 cus seers 0 2.0
‘The percentage of pure rubber in the whole trunk is 9.9, the whole
root 7.8, the branches and leaves 9. 7; and in the whole plant 9.5, * *
based on perfectly dry material. If ‘mill weight’ is taken as a basis, the
percentage of pure rubber in the whole plant is 7.8.’’ This result is found
to correspond very closely to factory experience and the more accurate
published results, and we may therefore adopt it as exact enough for the
present purpose.
The only figures available for irrigated plants are given in table 53
on the following page.
186 Guayule.
TABLE 53.—Analysis of irrigated plant two years old from transplanted stocks, Cedros.
Collected April 4, 1909. Plant weighing 4.5 pounds fresh.
(1) The original stump planted March 1907, divested of its subsequent growths.
(II) The growth of 1907 separated into wood and cortex: the wood (IIa), the
cortex (bark) (IIb). (III) The growths of 1908 intact, and therefore comprising
both wood and cortex. (IV) The growth of 1909, consisting of short new twigs
and their leaves, developed before the date of collection. (V) The lateral roots
intact.
Number. Rubber. | Number. Rubber. |
| away Gol sum
ber cent. || per cent. |
1 OE pear recies Bes } AISI rersrcet Ryo
| oie ee A CG. Sor WPL VAA ee: 0.67 |
Lhbfsastiens (wr 68i Hier Mets ees 3-95 |
The method by which the above data were obtained was worked out
by my former colleague, Dr. Whittelsey. The method was controlled by
myself microscopically, and the material was found after treatment to
have been thoroughly, though not quite entirely, extracted. The error
from this source, as shown by this control, is, however, extremely small,
and the figures may be accepted as practically correct.
For the purpose of appreciating the practical significance of the data,
we may compare the percentage of rubber in the new growth intact. For
field plants we have a 9.7 per cent rubber-content. In the twigs of the
irrigated plants studied the amount is 3.3 per cent, namely, a little over
one-third that of field plants. By comparing Ila and IIb, we note that
this low percentage is due, as shown in Chapter V, to the low percentage
of rubber in the wood and its relatively larger volume in irrigated plants.
Moreover, the ‘‘branches and twigs’’ of Whittelsey’s table can not be
directly compared with those of III in my own, but rather with II and
III taken together. If it were possible to compare the cortices alone
we should find, in all probability, a percentage of about 4 per cent of rub-
ber for irrigated plants against 15 to 20 per cent for field plants, so that
for the new growths under irrigation from the transplanted stocks in
question the amount of rubber formed by cortical tissues is about one-
fourth to one-fifth of that formed in the corresponding tissues in the
smaller branches and twigs of field plants. But the rate of growth under
irrigation is such as to result in the production of a volume of cortical
tissues, at the very least five times greater for the same length of time.
This factor would be very much increased if field and irrigated seedlings
were compared. The conclusion would therefore appear to be reached
that the difficulty attached to the problem of cultivating guayule for the
rubber is not that of obtaining rubber, but of properly handling the raw
material so as to extract the rubber from the tissues.
In the first place, we have repeatedly noted the relatively large vol-
ume of the wood cylinder in irrigated plants, and its density. We have
also seen that the branches are long and lithe. If this material is handled
in its entirety, the volume of barren material which must be handled by
machinery is considerably greater than in the case of field plants. The
suggestion (Whittelsey, 1909, p. 6) that the cost of manufacture could
be reduced by the use of decorticating machinery, as is done in the case
of “grass rubber’? (Funtumia spp.) in Africa, is still more pertinent for
The Origin and Occurrence of Rubber. 187
irrigated shrub, and the character of the growth lends itself to this. This
would seem to be necessary in the event that the relative amount of rub-
ber in the cortex can not be raised above 3.5 to 4 per cent, not only because
_ of this probable difficulty of agglomerating the more finely divided rubber,
but because of the interference with this of the fragments of splintery
wood, which will tend materially to obstruct agglomeration in any event.
In the second place, the individual masses of rubber in the irrigated
plant are smaller and further away from each other than in field plants.
Hence, as above said, it is more difficult to agglomerate the rubber. This
is noted in trying to isolate the rubber from irrigated tissues by mastica-
tion, a process more difficult than for field plants. It may be found neces-
sary to introduce a machine especially adapted to mastication of the
material after passing through the pebble-mill, in which rollers with differ-
ential speeds will cause the massing of the minute particles of rubber. But
the practical solution of such problems is not to be obtained merely by
reasoning about them. The laboratory and factory are mutually of value,
but the one does not always solve the difficulties of the other.
VARIATION IN RELATIVE AMOUNT OF RUBBER IN FIELD
BEAN TS:
I have already pointed out that rubber does not appear in newly
formed tissues for some time after the cessation of growth; it may be fora
period of some months. It therefore appears that the new growth of field
plants taken at some periods of the year has a content and distribution
of rubber similar to that in irrigated plants, aside from the relative bulk
of the tissues themselves. To illustrate, I take the following analysis of
seedlings, from Station 2, Quadrat 4 (plate 17, fig. A), collected April 1909,
germinated in 1908 (table 54). The leaves and stems with tap-roots were
analyzed separately.
TABLE 54.
| Rubber.
per cent.
SROMEAVESS Ft hates as 25 He SPH
The stem and tap-roots....| 2.40
Of interest in this table are the rubber-content of the leaves taken
separately and the low content of the stems and tap-roots. The leaves
probably represent the usual condition, as they were old, fully matured
leaves which had remained attached to the plants throughout a long
drought period. The plants, however, were of rapid growth, indeed
remarkably rapid for field plants, and the low rubber-content stands in
relation to this. There is no doubt that this rubber-content is much lower
than for seedlings of the same size of slow growth.
In this respect, therefore, there is no hard and fast difference as
between field and irrigated plants, norindeed is this the case for the relative
volumes of the tissues themselves, as I have previously shown (p. 117).
The response of the guayule under irrigation, therefore, is but an extreme
expression of what occurs in nature, correlated with the climatic differ-
ences which obtain from year to year, and in different localities.
188 Guayule.
RELATION OF RUBBER AND RESIN.
A notion has been widely entertained that the amount of rubber in
the guayule plant is in some way related to the amount of resin. This
naturally grew out of the fact that commercial rubbers always contain
more or less resin, and that resin is abundant in the guayule. In the
preparation of the commercial article from the guayule the resin becomes
intermingled with the rubber to the amount of 20 per cent (Whittelsey,
1909). There appears, however, to be no adequate evidence in support
of this notion, while on the other hand there is strong evidence to show that
the physiological processes involved in the secretion of these two materials
are quite distinct:
t. The canals which are laid down in the protogenic tissues become
actively secreting as regards resin long before rubber appears at all. This
is strikingly evident in irrigated plants, in which the amount of growth
is very much in excess of that in field plants.
2. Resin is secreted in other Composite in which rubber does not
occur. In the closely related mariola (Parthenium incanum) resin is
abundant, while rubber is very meager in amount; and this is true of
many others.
3. Inirrigated plants the amount of resin is correlated with the ana-
tomical conditions within the organism, while the secretion proper appears
to be neither retarded nor advanced by the presence of water. Water, on
the other hand, affects markedly, though probably indirectly, the rate of
rubber secretion, which lags behind growth. But the lagging behind of
rubber secretion is not in inverse relation to any possible increase which
may be shown to occur in the secretion of resin.
4. The distribution of starch appears to be connected with the secre-
tion of resin, as in other well-known instances (e.g., Pinus). The secretion
of resin appears, as above pointed out, to be extra-protoplasmic, and in
harmony with the view expressed by Tschirch, already alluded to.
5. Rubber, however, appears in the tissues independently of the dis-
tribution of starch referred to in (4) above. However, the starch found
in the young tissues near the growing apex may serve as a source of ma-
terial for the elaboration of rubber.
