I

^<*'

The
Root Habits of Desert Plants

BY

WILLIAM AUSTIN CANNON

WASHINGTON, D. C.

PUBLIbHED BY THE CaRNEGIE INSTITUTION OF WASHINGTON
191 I

922.

CARNEGIE INSTITUTION OF WASHINGTON

I'UBLICATIOX No. 131

Copies of this Book
were first issued

MAR 28 1911

PRESS OF GIBSON BROS.
W.ASHINGTO.V, D. C.

CONTENTS.

PAGE.

Introduction _,

Certain Features in the Attempted Classification of Roots 9

Scope of the Study and Methods 10

Environment of Roots i -,

Leading Characteristics of the Soil 12

Tumamoc Hill 12

Flood-plain of the Santa Cruz river 12

West Wash i ,

The Bajada i ^

Moisture in the Soil 15

Tumamoc Hill j c

The Bajada j ^

The West Wash 18

Flood-plain of the Santa Cruz river in

Temperature of the Soil 20

Record of Thermograph: 15 cm. depth 20

Record of Thermograph: 30 cm. depth 21

Soil Temperatures: 2.5 cm. depth 22

Summary of Environmental Conditions 23

Root Habits of Desert Plants 25

Summer Annuals 25

Amaranthus palmeri 25

Anoda thurberi 26

Aster tanacetifolius 26

Boerhaavia sp 26

Cladothrix lanuginosa 27

Datura sp 27

Ditaxis humilis 27

Dysoda papposa 28

Euphorbia glyptosperma 28

Kallstroemia grandiflora 28

Pectis prostrata 29

vSolanum eUeagnifolium 29

Trianthema portulacastrum 29

Vicia sp 29

A\'inter Annuals ,0

Amsinckia spectaljilis ^o

Astragalus nuttallianus 31

Bowlesia lobata -,1

Brodi;ca capitata 31

Daucus pusillus ^3

Eritrichium pterocaryum and Harpagonella palmeri 33

Erodium cicutarium 3^

Festuca octoflora and Hordeum rnurinum 34

Gilia bigelowii 34

Malva borealis 35

Medicago denticulata 35

Mentzelia albicaulis 36

Microseris linearifolia 36

4 CONTENTS.

Root Habits of Desert Plants — Continued.

Winter Annuals — Continued. pace.

Moni)Ic|)is chcnopodioides 3^

Orlhocarpus purpurascens 3^

Parietaria dcbilis 3^

Phacelia tanacctifolia 3^

Plantago fastijjiata 3^

Rallmsiiuia nco-iiiexicana 39

Strci)lantluis arizonicus, Sisymbrium canescens, Sisymbrium reflexum . 39

General Conclusions and vSummary of Studies on Annuals 40

Root Systems of Perennials 43

Plants from Tunianioc Hill 44

licliinocaclus wislizeni 44

Encelia farinosa 4^

Opuntia discata 4^

Opuntia Icptocaulis So

Opuntia versicolor 52

The Bajada 55

Carnegiea gigantea 55

Covillca tridentata 58

Foutiuieria splendens 61

Franseria deltoidea 64

Krameria canescens 67

Opuntia arbuscula 69

Opuntia fulgida 7i

Riddellia cooperi 73

Dasylirion texanum, Yucca radiosa, Yucca sp., and Agave sp 75

The Flood-plain 78

The Flood-plain of the Santa Cruz 78

Koerberlinia spinosa 78

Prosopis velutina 80

Condalia spathulata and Zizyphus parryi 81

Ephedra trifurca 82

Opuntia vivipara 83

Experimental Cultures 84

Physiological Features of Root-Systems 87

"Character' ' in Roots 87

The Relation of Roots to Soil Temperature 88

The Relation of Roots to Water 89

Relations of the Roots of Neighboring Plants 92

Root Habits and Plant Distribution 94

Summary 95

LIST OF PLATES.

Facing
„ Pagb.

Plate i.

A. Upper soil, adobe clay, of Santa Cn:z flood-plain, at place shown over 4 ni.

in thickness.

B. Adobe clay, the darker soil, overlying the caliche hardpan, with "rotten"

caliche, broken masses of the hardpan, between 12

Plate 2.

(a) Amaranthus palmeri, Tumamoc Hill, (b) Anoda thurberi, Tumamoc
Hill, (c) Aster tanacetifolius, Tumamoc Hill, (d) Aster tan-
acetifolius showing, at the left, root-formation at the base of

lateral of first order 24

Pl.ite 3.