6. The appearance of rubber in the canal-cells might be cited to sup-
port the view under discussion, but for the fact that the rubber is merely
accumulated in these cells and that this occurs later than the secretion
ofresin. Further, rubber occurs in other tissues, e.g., parenchyma rays,
far removed from resin secretion. Resin in the canal-cells has not been
demonstrated, but 7m the meatus only.
THE SIGNIFICANCE OF RUBBER.
The inevitable question as to the use of rubber to such a plant as the
guayule, subject as it is to the severe conditions of the desert, has been
raised and must be met in some wise. I have already briefly discussed
the matter (Lloyd, 1909) with but meager satisfaction, as will appear to
those inclined to find a use for everything in animate nature. I can only
repeat here what I have already said.
The Origin and Occurrence of Rubber. 189
The most obvious suggestion relates to the conservation of water,
and it seems quite possible that the rubber may act as a sort of blanket,
reducing to some extent the passage of water to the outer zones of tissue
_ and consequently to the outside of the plant, and as a storage material.
The slower deposition of rubber in irrigated plants and its behavior in
Castilloa elastica under similar circumstances lend a modicum of support
to this view. Rubber, as is well known, will take up and retain a certain
amount of water with considerable tenacity. One would be encouraged
to hold this view if rapidly grown field seedlings with much less than the
normal amount of rubber had not been known to pass successfully through
a long period of drought, indeed much longer than usual. Further, mari-
ola appears to be as well equipped for resisting drought as guayule, but
contains a very small amount of rubber. The obvious objection that the
mariola has some other means to the end would in this case, I believe, be
difficult to demonstrate, and as difficult to refute. We are here in the
field of teleological speculation.
Spence (1908), studying latex, found that this contains oxidases
capable of acting upon caoutchouc, and argued that this material may
therefore serve as a reserve food material.! Similar enzymes probably
occur in the guayule, but it is safe to remark that in this plant, once the
rubber is laid down, it is there to stay, as shown by its abjection in com-
pany with the bark-tissues. Even in the cells adjacent to the active cam-
bium, or other physiologically active tissues, the amount is never reduced,
while, if of use as a source of energy to the growing twigs, we should find
some evidence, analogous to that seen in the starch-content of growing
twigs, that there is translocation. But such evidence is quite lacking.
Whatever may prove to be true of latex plants, therefore, there does not
appear to be the slightest evidence that rubber is in any sense a food
material in the guayule.
This view has recently (1909) been again brought into question by
Spence:
The fact that the caoutchouc, or rubber, does not occur in any definite latex
system in the guayule, but in the parenchymatic cells of the medullary rays and
cortex, and further, that the amount of rubber from the dried plant varies con-
siderably from one period of the year to the other * * *, seems at once to
suggest to my mind that the rubber must have an important function in meta-
bolic processes. That the rubber is cast off partially and in a modified form in the
bark, as Professor Lloyd has pointed out, does not in any way weaken the evidence
of my theory, and from experiments which I have recently made I have found
that young Ficus elastica trees, grown in an atmosphere and soil free from carbon
dioxide, gradually drew upon their milk, which became nothing more than water
after a few weeks’ time.? In any case * * * the guayule plant shows very
clearly that we can hardly retain the theory that the latex merely affords protec-
tion to the plant against internal injury and moisture in time of drought; in
guayule there is no secretion on injuring the plant, and no reserve water-supply,
though the rubberisthereallthetime. * * *’’8
1 See also Cook, 1903.
? There has been a long controversy on the function of latex, for an account
of which see Tschirch, 1906.
’The quotation was printed in the past tense and third person. I have
made it into the first person, present. The italics are Spence’s. (Lloyd, rg09. Dis-
cussion, p. 141).
190 Guayule.
Dr. Spence adds that sugars are to be found in quantities in certain
barks, and that the physiological importance of these can not be doubted.
The answer would seem to be that whatever occurs in Ficus elastica
can only be of suggestive value with regard to the guayule. And the
behavior of sugar described means that the unused residue of the sugar
has been cut out by periderm, just as the unused portion of any other sub-
stance may be. But this can not mean that everything which appears in
the bark must have been of use to the plant. The statement made by me
that the amount of rubber varies from time to time in the year does not
mean that the absolute amount in a particular individual is now reduced
and now increased. It means that the amount of rubber relative to the
weight of the plant is greater at one time than another, and J myself have
shown this to be the case. The gradual accumulation in the tissues, unac-
companied by any reduction, of rubber which might serve a storage func-
tion, this accumulation following growth, seems to completely contradict
the view that rubber isa reserve food. We may very well say that during
growth energy is diverted from the secretion, or, as I should prefer to say,
excretion, of rubber, and this would accord with the fact that the more
energy is expended in growth the slower the secretion takes place.
In the statement to which Dr. Spence refers, when I speak of rubber
being cast off in a modified form I do not mean to say that this modifi-
cation is chemical, or that it takes place before the rubber is cast off,
but by virtue of (presurnably) oxidizing processes which take place in the
cork-cells, which are now dead. This change, it seems to me, can have, in
the light of the evidence, no significance to the plant. It remains, how-
ever, to show experimentally that my view is correct, but it can scarcely
be denied that the evidence against it is tenuous.
SUMMARY.
The studies presented in this chapter may be summarized as follows:
r. In the root, rubber is first secreted in the primary canal-cells (plate
41, fig. 6), the activity spreading from this region as a center, but more
rapidly along the radius. At about the same time, or, judging from the
size of the granules seen, somewhat later, it appears in the innermost cells
of the parenchyma rays. Rubber appears in the root earlier than in the
stem in the same plant.
2. Accumulation usually takes place in the oldest cells first; that is,
those in the outer zones. Thus, in the root the primary cortex contains,
before the maximum content for all the cells has been attained, more
rubber than the cells of the secondary cortex; and the outer cells of the
latter contain more than the inner. Accumulation (in irrigated plants
at least) is more rapid in the parenchyma-ray cells than either in the pith
or the.cortex.
In the primary cortex of the stem rubber may never appear, as, ¢.g.,
in irrigated plants in which growth and, hence, secondary changes are so
rapid that the primary cortex does not have time enough for secretion.
3. With one exception, namely, in the hypocotyl, the accumulation
of rubber in the stem takes place earlier in the pith than in the paren-
chyma rays or cortex, and earlier in the rays than in the cortex.
The Origin and Occurrence of Rubber. 191
At the apex of the stem of field plants more rubber is found in the
pith than in the cortex after prolonged drought.
In the hypocotyl (upper zones) accumulation of rubber takes place
more rapidly, if not earlier, in the inner parts of the parenchyma rays.
This appears to be due to a more primitive physiological condition of the
pith of the hypocotyl.
4. With questionable exceptions, the accumulation of rubber is
earlier in the ‘‘secreting-cells’’ of the resin-canals than in the surrounding
tissues. The exceptions noted were (a) in the apex of a very slowly grown
field seedling, in the resin-canals of which no rubber was noted, and (b)
in the new twigs, near the apex of field plants. Rubber may be noted,
however, in the canal-cells, as in a very rapidly grown irrigated seedling,
though it occurs nowhere else.
5. The amount of rubber in the cells of small seedlings! in the field is
relatively as great, or very nearly so, asin mature plants, except in those
seedlings (table 54) which have grown rapidly in the field, and which
have not had sufficient time for the accumulation of the full complement
of rubber.
6. Rubber occurs unchanged in the portions of the secondary cortex
which have been more recently cut out by inner periderm. In the cells
arising directly from the outer or inner periderm rubber does not occur.
In the bark proper the rubber-bearing tissues alternate with nearly bar-
ren suber. Volume for volume, therefore, the bark contains less rubber
than the contingent living cortex which still remains unmodified.
7. Rubber occurs in the pseudotylose tissue of the resin-canals in
quantities comparable to the amount found in adjacent cells.
8. The accumulation of rubber in the new tissues of secondarily
thickened roots and stems is analogous to that in those still in the primary
condition. It is for some time absent from the newer parts of the paren-
chyma rays, and secretion occurs first in the innermost and outermost
cells simultaneously. The march of the secretion of rubber is, therefore,
from the base toward the tip of new shoots and from the pith and cortex
toward the cambium in older stems.
g. In field plants, that is, in those subjected to the usual desert con-
ditions of their habitat, the accumulation of rubber is more rapid than in
irrigated plants. The maximum quantity is certainly not reached in four
months (June to September, incl., 1908) after growth commences, and it
is highly probable that six or more months must elapse.
In a given cell, the amount of rubber in a field plant will generally be
greater at the end of one year than in a corresponding cell in the irrigated
plant in two years. Also, cells containing a given quantity of rubber will
be found nearer the apex of the stem of field plants than of irrigated plants.
It is probable, again, that the total amount of rubber that a cell in a field
plant is capable of secreting is greater than in an irrigated plant, though
this is not certain.
1 Chemical analyses of entire small seedlings are misleading, because of (a)
the larger relative bulk of the leaves, and (b) the greater relative volume of tissues
partially filled with rubber, as in the case of seedlings taken after a period of growth,
but before the maximum rubber-content has been reached.
192 Guayule.
The determination of the time at which the maximum rubber con-
tent is reached is of economic importance, as the earlier gathering of shrub
involves a considerable economic loss, amounting approximately to the
quantity of rubber secreted in one year in the new parts. If consistent,
therefore, with other considerations, the gathering of shrub should not
occur during, or for some time after the close of, the growing season. It
will be understood that by new parts is meant the new tissues within the
already secondarily thickened roots and stems, as well as new accretions
in length. The time at which the maximum amount of rubber may be
expected differs with the length of the growing-season, which depends
upon the rainfall and the intensity of the drought following. It thus
happens that field conditions are sometimes such as to produce results in
field plants (seedlings, table 54) similar to those in irrigated plants.
to. The rate at which rubber is secreted by irrigated plants, under
the conditions described for the Cedros experimental plants, is such that
at the close of the second season’s growth (Sept. 1908), the amount in
the cells is sufficient to agglomerate into the large masses characteristic of
field plants. This condition was, however, approached after a succeeding
drought-period lasting till April 1909 (plate 43, figs. 1 and 2; table 53).
In plants grown at Caopas under irrigation, during the first season’s
growth (1908) and with a restricted amount of water during the second
season (1909), the amount of rubber was evidently greater than in the
Cedros material | and was great enough by October to agglomerate (plate
43, figs. 3, 4), forming dense masses, but not as large as in field plants.
There is, however, a large enough rubber-content in such plants for
mechanical extraction, though it is probable that some adaptation of the
process would be necessary. Although the amount of rubber may be as
low as 3 per cent, it must not be forgotten that the rate of growth under
irrigation is enormously in excess of that under field conditions.
11. There appears to be no direct physiological relation between the
secretion of rubber and of resin.
12. Rubber appears to have no physiological function in the guayule
plant.
‘The slowness of secretion in well-watered plants offers an interesting analogy
to the behavior of the rubber-bearing latex plant Castilloa elastica. (Collins, and
Pittier; see Cook, 1903). Olsson-Seffer has also pointed out that the secretion of rub-
ber in this plant is retarded by irrigation, and in consequence it must be deprived
of water for some time before it can be tapped to advantage.
Base of 1908 growth, August. Cedros, irrigated.
Growth of 1908 in April 1909. Cedros, irrigated.
Cortex, 2-year old stem. October 1909. Caopas, irrigated.
Pith of same plant.
Epidermis of an old leaf, field plant, April 1909.
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CHAPTER VIII.
VEGETATIVE REPRODUCTION.
In attempting to solve the problem of the cultivation of a hitherto
totally feral desert plant, it became necessary to determine quantitatively
the possibilities of the plant for reproduction vegetatively as well as by
seed. As has been mentioned, the percentage of germination is small,
even under the best cultural conditions, so that any haphazard field
method of sowing seed, in the hope that nature will do the rest, is prac-
tically out of the question. In the hope that cuttings could be made to
grow readily and in sufficiently large quantities for cultural purposes, this
was gone into thoroughly. The net result of all the experiments is to
show that only a short zone of the stem is capable of root-regeneration,
namely, that immediately above the tap-root, but including some portion,
difficult to delimit, of the epicotyledonary region in seedlings and an anal-
ogous portion of the stem in retofios (fig. 11). The ability to produce
roots in plants from seed is, however, not restricted to the main stem, but,
as will be shown, resides also in branches springing from the root-producing
zone. This fact is of rather special biological as well as economic interest,
and as it throws light on the failure of attempts to grow cuttings I shall
first present my observations leading to the conclusion stated.
INDUCED ROOT-REGENERATION.!
Both the Mexican guayule (Parthenium argentatum A. Gray) and its
congener, the mariola (P. incanum H. B. K.), exhibit methods of vegeta-
tive reproduction which, while shared by other plants, are not common to
these under the normal conditions of growth. A somewhat detailed ac-
count of the matter has already been published,” but a brief restatement
will be necessary to make clear the point of the present discussion.
The mariola is a low shrub with rather numerous branches rising
immediately from the base of the chief stem. These branches arise sub-
sequently to the development of the chief shoot, and not unusually, during
the first season of growth, from the seedling. Each following period of
development sees new lateral shoots of this kind arise again from the base,
either of the main shoot or, secondarily, from an already well-developed
basal-lateral shoot. Long continuation of this process results in the dense
group of stems arising near the surface of the ground which characterizes
the mature plant of the mariola.
It is to be further noted that nearly all of these basal-lateral shoots
are provided with their own root-systems (plate 44, fig. B). From the
base of each new shoot, soon after it has accomplished a fair amount of
development, there spring adventitious roots, one of which, by the direc-
1 Presented before Section G of the American Association for the Advance-
ment of Science at the Baltimore meeting, 1908.
? Lloyd, r908b.
13 193
194 Guayule.
tion and amount of growth, becomes, in effect, a tap-root of the branch
from which it springs. Subsequent development of roots of the second
and higher orders results in the ultimate elaboration of a complete root-
system.
We find furthermore that, while the caliber of the basal-lateral stem
increases with age, the isthmus of tissue between this and the chief stem
increases only slowly, so that there is never more than a weak connection
established, and this ultimately becomes disintegrated. In this manner
the basal branches in question are set free from the parent stock. There
results, therefore, from a single original stock, a group of independent
individuals closely crowded together.
A departure from this behavior is sometimes to be found. A glance
at the root-system of a single stock will show that the lateral roots run ob-
liquely into the soil, so that they soon attain a considerable depth. From
the upper portion of these lateral roots retofios occasionally arise which
behave much as do the basal-lateral branches above described, and the
net result is the same, namely, to produce a crowded group of individual
plants.
The root-system of the guayule, on the other hand, consists of a
strong tap-root and several strong laterals, which arise at a short distance
below the surface of the soil (plate 9, fig. A). These follow a horizontal
path for a distance,it may be, of 2 meters or more from the plant, and con-
stitute a water-collecting system by which the plant derives water from
rain-water which does not penetrate deeply—a feature shared by many
desert plants (Cannon, 1911). These shallow roots frequently produce
root-shoots (retofios) at various distances from the parent stock. I have
found them at a meter distant, and it is likely that they may arise still
farther away, though I believe less often than at shorter distances.