Root habits of summer annuals, Tumamoc Hill, (a) Boerhaavia sp. {b) Cla-

dothrix lanuginosa, (c) Datura sp. (d) Ditaxis humilis 28

Plate 4.
Root habits of summer annuals, August, 1909. («) Dysoda papposa, flood-
plain of the Santa Cruz (b) Euphorbia glyptosperma, Tumamoc
Hill, (f) Kallstroemia grandiflora, Tumamoc Hill, (d) Pectis
prostrata, Tumamoc Hill, (e) Solanum elEeagnifolium, Tumamoc
Hill 28

Plate 5-

A. Root habits of summer annuals, (a) and (6), Aug., 1909. (a) Trianthema

portulacastrum, flood-plain of Santa Cruz, (ft) Vicia sp., Tuma-
moc Hill.

B. Root habits of winter annuals, (c) and (d), March, 1906 and 1907. (c)

Amsinckia spectabilis, Tumamoc Hill, {d) Daucus pusillus,
Tumamoc Hill 32

Plate 6.

Root habits of winter annuals, Tumamoc Hill, (u) Festuca octoflora. (6)
Gilia bigelowii. (f) Malva borealis, showing development of root-
system, ((i) Medicago denticulata. (e) Microseris linearifolia.. . . 32

Plate 7.

Root habits of winter annuals, Tumamoc Hill, (a) Monolepis chenopodioides.
(b) Orthocarpus purpurascens parasitic on Astragalus sp. (c) Or-
thocarpus purpurascens on Lupinus sp. (d) Parietaria debilis.
mature plants from favorable and unfavorable moisture condi-
tions, (f) Phacelia tanacetifolia. (/) Plantago fastigiata 36

Plate 8.

Root habits of winter annuals, Tumamoc Hill, March, 1907. (a) Sisymbrium

canescens. (6) Streptanthus californicus 36

Plate 9.

Root-system of Echinocactus wislizeni. (a) Portion of system showing an-
choring and absorbing roots in natural position, from West Wash.

(b) Bird's-eye view of roots from which overlying soil has been
removed, Tumamoc Hill 44

Plate 10.

(a) Root and shoot habit of a specimen of Opuntia leptocaulis growing under
the protection of Acacia constricta, Tumamoc Hill, (b) Optunia
versicolor, showing habit of plant and character of environment,

(c) Bird's-eye view of a portion of the central part of the root-sys-
tem of the plant shown in b. (d) This, and figures on plate 11, are
of one main lateral and some of its branches of the plant shown

above 44

Plate ii.

Plate 10, continued; bird's-eye view of root-system of Opuntia versicolor. . . 44

Plate 12.

(a) Vertical and (b) horizontal extensions of root-system of Opuntia versi-
color, views of which are shown in plates 10 and 11, Tumamoc
Hill, 1907 52

6 LIST OP PLATES.

I'ACIHO

Page.
Plate 13-

Root habit of Jatroptia cardiopliylla of which the horizontal extensions of the

rool-syslcin were shown in plate 12 52

Plate 14.

Horizontal and vertical extensions of root-systems of Carnegiea gigantea,

Covilka tridcntala. and Parkinsonia microphylla 56

Plate 15.

(a) The anchorinK roots and bases of sonic lateral roots of Carnegiea gigantea
6.8 in. high. {/>) Tap root and secondary roots making up the
anchoring system, and the bases of some of the superficial roots
of a specimen of Carnegiea 1.2 m. high from same habitat as cactus
shown in (a), (c) Tap root and bases of laterals of Parkinsonia
microphylla growing near Carnegiea shown in (6) and which

appears in plate 14 56

Plate 16.

(o) Covillea tridentata. Horizontal and vertical extensions of roots of this
plant arc shown in fig. 8. (6) Covillea from flood-plain near West
Wash, showing the large number of slender adventitious roots
springing from upper portion of the main roots and long tap root
of which only a i)art appears, (c) Fouquieria splendens from
bajada where Covillea shown in (a) was growing. Horizontal

extension of roots of this plant shown in fig. 8 64

Plate 17.

(a) Franscria deltoidea from flood-plain near West Wash, showing part
of tap root, (b) Lateral of the first order bearing filamentous
roots in groups, which, at time photograph was made, were no
longer functional, (c) Krameria canescens from the flood-plain
near West Wash, showing essential superficial placing of roots.

(d) Lycium andersonii from near West Wash 64

Plate 18.

Bird's-eye view of superficial roots of Opuntia arbuscula from bajada a mile

east of Santa Cruz, showing tieir fleshy character 68

Plate 19.

(o) Riddellia cooperi showing the large number of slender laterals arising
from crown of tap root, and the branching feature of the bases of
the laterals, (b) Riddellia from red clay soil, to illustrate lack of
slender roots at crown of tap root, (c) Dasylirion texanum, with
root-system partly exposed, (d) Shoot and root habit of Yucca

sp. (e) Root character of Yucca radiosa 72

Plate 20.

Root habit of Koeberliniaspinosa. (a) Isolated plant with prominent tap root.
(b) Several shoots arising from a single sucker which still keeps its
connection with parent tap root, (c) Adventitious absorption
roots arising from a sucker which has not formed an independent
tap root, (d) Secondary formation of tap root in a group with

connecting sucker root no longer living So

Plate 21.

(a) Peniocereus greggii showing fleshy main root and the most important
laterals, (b) Condalia spathulata. (c) Root-system of Prosopis
velutina partly exposed by the caving bank of the Santa Cruz.
The vertical distance from the ground surface to the water is
approximately 5 m. (d) Young plant of Prosopis. The squares
are centimeters, (e, f, g) Cuttings of Opuntia arbuscula (Tucson),
O. arbuscula (Sacaton), and O. vivipara, respectively, all grown

under similar conditions and with a large water supply 80

Plate 22.

(o) Cultures of Fouquieria splendens and of garden water-melon showing
differences in character and development of extreme types of
root-systems, (b) Shoot of Opuntia vivipara springing from a

fleshy root 84

Plate 23.

Brodiaea capitata grown in adobe clay (a) and in sand (6) in plant-house cul-
tures. Main root of clay-grown plant curved away from soiu-ce
of water supply 88

The Root Habits of Desert Plants.

INTRODUCTION.

It is now generally recognized that the arid and semi-arid regions are
especially favorable areas in which to study the habits of plants. This is
partly because the vegetation of the less humid countries represents the
most advanced type of land forms — that is, those farthest removed from
the primitive water-loving plants — and partly because, as the environ-
mental features are severe, so the response on the part of the plants is corre-
spondingly effective in order to bring about survival. The shoot habits of
desert plants have received considerable attention from botanists, but the
reaction of roots to desert conditions has, in a large measure, been neglected.
That this neglect is illogical and without good reason is apparent when it
is recalled that the connection between the plant and a most important
feature of the environment, the soil and its water content, is sustained
onlv through the roots.

The fact is well known that the roots of plants have a twofold function.
They at once afford safe anchorage and su[)port and at the same time are
the means by which water and inorganic food materials are acquired. The
dual nature of roots does not find separate organic expression in the most
primitive plants, and in certain of them, the alga?, organs answering to
roots serve the purpose of anchorage only. In the low land plants, the
roots, morphologically rhizoids, are, from this point of view, quite imdif-
ferentiated, but as the scale of plant life is ascended we find these functions
separated both in time and in space, at first on the same root and its
immediate branches, and finally there is differentiation in the root-system
of the plant, by which the anchorage is largely taken over by one set of
roots and the absorption by quite another set. The last is the advanced
condition found in the extreme xerophytic desert plants, such as many of
the cacti. Thus the extent and the character of root development will
reveal, in a measure, the degree of xerophily of a plant.

The prevailing idea that the roots of plants of the deserts, or of semi-
arid regions, are of great length, especially that they penetrate the ground
to great depths, doubtless has its origin largely in the belief that desert
plants are obUged to develop a deeply placed root-system in order to obtain
water during long dry seasons, and also in the few striking examples of
really long roots of plants of arid countries which are accepted as repre-
senting the root condition of all desert plants.

7

8 ROOT HABITS OF DESERT I'l.ANTS.

On this point Schimper (Plant-geography, Enghsh idition, \). 612) says:

This second category of desert plants exhibits its dependence on subterranean water
nearly universally by the immense length of its root-system, which the depth of the
level of the subterranean water renders vitally necessary. A considerable length of root
is, to a more or less extent, common to all desert plants and has attracted the notice
of all travelers.

This statement is substantiated by a quotation from Volkens, who says
(Die Flora der aegyptisch-arabischen Wiiste auf Gnmdlage anatomisch-
j)hysiologischer Forschungen, p. 7) :

Often as I have tried to dig up old bushes of perennial plants to the extremity of
their roots, I have never succeeded in doing so. The most that I could establish was,
that the root was thinner at the depth of one or two meters than at the surface of the
ground. One can safely assume that, in this case, the length of the sul)lerranean part
was at least twenty times that of the epigeous part.

\''olkens then goes on to say that certain species of Acacia were said to
have been seen at the time of the digging of the Suez canal, whose roots
were found in its bed, although the trees to which they belonged were
growing on eminences on its banks.