It may be presumed that shoots, arising, as they not infrequently do,
from the basal portion of the main axis, may occasionally strike root as in
the mariola. Many thousands of plants, however, have been examined,
and only one or two cases have been found which may be permitted this
interpretation. We may therefore regard the method described as the
only normal method of vegetative reproduction under natural conditions,
though it has been observed to occur in the field (Station 5) in two cases
in which the aerial portion of the plant had been removed.
On observing for the first time the conditions above described in the
mariola, it occurred to me that it ought to be possible to induce the guayule
to behave similarly. The fact that a guayule retofio strikes new adven-
titious roots from its basal zone (fig. 11), and that this,in common with
that part of the chief axis above the cotyledons, has a different ana-
tomical structure from other stems, gave color to the notion that there
are physiological grounds for entertaining the belief that, with proper
treatment, the possibility might be realized.
As experiments to this end would have necessarily involved a long
period of time, it was fortunate that I had under observation at Cedros
plants which had been growing for the major part of two seasons under
irrigation. This was in September 1908. These plants had grown from
stumps which were planted in March 1907, by Mr. C. T. Andrews. The
Vegetative Reproduction. 195
parent plants had been taken from an old stack-ground in Saltillo, at the
guayule factory of Martin Brothers, and had started there from seed
which had fallen from the stacked guayule. Before being transplanted,
they were variously trimmed back, leaving only the lower portions of the
main stem and, in some instances, of the lowermost branches. During
1907 the new growths attained a length of about 25 cm., making rounded
bushes about 15 cm. in diameter. By September 1908 another 25 cm.
of growth brought them to a spread of a meter for the largest plants.
It was then discovered (on the 19th of September) that the lowermost
new shoots in certain of these plants had struck root, quite after the
manner described for the mariola, and it was further observed that this
had not occurred in all of the plants, but either in those plants which had
been trimmed back so as to leave only a very short basal portion, or in
those new shoots which had arisen close to the tap-root (plate 44, fig. A).
In several instances the whole of the lowermost branch was buried by
chance in the soil, and in others a part, but neither in these nor in some
layering experiments by Dr. Kirkwood! was any response observed. The
behavior of guayule in this respect is similar to that of certain plants
which are subjected to mound-layering. Whether it is possible to compel
every plant properly treated to behave in the manner described can not
be said, as circumstances prevented a more careful study of the matter.’
If this should prove the case, it is evident that the branches which are pro-
vided with their own root-systems could be removed and transplanted
with ease.
PROPAGATION BY CUTTINGS.
The general conclusion suggested by the above experience was that
only cuttings taken from the root, or from a portion of the stem near the
top of the tap-root, would succeed, but as the time of my stay at Cedros
had drawn to a close it was not possible to direct experiments to test the
latter of the alternatives. Table 55, summarizing the results of my study
of cuttings, did not include this particular condition, which could hardly
have been anticipated. I early found, however, that the stem-cuttings
made did not respond, and that recourse must be had to cuttings in which
a portion of root-tissue was involved. The scheme of splitting the butt of
the plant so as to get two to four pieces was seized upon, the only method
of those used which secured positive results aside from pure root-cuttings.
The following conclusions may be drawn from the data in table 55:
1. Cuttings involving stem-tissues alone, with a possible exception of
stem-tissue close to the root in seedling or rotono, do not regenerate roots
under the treatment given. It remains theoretically possible, by special
and more refined methods, to induce root-regeneration, but for the pur-
poses toward which the experiments were chiefly directed, this is not
practicable.
2. Stem-cuttings may be kept alive, after being planted, for a con-
siderable period, particularly during the cooler season, by using careful
1 Exp. 181, 182, in which either branches or whole plants were layered.
2 I have noted the same behavior in guayule from Texas planted by me at the
Desert Botanical Laboratory and at Auburn, Alabama.
196 Guayule.
TABLE 55.—Experiments in propagation by cuttings.
Exp | | No. ae |
Nore Date | Parts used. of cut- Conditions. | Results.
: | tings.
| |
1907 |
1-6 | Aug. 2) Stem cuttings of vari- 65 Set vertically or hori- | Negative.
| ous ages. zontally in trays,
| watered.
7 | Aug. 2 | Root cuttings 2 to 5 25 Laid horizontally..... | Negative.
cm. long.
8-15 | Aug. 3 Various parts of | 116 | Gardensoil, sand and | Negative.
stem. soil, manured soil.
16-17 | Aug. 3 | Root cuttings from ra- 20 Laid horizontally..... Negative.
pidly grown plants.
83 | Oct. 25 | Lateral roots of field 2 Laid horizontally, | Dec. 3, new shoots 1 to
plants. lightly covered| to mm. long, on one.
with soil (garden Dec. 24, shoots on
soil). both. No new roots.
1908 | Died later.
130a | Jan. 24} Field plants. 1907 100 Planted in limestone | Apr. 6, 12 still alive, but
growth pinched off, soil in 1-inch paper all died later. No
| leaves trimmed. tubes in tray. roots formed.
Transplanted Apr.
| 6, into prepared |
| | bed of limestone
| | soil, watered and
| shaded.
1300 | Jan. 24 | Ditto, roots......... 40 DiGtOj-clate-cyet tense leueeond: Mar. 3, 2 buds on one
cutting. Apr. 6, 9
living. 2 more started
after transplanting.
Aug. 28, 3 growing
well.
130¢ | Jan. 24 | Ditto, 1907 growth} 100 Ditton «oe | Apr. 6, 13 alive. All
cut off and leaves | | died later, no roots
trimmed off. | having been formed.
130d | Jan. 24/| Ditto, twigs 2 to 3 40 DittOs «cis 1 cle ive ADE Oesealive: ibitacll
years old. died later. No roots.
130e | Jan. 24) Ditto, 1907 growth 20 DitstO tere (eisieisrs een eee Apr. 6, all dead. No
pinched off and roots.
leaves on left.
130f | Jan. 24] Ditto, cut off ob-| 100 DittO's fester sot ate Apr. 6, 21 alive. All
liquelythrough died later. No roots.
growth of 1906.
Leaves trimmed
away.
131 | Jan. 27 | Field plants, growth 40 Moist sarndiy.0. 12 cere | Jan. 21, all dying, rot-
of 1907 broken off; ting off.
leaves trimmed
away.
144 | Feb. 9| Twigs 2 to 3 years TO, UWhateGrcutecis eros tsecers Apr. 25, all dead.
old, field plants.
148a | Feb. 24 | “‘Root-shoot” cut- 31 Prepared bed. lime- | Apr. 25, growing vigor-
tings; lower part stone soil. ously 11, starting 8,
of stem and upper well started but wilt-
part of tap-root, ing, saved by heaping
split into 2 to 4 soil about them 2.
pieces. Started well but died
later 1; failed to start
9. May 2, one more
started below surface
of soil; May 19, an-
other, which later
died. Aug. 28, 25 to
32 cm. growth in the
above living cuttings.
VASO MPR ED). 249| MROOLS He yore cera eee 2 DDIGEOKteis/e ere teyeierer erate Both started, one dying
after making 3 cm.
growth.
152 Apr. 4 | Twigs3tos5 years old. 25 Ditto, with shade of | Apr. 16, all alive. May
Leaves trimmed cloth. 19, 5 budded out.
away. May 25, all appear
dead. May 31, one
with fresh buds start-
ing. June rs, all
dead.
155 Apr. 11 | Root-shoot, asin exp. 50 DD GEO saaorreeierac After a number had
148, but from small
plants.
Norre.—Exp. 1 to 17 were done jointly with Dr. J. E. Kirkwood. These were preliminary, and not
under critical conditions.
made a start (Apr.
25), all died later.
Vegetative Reproduction. 197
TABLE 55.—Experiments in propagation by cuttings—Continued,
No
.