Isolated observations indicate that the roots of certain trees of the arid
regions of the southwestern part of the United States ma}^ under favorable
conditions, form long or deeply penetrating roots. Thus Prof. R. H.
Forbes, director of the Arizona Experiment Station, informs me that he
has seen roots of the mesquite {Prosopis vcluiina), by an irrigating ditch,
extend ver^' near the surface of the ground as far as 1 5 meters, and roots
of the same species, where exposed by the washing away of river banks,
which penetrated as deep as 8 meters. These figures may be taken as
probably representing the deepest root penetration in this vicinity, although
it is no unusual thing to see mesquite roots 5 meters in length. Dr. V.
Havard is quoted as writing that sometimes in the Southwest camps were
pitched on the plains where there was no fuel of any sort to be seen. It
is there that the frontiersman, armed with a spade, went digging for wood.
Speaking of the deep-penetrating roots of mesquite Havard savs :

Of the vertical roots, the tap root is the only long and conspicuous one. It plunges
down to a prodigious depth, varying with that at which moisture is obtainable. On
the side of gulches one can trace these roots down thirty or forty feet. (American
Xaturalist, vol. iS, p. 451, 1884.)

It will appear from facts given in this paper that the root-systems of
different perennials growing in the vicinity of the Desert Laboratory are
extremely variable as regards depth of penetration, lateral extent, and
other characteristics, and that no one type of root can be said to be the
prevalent one.

The opposite extreme in the position of the roots of desert perennials is
to be found in a highly specialized class, the succulents, in which the roots
are uniformly near the surface of the ground. Volkens, for example (Die
Flora der aegyptisch-arabischen Wiiste, p. 24), states that Euphoihia has

INTRODUCTION. 9

laterals which are superficially placed, and Weiss and Yapp (vSketches of
vegetation at home and abroad, III. "The Karoo" in August. The New
Phytologist, vol. v. May and June, 1906) have reported the formation of
superficial in addition to deep-growing roots in Mcscmbnanthemiim. In a
brief account of the habits of several cacti of Arizona Preston also describes
the shallow placing of the roots of several of them (Botanical Gazette, vol.
30, p. 348, 1900).

The dwarfing of the shoots of the perennials of the deserts, resulting from
excessive evaporation and inadequate water supply, does not indicate the
best conditions for maximum root development. It is probable, on the
contrary, that the longest or the most deeply penetrating roots are found
where there is considerable rainfall and where the penetration of the rain
is considerable and the water table relatively deep. In California, under
field conditions, the roots of the grape may reach a depth of 22 feet (Hilgard ;
Soils, p. 167), and in Nebraska the roots of Shcpherdia are said to attain a
depth of 50 feet (Merrill: Rocks and Rockwcathering, p. 181).

CERTAIN FEATURES IN THE ATTEMPTED CLASSIFICATION OF ROOTS.

From the point of view of this study any classification of root-systems
which has for its final aim a better understanding of the habits of plants
should receive attention, while those classifications which are purely sys-
tematic, although of value in other lines of research, may in this place be
neglected. Attention will be called, therefore, merely to the work of Rim-
bach, Biisgen, and Freidenfeldt, mainly as reviewed by von Alten (Wurzel-
studien. Hot. Zeit.,vol. 67, 1909, p. 175), which is to be largely interpreted
in physiological terms.

The researches of the authors referred to indicate that the root-svstems
of flowering plants may be separated into two groups according to the
character of the terminal roots — they are either intensive or extensive.
Intensive root-systems are such as have fine terminal roots ; they are richlv
branched and occupy a relatively small soil volume. Extensive root-
svstems, on the other hand, are such as have coarse ultimate rootlets, are
not richlv branched, and occupy a relatively large soil volume. An im-
portant additional distinction, advanced by Freidenfeldt, is that of the
diameter-quotient, or the relative diameter of the central cylinder of such
rootlets to that of the entire rootlet. Von Alten states that the diameter-
quotient in intensive root-systems is greater than in extensive root-svstems.
I-'or example, the difference may be from one-third or one-fourth to one-
fifth or one-sixth, respectiveh', for the two types. In addition, there are
structural diff'erences which need not be given in this place.

Freidenfeldt, in place of the term intensive, classifies plants having these
root types as xerophytes, and in place of the term extensive, classifies
plants with this type of root-system as hydrophytes. The beech is given
as an example of the former and the ash as an example of the latter, but

lO RdoT HABITS OF DKSKKT I'l.ANTS.

the tcrius xerophyte and hydrophyte denote more than intensive and exten-
sive, as von Altcn jwints out, and from a ])hysiological standpoint, the
latter terms are more useful.