Now Date. | Parts used. | of cut- | Conditions. Results. |
Y tings. |
1908
160 May 19 | Twigs 20cm. long ...| 356 Planted reversed! | May 31, 3 started; June
| in a “‘melga,’’2 2| 5, 46 started; July 9,
| | to 5 cm. projecting 55 started. In some
| above surface of| cases buds started 10
soil. | cm. below surface.
: Nonelived. Noroots |
| J | in any case. :
161a | May 19! Twigs 15 cm. long,| 19 Prepared bed of lime- | May 29, 5 started, but
| leaves notremoved. | stone soil. Planted all died later. No
| reversed. roots.
LOCO eMaws ro |Dithon puleavess “re= |< 35) jDittor te... .22- ty | May 20, 4 started, but
| moved. all died later. No
roots.
162 May 19 | Twigs 15 cm. long, 20 | Ditto, not reversed...) May 26-June 5, 19 |
| foliage trimmed | started, all dying
| away. later. No roots.
163 May 19 | Roots, 2 to ro mm. 35 Dire eee Bice eee eres eee June to, 7 started, of
| diameter. which 6 died. One
| grew well.
1QOn the theory that newer tissues might be able to regenerate roots.
2A melga is a bed with a deep border to facilitate irrigation by flooding. Alfalfa is frequently
irrigated in this way.
methods. In many instances they will produce new shoots, the size of
which varies directly with the volume of the piece. Consequently, exami-
nation of the above-ground parts might
easily persuade the uninitiated that
growth, including that of the roots, had
taken place. The fact remains that in
no case had the pieces regenerated roots,
and in consequence the cuttings all died
sooner or later.
3. Root -cuttings may live and
become permanently established, but
under the conditions used the number
was small (plate 20, Ar). In these, too,
new shoots may be produced without a
commensurate growth of new roots, and
the cuttings may therefore die after
starting.
4. Sectorial cuttings made by split-
ting the lower part of the plant in such
a manner as to involve root and stem
tissue grow most readily (plate 20, fig.
A,2to 4). The pieces heal completely
without decaying (fig. 18), and new
growths of normal extent under irriga-
tion will be formed, these flowering
abundantly the first season. Under
oe Fic. 18.—A successful sectorial root-stem cut-
favorable conditions about 75 per cent ting, showing complete healing.
may be expected to live.
5. Stem-tissue may be forced to regenerate roots by planting the
basal portions of plants, trimmed close to the top of the tap-root (plate
44, fig. A). Branches which then start out will generally behave as the
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. ;
)
198 Guayule.
basal shoots of mariola do normally, namely, each will send out a root
from near its base (plate 44, fig. A), which becomes, in effect, a tap-root.
Thus these branches become established independently of the parent stock,
and may be separated from it and used for propagation. This occurs in
nature very exceptionally, but more readily when the top has been removed
by design or accident; in the field, however, roots are normally produced
from the basal portion of the retofio chief shoot (fig. 11), from which its
root-system proper is derived. The more ready production of roots in this
manner in irrigated plants is connected with the larger supply of water.
It is seen that the guayule displays a marked polarity analogous to
that found in plants which will not regenerate roots from stems when ma-
ture, but will do so when young (Cupressus, vide Goebel, Organography,
Engl. ed., p. 45, I), and to certain lilies (Hyacinthus sp., Goebel, zbid.), in
which bulbils are formed from the lower portion of a severed stem, but
not above. That is to say, the expression of polarity by root-regeneration
from the stem is definitely restricted to a particular region of certain
stems only, namely, to the lowermost zone of the branches of the second
and (probably) higher orders, which themselves arise ftom a narrow zone
of the chief stem just above the tap-root.
Shoot-regeneration is, by contrast, easy, and this is true for the root,
from which stem-primordia are absent. It does not appear that external
stimulus is necessary, for wounding the cortex of the root 7m situ is not
followed, in any of my experiments, by shoot-formation at the point of
wounding. Nevertheless, as in many plants, a complete severance of a
root left zm situ is frequently followed by shoot-formation, but in a posi-
tion determined by other conditions, such as dying back, resulting from
drought. Thus it appears that the notion formulated by Miss Kupfer
(1907), that the disposition to form roots is much more generalized than
to form shoots, does not include cases like this before us, which need eluci-
dation as much as any. And as McCallum (1905) has well said, the prob-
lem of regeneration is more especially to determine the cause of non-
development “of parts’’ in the normal life of the plant.
CHAPTER AX.
LHe CULTIVATION. OF GUAYULE.
Under the cultivation of guayule must be included all operations
intended to modify the relation of the plant to its environment. These
operations may be forestal or cultural, in the narrower sense. It is the
purpose of this chapter to set forth the conclusions as to the possibilities
which have presented themselves in both these directions. Although only
a beginning has been made in the solution of the many difficult practical
problems which have arisen, the more immediately insistent questions
involved have been fairly if not completely answered. The difficulty of
practice is not lessened by the fact that the problem before us is distinctly
a desert one, and the final answer to many questions may not be obtained
for many years.
FORESTAL OPERATIONS.
PRESENT FIELD OPERATIONS.
Up to the present time, with only very few experimental exceptions,
field operations have been confined to the collection of shrub in the great-
est possible amount with the greatest ease, for the sake of the immediate
monetary return. This has had both a bad and, in less degree, a good
result. In many places where shrub had been taken there were so many
small plants that it was thought that it would not pay to collect them,
and these will serve to repopulate the areas so treated. In other places,
where the stand consisted only of large plants, nearly every vestige has
been removed, leaving at most only the occasional small plants to lay the
foundations for the future. If in such places a few healthy medium-sized
plants had been left to produce seed, as common sense should have dic-
tated, ground that will be barren of guayule for many years might have
been repopulated, at any rate to some extent.
The method which has ordinarily been used is to pull up the plant by
hand, and, while the method of cutting it off at the surface of the ground
has been advocated and to some extent practiced, pulling has been most
largely used. But in very rocky areas, where the plants frequently grow
in the fissures of the rock, from which it is often impossible to pull them
out, the peons will break or twist off the top, leaving the butt in the
ground. A specific case of this kind was noted by me in a part of the
Sierra de Ramirez, a range of mountains lying partly in each of the States
of Zacatecas and Durango, opposite Tanque de la Pendencia. On first
entering the guayule area, which had been worked in the winter of 1907-
08, scarcely any guayule was to be seen, but further search discovered
numerous young growths, visible with difficulty on account of their color
when seen in April 1909, which had come up from the basal portions of
plants which had been twisted off. Bare as this ground appeared to be
of guayule, there is little doubt that in time the stand will be replenished
to a large degree, if not fully.
199
200 Guayule.
In the Lomerio de Zorrillos, some leagues further south, where the
substratum is made up of fine limestone soil containing stones of various
sizes, it is easy to pull the plants up, and here all the larger ones were
taken. As the number of small plants was, however, very great, all these
were left, and number 600 to 800 to the 100 square meters, weighing 1 to 2
tons to the hectare. This condition probably represents the best that may
occur under the old methods, and is but seldom found. In many spots
from which larger plants had been taken, pieces of root left by breaking
off were found to have produced retonos.
The work of pulling up the guayule is done by peons who tie the shrub
into bundles, make up burro-loads, and take it to a neighboring “‘ campo de
guayule,” a field-center of operations, where the shrub is baled in hand-
presses. From here it is hauled in wagons to the most accessible shipping-
point on the railroad, and so by rail to the factory.
In undertaking to harvest the shrub from a particular region, the
usual method is to let a contract to local agents who can command the
conditions, which, as may well be imagined, are often severe on account
of the great distances and lack of water. The easiest time to work is
while the ground is still soft from the rains and when water is relatively
plentiful, and it happens that this is the worst possible time to take the
plant as regards its rubber-content. At that time also the shrinkage in
weight is much greater, both by the loss of a greater amount of water in
the plant and the larger bulk of the foliage.
SUGGESTED RULES OF PRACTICE.
The statement will not need defense that an immediate desideratum
is a rationale of forestal operation, in order that the present field supply,
already much reduced from the original stand, may be kept from being
well-nigh wiped out. The data upon which rules of procedure must be
based, in the absence of still necessary extensive quantitative study of
treated areas, have been presented in Chapter IV. The general practice
indicated by the experiments recorded will therefore be stated here.
1. Guayule should be gathered by cutting it off at the level of the
ground. That which is allowed to project above the surface will die back
more or less and be an economic loss, as these parts represent a substan-
tial proportion of the weight of the plants. The cutting should be done
with a sharp grubbing-hoe (talacho), a method which is easier on the men,
as well as contributing to the preservation of the stand of plants. It is
practically certain that new shoots will arise from many of the parts left
in the ground, and these, during the first season, will produce flowers, the
seeds from which will help to repopulate the area.
It has recently been suggested by Escobar (1910) that, after cutting,
a shallow depression be cut in the soil about the remaining root, for the
purpose of catching the run-off, thus increasing the water-supply. Further
operations (terracing or furrowing along the contour lines), designed to
hold back the run-off, are also recommended. In many situations it
would be difficult to carry out schemes of this kind.
2. Only plants 40 cm. or more in height should be taken on the first
cutting. Five years later there should normally be a crop of 40 cm. plants,
The Cultivation of Guayule. 201
which may then be taken. Between 30 and 40 cm. the maximum econo-
mic efficiency of growth obtains, and this lies between 10 and 15 years of
age. Fifteen years is therefore the rotation period, but as the growth effi-
ciency of a plant falls after this age has been reached, these plants must be
removed each fifth year. The advantage of this rule is to be expected not
only in the growth of the plants already there, but also in the great effi-
ciency of seeding. The question has been raised as to the possible increase
of efficiency of germination by partial or total clearing of the land, thus
removing the factor of competition.
3. Removal of the vegetation other than guayule. It is too early to
make any definite statements as to the value, even with regard to the
well-being of the mature plant, of clearing operations on guayule lands.
The experiments which have been initiated involve an area of about 75
acres, which were well cleared of all vegetation excepting the guayule, the
‘“‘palms’’ (palma samandoca) which produce fiber, and the few cacti, of
large species, which occupy little area and do not constitute an aggressive
element in the vegetation. The clearing of the land has the effect of loos-
ening the more superficial layers of soil generally, and to some depth in
spots. On general grounds this ought (1) to remove competition with
other plants, which, as has been shown elsewhere, is not insignificant and
frequently constitutes a real menace to the guayule, necessitating partial
clearing, at least. This competition of course relates especially to the
water-content of the soil. Unless the removal of the covering allows
greater washing than in any event occurs, it should render more water
available for the guayule and thus enhance growth. It must not be for-
gotten, however, that a much greater growth is correlated with reduced
activity in secretion of rubber, either directly or by reducing the volume
of the rubber-bearing tissues, as has been brought out in the discussion
of plants under irrigation and from areas of greater rainfall (Chapter V).
(2) It is important also to know what effect the removal of the vegetation
has upon the crop of seedlings. The evidence so far obtained appears to
favor the clearing of theland. I refer especially to the census of seedlings
made at Station 2 (page 70), in which are recorded numbers of seedlings
far in advance of those found elsewhere. As to the question of protec-
tion afforded young seedlings by the shade of other plants, of no small
importance in many cases, as has been repeatedly indicated by studies at
the Desert Botanical Laboratory, it may be concluded that the number
of seedlings which survive a six months’ drought, as observed by myself
in April 1909, is sufficient to warrant the statement that not enough suc-
cumb to unfavorable conditions to neutralize the good effects, as seen in
the surviving numbers. It seems probable, therefore, that clearing the
land of other vegetation, saving the species above mentioned, is, on the
whole, beneficial to the guayule.
As to the specific question of the response in growth, all that can be
said at this time is guesswork. The areas which were cleared, as it hap-
pened, were subject to severe droughts from the time of clearing in the
winter of 1907-08 till the summer of 1909. It is hoped that the abundant
rainfall of the season now drawing to a close will enable us to form some
conclusion on this point.
202 Guayule.
HARVESTING PERIOD.
The question of the variation in the relative rubber-content of the
guayule according to the time of the year is undoubtedly of more impor-
tance than is at present appreciated. The loss arising from this cause,
moreover, can not be detected by the chemical control of a factory labora-
tory, for the reason that the new succulent growths when dried add but
little to the weight of the plant, while their capacity for rubber-secretion
is indicated by their living volume. The “shrinkage’’ between field and
factory referred to by the manufacturer is equally inefficient as an indi-
cator of the loss, in a practical sense; shrinkage consists of all kinds of loss
in handling and transportation from the field to the factory, an important
economic factor which, while including the loss under consideration, leaves
it undiscoverable.
An element of uncertainty arises from the different moisture-content
of the shrub at various seasons. Thus, the shrinkage in weight from dry-
ing in field plants is from 20 to 25 per cent (exactly in my determinations
between 22 and 23 per cent) during drought; in irrigated plants it is as
high as 50 percent. In August 1908, at the height of the growing-season,
the water-content ranged between 25 and 50 per cent, averaging in the
neighborhood of 35 to 4o per cent, as high, nearly, as in irrigated plants,
in which it rarely falls below 40 per cent, and is usually about 50 per cent.
In addition to this, the weight of the additional leaves in summer is not
negligible. I shall therefore venture to state with some insistence that,
assuming normal distribution of rainfall, the gathering of shrub during
summer months and for several months thereafter can mean, practically,
only the total loss of the rubber accretion of a whole year. The small
amounts of rubber undoubtedly present in the newer growths can scarcely
be recovered by mechanical means, while the ready breakage of the slender
and weak twigs of recent growth would in any event result in a loss.
Another consideration is involved also. The germination during the
growing-season results in the annual crop of young seedlings, the greater
part of which, on account of the numbers and small size, would undoubt-
edly be destroyed by the peons at work collecting shrub. Aside from this,
the peons should be not only instructed but compelled to work carefully,
so as not to destroy the small plants.
RESEEDING BARREN GROUND.
Land from which guayule has been completely removed may, under
favorable conditions, be restocked by the simple operation of reseeding.
Whether the cost of doing this would be justified, however, is doubtful,
since an area of any size would require an immense amount of seed, which
at present it is difficult to obtain in quantities, and since the percentage
of germination under natural conditions would be very small.
Whether the business view will see a sufficient monetary recompense
in the returns from following the procedure above recommended is not
the present problem. Local conditions vary too much to solve it in general
terms. This much, however, may be said: that the rules of operation out-
lined are dependable in the degree indicated, and that the disregard of
them, or of some equally or more efficient ones, will only lead to the prac-
tical extermination of the plant.
The Cultivation of Guayule. 203
CULTURAL OPERATIONS.
Although it will be granted that forestal operations are of immediate
and great importance for the preservation of the natural stand as a source
of revenue for as long a period as possible, the ultimate and adequate
solution of the production of guayule shrub lies in its successful cultiva-
tion. That this is possible seems at the present not to be overstating the
case. The abundant and ready growth of guayule under irrigation, its
drought-resistant qualities, its consequent adaptability to comparatively
meager irrigation, if this condition is imposed, and its ability to secrete
rubber, though in relatively smaller quantities per unit-volume of tissue,
under irrigation properly alternated with drought, are established facts.