How susceptible to modification, through a variation in the soil medium
or its water content, the two classes of roots may be, has not yet been
determined. It is believed, however, that the conservative inheritance
tendencies are more powerful here than the influence of changing environ-
ment, and that the types will remain essentially constant under whatever
condition they may be found.

Among plants with intensive form of root-system are: Mentha piperita,
Pi<)itiilis purpurea, Artemisia vulgaris, Imperatoria ostruthium. Plants hav-
ing the extensive form of root-systems include: Hellehorus purpurascens,
Solidago canadensis, Polysoma integrijolium, Valeriana officinalis, Arnica
cltamisouis, Rnnuneuhis acer. Adonis vcrnalis, and others.

SCOPE OF THE STUDY .4ND METHODS.

When it was proposed some time ago to take up the study of the roots of
desert plants, the work was laid down on broad physiological-ecological
grounds, and it was realized that as an introduction to such studies, which
would be in the nature of things mainly experimental, a knowledge of the
habits of annuals and of perennials afield was a prerequisite. As an exact
description of the root-systems of the most characteristic forms was wanting,
the prosaic task of excavating roots was undertaken, and the present paper
includes an account of the results of this work.

WTiile the plants selected for study have been such as arc presumabh'
representative of all tvpes, they do not include many which would be of
interest, but an account of which would in a degree duplicate results already
attained. Aside from the usefulness of the bare descriptions of root-
systems in later experimental work, they will probably be found of value
in comparative root studies, where the root-sj-stems of other arid regions,
in which the physical conditions are otherwise different from those of the
Southwest, are carefully worked up and mapped. ^Much interest has been
found in comparing the nature of the root-systems and their variability as
observed in the field with the leading and obvious characters of the environ-
ment, and it has been fotmd. as will be related later, that in certain instances
there is a verv clear relation between root type and plant distribution, as
well as between root type and other habits of the plant. The root-systems,
therefore, have been studied only in the field, and natural conditions h ve
uniformly been described.

In the course of the research different methods have been developed to
suit the particular form of roots studied. In the case of annuals, the entire
root-system, or as much of it as could be removed from the ground, was
measured and photographed, the photographs being somewhat under life
size.

IXTRODl'CTIdN. I i

The root-systems of perennials had to be studied in another way and
always in situ. After the earth above the most important roots was
removed, the root-area of the plant was considered as a square and was
surveyed in the following manner. Tapes divided according to the metric
system were stretched along the east and west sides of the imaginary square,
in a north-and-south direction, always at a certain distance, 1.5 meters
from the base of the stem. These tapes were firmly fastened to the ground.
A third movable tape was extended in an easterly' and westerly direction
so that it connected the two permanent tapes. This was also ruled metri-
cally. By means of moving the latter tape and noting its position on the
stationary tapes, the north-and-south as well as the east-and-west exten-
sions of the roots were learned with fair accuracy. The record was made
on a square of metrically ruled paper, each square on the paper correspond-
ing with an imaginary square of the root-area as delimited by the divisions
of the tapes. Wherever possible a reduction to one-tenth was employed in
sketching the roots and preparing the field charts.

The data on the temperature of the air and soil, on water in the soil, and
on its physical nature are taken from records made at the Desert Labora-
tory or from studies made elsewhere but now available at the Laboratory.
The soil temperatures are mostly from the continuous soil-thermograph
record which has been kept since 1904-05, and partly from thermometer
readings made by Dr. V. M. vSpalding. The other data on the soil are
mainly from studies by Dr. B. E. Livingston (Distribution and Movements
of Desert Plants, Carnegie Institution of Washington, Publication 113, pp.
83""93)- Acknowledgment should also be made to Prof. J. J. Thornber,
University of Arizona, for the determination of several of the plants studied,
as well as for much information on the distribution of the plants of the
domain of the Desert Laboratory (Vegetation Groups of the Desert Labora-
tory Domain, Carnegie Institution of Washington, Pub. 113, pp. 103-112).

ENVIRONMENT OF THE ROOTS.

LEADING CHARACTERISTICS OF THE SOIL.

In the vicinity of Tucson there are several sharply distinguishable phys-
iographic areas which have well-defined characteristics as to soils, water
and temperature relations, exposure and plant covering. For the present
])urpose these areas may be described as follows: (i) Tumanioc Hill, upon
which the Desert Laboratory is situated; (2) the flood plain of the Santa
Cruz river; (3) the bajadaor mesa*; (4) West Wash. In addition to these
areas a portion of the high bajada which lies about 15 miles east of Tucson
and the upper bajada slopes at the western base of the Rincon mountains,
an altitude about 1,300 feet above Tucson and 20 miles distant, were
observed. These areas are among the most sharply defined of all of those
of low altitude in the vicinity of Tucson.

TUMAMOC HILL.

Tumamoc Hill is a low mountain rising about 800 feet above the sur-
rounding bajada, and is an isolated member of the Tucson range. The
northern slope is fairly gentle ; the other sides are more or less precipitous.
. A wash or arroyo, which heads in the southeastern part of the mountain,
runs along the eastern base, and another wash, called here West Wash, is
along the western side. Bold outcrops of volcanic rock occur on the west,
south, and east sides particularly, and irregular masses of rock form descend-
ing steps along the northeastern face. In crevices of the rock and incrust-
ing thinly the surface of the upper rocks which are not exposed is a hardpan,
the caliche, which is practically impervious to water.

The soil of Tumamoc Hill is an adobe clay, malpais, which is derived
from the lava rock. It contains small particles of volcanic rock and of
caliche, about equaling the malpais in volume. The soil as thus constituted
varies greatlv in depth. In places it merely covers the rocks, while at
others it lies in pockets and may be 50 cm. or more deep.

FLOOD-PLAIN OF THE SANTA CRUZ RIVER.

The flood-plain of the Santa Cruz river at Tucson is about a mile in width.
Toward the eastern side of the plain the river has in recent years cut a
channel 3 to 5 meters deep ; formerly the water which washed down from
the mountains to the south, from the Tucson mountains, or from the bajada
to the east and north, spread over the entire floor of the flood plain. This
alteration of the river course has probably affected the water table of the
plain in a marked degree, causing it to sink to a level lower than that for-
merly occupied.

The soils of the plain have been exposed along the banks of the river so
that examination of them to the depth attained by the river is an easy

*The term bajada is used to designate the gently sloping masses of detrital material,
which depend from the bases of mountain masses, and form such a prominent feature
of deserts.

;•«»;«< ':

A. Upper soil, adobe clay, of Santa Cruz flood-plain, over 4 m. in
thickness at the place shown.

B. Adobe clay, the darker soil overlying the caliche hardpan with
"rotten" caliche, broken masses of the hardpan between. From the
bajada, 1 mile east of the Santa Cruz River.

ENVIRONMENT OF THE ROOTS. 1 3

matter; and through the digging of two wells, one on the pump lot and
one on the experimental garden of the Desert Laboratory, both within
70 meters of the western side of the flood plain, a soil section to a depth
exceeding 12 meters has been obtained.

Along the course of the river the soils are adobe clay and apparently
homogeneous to the depth examined. At the western side to the same
depth, that is, about 5 meters, the soil is of a character similar to that near
the river, but below 5 meters the adobe gives place to stratified sands and
gravels. Caliche was not found in the soils of the flood-plain to the depth
observed and probably none exists there.

WEST WASH.

West Wash lies along the western base of Tumamoc Hill and separates
it from the benches of the bajada which stretch farther west to the main
part of the Tucson mountains. It receives the drainage from the western
face of Tumamoc and from a small portion of the Tucson range. The
wash, for the present purposes, may be said to be differentiated into a
channel from 5 to 10 meters in width and a small plain which separates
the channel from the benches to the west.

The soil of the channel is a coarse sand which reaches to an undetermined
depth. The soil of the plain is a sandy loam to a depth exceeding 2 meters.
In both channel and plain the water quickly disappears from the surface ;
hence, as will appear repeatedly, the conditions of plant life here are mark-
edly difi'erent from those of the river flood-plain, as well as from those of
the other habitats to be described.

The only plant seen growing in the channel of the wash was a specimen
of Curcuhita digitata, with a fleshy root. Along the banks of the channel,
which are less than a meter below the level of the adjacent plain, there is a
fairly heavy growth of Acacia consiricta, A. greggii, Covillea tridentata,
Ephedra trifurca, Parkinsonia torreyana, Prosopis velutina, and Zizyphus
parryi, with occasional specimens of Echinocacius wislizcni. The flood-
plain of the wash contains all the species named as occurring along the
channel, but the growth is more scattering and the plants may be some-
what smaller.

THE BAJADA.

The bajada is the drainage slope of the mountains and constitutes the
mesa, or Covillea plain. In places in this vicinity it extends in a gentle
gradient, said to be about 4 per cent, for distances of 10 miles or more.
The slope of the bajada from the base of a mountain range, when viewed
in profile and at a distance, constitutes one of the most striking features
of desert topography. Wliere the slope of the bajada is short there are
practically no cross-drainage channels, but where it comprises a wide extent
of territory, physiography, soils, and other physical characteristics peculiar
to it are developed which serve to greatly increase the diversity of this

14 KOilT lIAlilTS i)K DliSlvKT PLANTS.

formation. No pliysiographic area of this vicinity lias so great diversity
as the bajada.