It remains, therefore, to test, on a larger scale than has hitherto been
attempted, what may be done to establish the culture of the plant on an
economic basis. Butin stating the positive basis for success the difficulties
must not be underestimated. These will be indicated in what follows,
and it will suffice here to point out, in a word, that the great difficulty lies
in the initial work of establishing the plants, which necessitates water. It
would be useless to attempt cultural operations without it.
SEED.
Should it turn out finally that the raising and the transplanting of
seedlings is a desirable method of procedure, the obtaining of a sufficient
amount of seed will be an important desideratum. At present it would
be necessary to collect seed from the field. This is costly and uncertain.
Experience has shown that the ripening of seed in the fieldis uneven; much
of it quickly falls off, and the most satisfactory places to collect are fre-
quently far removed from habitations. Hand-picking is slow, but could
be rendered more efficient by the arrangement of a device of wire and
cloth, in two pieces, to be held under the shrub, from which the seed could
then be dislodged by light beating. It seems, however, unlikely that any
field method of collecting seed will be as satisfactory as its production by
irrigated plants, which, in the climate of North Zacatecas, will ripen seed
for the greater part of the year. The ripening of a large amount at one
time will render rapid collection easier. Some such device as suggested
willin any event be necessary, as the seed must be collected and submitted
to optimum conditions in order to get the maximum germination. It has
been found that the ordinary conditions of growth, even under irrigation,
are not efficient for this result. The advantage of growing plants under
irrigation is in the convenience and economy in obtaining seed, and not
in its superior quality. Kirkwood (1910a) found that the number of good
seed from irrigated plants does not exceed 17 per cent, and this is some-
times, but not often, surpassed in field plants.
THE RAISING OF SEEDLINGS.
The small size of the seedlings and their tender character when young
make it necessary to handle them with considerable care. The precise
conditions for their successful culture have been studied by Dr. Kirkwood
(1910a) and by myself, and from these experiences, but more particularly
from my own, the following practical suggestions are made:
204 Guayule.
The probable necessity of transplanting large numbers of seedlings
at a very great risk of loss led me to adopt experimentally a scheme used
in the tropics, where the hollow joints of bamboo are used as flowerpots
in which to raise cacao, coffee, and other seedlings. When ready for plant-
ing in the grove the whole pot is planted, and the decay of this, aided by
fracture, sets the roots free without any disturbance. In a preliminary
way the joints of “carrizos’’ (Arundo donax) were tried, but proved too
small. Combining this idea with that of the paper flowerpot, a unit sys-
tem of wooden trays and paper tubes was devised,! the tubes being 1
square inch in transverse section and 6 inches long (plate 45, fig. A). As
trials with these taught that they afforded too little room for the horizon-
tal development of roots, a comparative test with similar tubes of 4 square
inches transverse section was carried out under identical conditions. ratures
recorded by Kirkwood (1910a), seeds germinate more pror —_ ud, what
is more important, the seedlings make a much more rapid growth during
the summer months, as my experiments in July and August 1908 showed.
In winter, also, the seedlings were frequently killed in part by frost,in part
by a storm of hail, and were more subject to damping-off. The heavy
rains of summer also prove more or less destructive, and it was found that
the seedlings with the shortest hypocotyls survived the best. For this
reason as thin a shade as possible is desirable, the object of this being to
preserve the superficial soil-moisture and to cut down the light as little as
possible.
206 Guayule.
When it is desired to transplant the seedlings, the tubes will be found
to be soft and partially decayed, so that they may be torn by slight pres-
sure when being placed in the ground. This will favor a prompter adjust-
ment to the new conditions. The loss will vary and can not be foretold,
3
Fic. 20.—The same root-system as shown in fig. 19, B, projected on a
horizontal plane.
but with care should be small. As time did not permit an extended trial
of this, however, I am unable to state economically valuable results,
though some indication has been had from the following:
Experiment 79.—Of 449 seedlings (small plants 1 to 5 years old),
transplanted into irrigated ground by a peon, 300 lived. The
transplanting was done Nov. 26, 1907, care being taken to pre-
vent the roots from drying out, and the ground was well irri-
gated. On Feb. 16, 1908, the first indications of growth were
seen, but the plants started unevenly, some not showing signs
of new growth until Apr. g.
Experiment 159.—May 3, 1908, 5 seedlings were transplanted in 1-
inch tubes, the upper 5 cm. of the tube only being preserved.
All lived and grew well till last observed, Sept. 1908.
Experiment 153.—Apr. 8, 1908. Of 14 small seedlings (epicotyl 10
mm. long in the largest) transplanted into a prepared bed, 12
grew well, 2 died.
Experiment 164 (J. E. Kirkwood).—On May 13 a bed was prepared
by digging up the soil and flooding to a depth of 4 inches. On
the following day 1-square-inch paper tubes containing seedlings
an inch high were set in the wet ground their full length. These
plants had been grown in the tubes from seed and were some
two months old. 64 of these were planted, and nearly all lived.
Experiment 165 (J. E. Kirkwood).—so tubes containing plants of
the same lot as the preceding were set in relatively dry soil
which showed visible moisture an inch or so below the surface.
This was done May 14 and the ground received water to saturate
several inches on the 18th and roth. A few of these survived.
The Cultivation of Guayule. 207
Experiment 166 (J. E. Kirkwood).—Bed prepared as above and cov-
ered with 4 inches of water. On the following day 250 plants
(seedlings) of three months or less were transplanted into this
bed. These plants received no more water than what was given
at the start, in order to test this practice in the transplanting.
In five days these plants appeared to be dead.
Experiment 167 (J. E. Kirkwood).—Bed prepared as before and
watered to saturation. Into this 15 young plants were set on
May 15 and immediately watered by flooding. The bed was
watered again on May 16. Nearly all of these lived. It resulted
that in 164 and 167, in which abundant watering was had at the
start, nearly all of the plants lived. In the others nearly all
failed.
Transplanting cultivated seedlings into the natural habitat was tried,
but the plants were destroyed by goats. The operation is fraught with
much difficulty on account of the character of the ground, and would not
justify itself practically.
It may properly be said that the raising of guayule seedlings, more
particularly during the first few weeks, is not a mere rule-of-thumb pro-
cedure. One has to watch them with care and learn their idiosyncrasies.
Later they become quite resistant and may be handled much more easily.
The best soil for them,so far as experiments by Kirkwood (1910a) and
myself have shown, is the limestone soil of their natural habitat (plate
16). Soil which contains a good deal of humus appears unfavorable for
young seedlings, as, among other difficulties, damping-off is very preva-
lent. However, they were found to have germinated abundantly after
lying in such soil for seven months, and grew well, though it must not be
forgotten that the soil must have suffered considerable change by leaching
and chemical action in the interval. The action of fertilizers has not been
tried as yet either on seedlings or mature plants. Recent experiments
have, however, shown that guayule will grow wellin a noncalcareous soil,
and respond readily to sodium nitrate.
The presence of small stones in the soil appears on the whole to be an
advantage. The following experiments were done to test this point:
Experiment 138, Jan. 24, 1908.—Into three root-cages with sloping
glass sides three lots of seedlings of about equal size were trans-
planted. One (I) of these contained very finely sifted limestone
soil; the second (II), similar soil mixed half and half with fine
angular gravel of limestone; the third (III) was filled with the
same fine soil and coarse gravel (1 cm. ave. diam.), the gravel
occupying the space that it would without the soil. Feb. 22,
length of tap-root in (I) 50 mm.; (II) 50 to 60 mm.; (III) 60 to
120 mm. These measurements represent differences in rate of
growth of the tap-root, which was about the same length in all
at the beginning of the experiment.
April 5, the individual measurements of the roots were as follows:
(i) 220s. so; To, 2o; 1295; 04, 77 em... Average; 11.9 cm:
G))ieoeny, 16,\13) 14, 15 ems Average, 15.8 cm.
GLI), a6..37..26; 93, 26, 19,14, .14, 20,25, Gin, Average, 22.6 cm.
These differences were reflected in the aerial development, those of
III being obviously in advance of the others. -
208 Guayule.
Experiment 139a.—Two seedlings of nearly equal size were planted
January 24, 1908, in a 5-gallon oil-can, half of which contained
a soil made up of coarse gravel and fine soil (of the latter only
so much as would go between the gravel), while the other half
contained uniform, finely-sifted soil of the same kind. The
watering was equal for both sides, and sufficient to keep an
abundance of water available. The subsequent growth in the
plant in gravelly soil was very much more marked, as shown in
the left-hand plant on plates 18 to 20, the limit of growth for
the year being nearly reached in fourmonths. This plant, which
weighed 8 ounces, produced fully 2,000 seeds. The development
of roots was correspondingly greater in the gravelly soil, and
careful removal of the roots showed that they were confined
chiefly to this soil, though occasional roots of each plant reached
over into the territory of the other. However, it should be noted
that there appeared to be a tendency of the roots in gravelly soil
to grow toward the fine soil, as seen in plate 20, fig. A, in which
the plants are oriented with respect to each other as they grew.
In these experiments, therefore, the gravelly soil was more favor-
able to root-development, a result which appears to harmonize
with agricultural practice.
IRRIGATION.
If large numbers of seedlings are to be raised, the method of watering
will introduce a material element of expense, aside from the cost of the
water. Hand-watering of the surface would prove to be laborious and
expensive. For this reason a method of subirrigation was tried, with the
results as stated above. Additional evidence is as follows:
Experiment 141.—To test the relative value of subirrigation, with
and without shade, as compared with surface watering. Four
trays with 1-inch paper tubes (plate 45, fig. A) were filled with
limestone soil mixed with gravel, each sown with 1 ounce of seed.
(1) Placed on the surface of the ground and shaded by a thin
white muslin screen.
(11) The same, but without shade.
(III) Placed in a melga, and shaded as above.
(1V) The same as III, without shade.
III and IV were watered by subirrigation; I and II by surface water-
ing, and served as a check on III and IV. It was noted that it
was very difficult to keep II wet enough. The surface of IV
was never dry.
In both shaded trays the germination was far in excess of that in the
control. In both the subirrigated trays taken together, the germination
was over twice that in the surface-watered trays, though it was slightly
more in the shaded, surface-watered tray than in the unshaded, subirri-
gated tray. The result indicates clearly that subirrigation with shade is
the most favorable of the four conditions. It should be noted that tray III
was left unshaded after February 13, in order to avoid extreme etiolation,
and this may have lowered the subsequent rate of germination without
vitiating the general result. «i
The Cultivation of Guayule. 209
TABLE 57.
Numbers of seedlings in—
Date of count. _————— |
Tray I. | Tray II. | Tray III. | Tray IV. |
Rebate te i: Be ciiws 2 | 6 | 5 18 9
Oli raes clectey rat « teens cx 2 ° 34 6
IS), cme o Sep ACMOMC =xeuce ct 20 | 2 42 3
BOG oot od a RITA SI 21 12 SSIS) 250d
Teas Seb ee eyes 28 mit 40 bce) |
1 Se Sa ae ae 16 3 25 II
Ti} aes. f Eier Specs. £ 6 3 13 4 |
DA vio Mee os Soostn ste 8 Gi 8 3
7 Oe cee oe T5 2 15 4
ROP oe aoe ce 3 ° 5 ris
2 QUA ea. eter 6 2 12 9
BAW scat .cte Ae = 5 ° ° 12
Mae 5G os... ee oie 21 19 8 EEE |
Ota lSie 4, ete uy « ly 2x70 57 283 184
Less loss to Mar. 16 .. 13 38 35 40 |
Rotaltalivesshek.«. a-k er) 19 248 144 |
TRANSPLANTING.
Another method of getting a stand of guayule started and having the
advantage of speed is by transplanting field plants into irrigated ground.
Experience has taught that it is of little use to attempt to preserve the
aerial part of plants of any size, and that even small ones frequently die
back. Of a plantation of some hundreds of individuals so treated (at
Caopas), scarcely 25 per cent grew, but upon cutting them back a consid-
erable additional number revived (plate 46, fig. B). If it should be found
desirable for any reason to start a crop of guayule from field plants, the
best method is to cut back to the top of the tap-root and send the tops to
the factory for extraction. The returns from these would go far toward
the expense of the operations. It is difficult in any event to start stocks
unless previously pollarded.
The portions to be planted should be handled as rapidly as possible,
being kept from drying out by means of wet burlaps, or some such means.
They should be planted deeply, the cut surface being no higher than the
surface of the soil, and they should then be thoroughly irrigated.. The
question as to the amount of water which may be used without doing them
damage is answered by the simple experiment (exp. 145, Feb. 9, 1908) of
putting a number of plants into water with their roots and basal part of
the stem totally submersed. In four days numerous actively growing len-
ticels were to be seen on the submersed stem, and on March 14 a rootlet
to mm. long had grown from one plant, while others had started. By
February 24 rootlets 6 to 8 mm. long occurred on the upper parts of the
tap-root, and even roots of the third order were subsequently formed.
There was no sign of disorganization, so that, unless the soil itself should
introduce unfavorable elements, we may believe, as indeed experience in
general shows, that the guayule can stand abundant water.
14
210 Guayule.
The best time of the year for transplanting, as shown by the prompter
responses of the experiments cited in Chapter VI, is in late spring and
in summer, when the warmer night-temperatures aid in stimulation. The
differences in this regard were very noticeable and showed conclusively
that winter, in North Zacatecas at any rate, is unfavorable for cultural
operations of any kind.
The advantage of cutting back to the region of the tap-root, in addi-
tion to avoiding the loss from dying back, is to be had in the behavior
which I have described at some length in Chapter VI, namely, the produc-
tion of basal shoots which root independently. These shoots will be pro-
duced the more frequently the nearer the tap-root the cut is made. As
also the guayule frequently sends out new shoots before any new roots
have been formed, there is less likelihood that these will exhaust the avail-
able moisture when the whole of the transplanted portion is covered with
soil.
HARVESTING CULTIVATED GUAYULE.
It is almost gratuitous to say anything about this topic, as up to the
present time the facts have not warranted cultural trials on a scale suffi-
cient to make available a crop of anything but limited experimental size.
We are justified, however, in drawing a few conclusions from the facts
which have been brought to light in the present paper.
Assuming that the amount of rubber ultimately produced by guayule
under irrigation is sufficient to warrant its culture, it seems clear that the
methods of harvesting should be approximately as follows: The new
growths, say of two years, of plants about a meter in spread,’ may with
advantage be removed by a cutting instrument, so as to leave the butt
undisturbed to shoot out afresh. The branches which have rooted can
then be removed by hand simply by breaking them away, and replanted.
These are usually supplied with a strong root which can be pulled up with-
out severe damage. In this way the cultivated stand may be increased
ad libitum, provided areas with sufficient water are at hand.
CATCH CROPS.
Immense areas of land are available in the Mesa Central of Mexico,
and doubtless elsewhere, where ‘‘riego temporal’’ is practiced. This sys-
tem of irrigation consists of ditches to catch the run-off, leading it to the
fields. The behavior of guayule would seem to justify the belief that this
plant could be grown for a sufficient period, say two or three years, in such
irrigable areas, and the expense, in part at any rate, offset by growing corn
or some other suitable plant, asa catch crop. The guayule, when of suffi-
cient size, should then be ‘‘laid by”’ to endure a period of drought till it
becomes usable, when it could be cut as suggested, and restarted. This
suggestion, and it is that and no more, deserves a serious trial.
1 Assuming the conditions which have constantly been referred to in this work.
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