The portions of the bajada brought into this study comprise the upper
slope at the north base of Tumamoc Hill, the drainage slopes west of West
Wash, the high mesa about a mile cast of the vSanla Cruz river, a certain
locality about 15 miles east of Tucson, and the upper reaches of the bajada
at the west base of the Rincon mountains. The range of the bajada in
altitude at the stations mentioned runs from 2,500 feet, at the north l)ase
of Tumamoc, to about 3,500 feet at the west base of the Rincons.

The soils of the bajada are unHke in the localities mentioned, but a study
of them, aside from observations in the field, has been confined to that
portion which lies just north of Tumamoc Hill. The soils of the different
localities, however, have certain features in common, some of which are as
follows: The upper soil laver, to a depth of 30 cm., more or less, is of adobe
clay. Underlying the adobe is caliche, a calcareous hardpan, which extends
to an indefinite depth. The lower portion of the adobe, perhaps 10 cm.,
is composed of fragments of caliche and frequently of rock, and for con
venience is here referred to as "rotten" caliche. It is a common occur-
rence that the caliche hardpan is cracked, or is wanting in small areas, so
that the adobe which replaces it is consequently of considerable thickness.
Plate I, taken from a photograph of a cut in the bajada a mile east of the
river flood-plain, shows variation in the depth of the soil which had been
brought about in the manner indicated. The adobe is also of greater depth
where it has accumulated in depressions as a result of the erosion of higher
areas.

Although certain general conditions are shared by all of the bajada soils
examined, even a superficial examination and comparison of them shows
great differences. On the portions of the bajada where the distance from
the mountains is relativelv great, the adobe is practically homogeneous;
but near the base of the mountains, as at the north base of Tumamoc, it
may be shot through with fine rock fragments and caliche which may about
equal half the volume of the whole. On the slopes west of West Wash
there is httle adobe, and what is comparable to the rotten caliche of the
other parts of this formation there comes very close to the surface, thus
causing a larger percentage of caliche and rock fragments at this place.

A special characterization of the soil conditions in the habitats of each
plant examined will be given with the account of the root-system of the
plant.

MOISTURE IN THE SOIL.
TUMAMOC HILL.

The water relations of the plants of Tumamoc Hill are in part condi-
tioned by the configuration of the hill, in part by the character of the soil
and the vegetal covering, and in part by its relation to the rest of the
Tucson mountains.

Tumamoc Hill is an outlying spur of the Tucsons and is lower than
many peaks of this range, for which reason the rainfall on Tumamoc is prob-
ably less and the temperature probably higher than on the more elevated
parts of those mountains. Owing to its isolation, Tumamoc, although lower
than the rest of the range to which it belongs, does not profit from the run-
off from these mountains.

Tumamoc is fiat-topped and has a gently sloping northern side, where
there are two or more shoulders, and sharply descending eastern, southern,
and western faces. The soil is deepest where the slope is least, that is,
on the northern shoulders. Here, also, because of the slight gradient and
because of seepage and superficial run-off from higher portions of the Hill,
the water relations are most favorable.

Besides these factors the character of the rains, which are seasonal and
often torrential, and the want of a heavy plant-covering operate to render
much of the water which falls of no avail to the plants on the hill. The
rainfall and its amount and character for a period of 15 years at the city of
Tucson are given month by month in the following table.*

Table i. — Rainjall at Tucson {in Inches).

Month.

Rainfall.

Month.

Rainfall.

January

0.79

July

2.40

I'eljruary. . . .

0.90

August

2.60

March

0. 27

September. . . .

1. 16

April

n.14

October

0.64

May

0.14

November. . . .

0.87

June

0.26

December ....

1 .00

Thus the heaviest fall of rain occurs in midsummer and in midwinter,
with little or no rainfall between. The character of the rains of the tw()
seasons is imlike, that of summer is frequently torrential, while that of
winter comes with less force. Much of the rain in the summer season,
therefore, is lost as superficial run-off. Consequentlv, rain falling in equal
amounts in winter and in summer probablv produces imequal effects in
moistening the soil and in providing the plants with water.

The penetration of the soil by the rain is not a fixed amoimt.but depends
on a variety of factors, among which are the amount and duration of the
rain. The depth of penetration is usually from 2 to 5 times the registered

*Coville and MacDougal: The Desert Botanical I.aljoratory, Publication No. 6,
Carnegie Institution of Washington, 1903, p. 26.

15

l6 ROOT HADITS OF DKSKRT PLANTS.

precipitation, hut thf total ])enetrati()n naturally does not exceed the
thickness of the adol)e, that is, 50 cm. more or less, iisuallv le.ss.

The studies on the movements of the moisture of the soil, on which this
account is mainly based, comprise observations from October 3, 1907, to
April II, 1908, and include, therefore, only the dry autumn and dry earlv
winter, the winter rainy season and the early spring. .Studies on the condi-
tions for the remainder of the year are not now available. The special
studies were made on soils at two depths, namelv, 15 and 30 cm. The soil
was removed at frequent intervals, placed in stoppered bottles, and dried
in an oven at the laboratory.

The following summary gives the water movements during the period
under consideration, together with the rainfall recorded on Tumamoc Hill
at the time:

From October until the latter part of January 1.68 inches of rain fell,
and the curves of soil moisture at this time were fairly flat ; that for the
deeper soils fluctuated less than that for the shallow soils. Between
January 13 and February 11, 1.54 inches of rain were reported at the
laboratory. The soils on the first of February were the driest of the period
under observation. At the 15 cm. depth they contained 15 per cent water;
at the depth of 30 cm., 14.5 per cent (dry weight). During the period
between February i and February 1 1 , when the next soil obser\-ation was
made, 1.49 inches of rain fell. On February 11 the amount of water in
the soil at the 15 cm. depth was 34.8 per cent, which was the largest amount
at that depth found at any time during the season. At this time there was
19.1 percentwateratthe30 cm. depth. Between February 1 1 and March23,
0.87 inches of rain fell, in small amounts. During this period the amount
of water in the soil at the higher level had fallen to 18.9 per cent, but the
water content at the depth of 30 cm. had slightly increased.

Although the data for the year are not available to show it, it is fair to
assume that, as the dry summer approached, the soils at both depths
became more dry and reached their extreme desiccation in July when the
summer rains began. The per cent of water in the soils at the time is not
known, but observations on the soils at the laboratory- made 4 years pre-
viously in July, gave the following results: The water content was 17.83
per cent at a depth of 30 to 40 cm. ; at a depth of 40 cm. it was 15.8 per
cent of the dry weight of the soil. At the same time, at the 15 cm. depth
the moisture content was only 9.1 per cent. The significance of these last
figures becomes apparent when it is recalled that the minimum moisture
content of this soil which can be available to plants not possessed of water-
storage organs is 10 per cent its dry weight.

From these data, admittedly inadequate, we can understand some of
the characteristics of the environment and the conditions of growth of the
flora of Tumamoc Hill. The soil to a depth of 15 cm. probably does not
retain sufficient moisture for absorption by plants whose roots do not

MOISTURE IN THE SOIL. 17

reach deeper than this, for a period much exceeding six weeks following
storms. This defines the limit of life of most annuals, both those of winter
and of summer, and probably also the season of absorption of perennials
with shallowly placed roots. Plants having roots which reach to greater
depths than 15 cm. can obtain some moisture at all seasons. In order to
survive, seedlings must send their roots below 15 cm. within six weeks
following the close of a storm}' period.

THE BAJAD.^.

The water relations of the bajada are very diverse, because of differences
in soils, in topography, and in its relations to other physiographic areas.
The nearly impervious caliche hardpan which underlies the bajada every-
where prevents at once the deep penetration of the rains and the oppor-
tunity of tapping subterranean water. .Save where the bajada constitutes
a drainage slope from a higher area, and well-defined channels are not
formed, the only water available to the plants growing on it is what falls
upon it directly. The water table of the bajada varies considerably in its
position \vith relation to the surface of the ground. That in the vicinity
of the University of Arizona, about a mile east of the Santa Cruz river, is
approximately 25 meters deep. In other portions, where the general level
of the bajada is higher, the perennial water is considerably lower than this
figure.

The depth of the penetration of the rains is apparently entirely condi-
tioned on the thickness of the upper soils and on the presence of cracks or
rifts in the caliche hardpan which are filled with adobe. In favorable
places on the mesa, where there has been a considerable accumulation of
adobe soil, enough water is present to allow the growth of small specimens
of such perennials as are most abundant on the flood-plain, such as Prosopis
velutina, Acacia grcggii, and Ephedra trifurca.

Where the bajada nears higher areas, as Tumamoc Hill, the most favorable
water relations are to be found. This may be attributed partly to the
larger rainfall in such areas, but mainly to seepage and to superficial run-off
from the higher ground. The adobe soil from the bajada north of Tumamoc
Hill has an admixture of fragments of caliche and of rock so that its moist-
ure-retaining capacity is about 20.1 per cent of its dry weight. For this
reason it is air-dry, to a depth of 20 cm., most of the year, and the perennials
which are to be found on it, mainly CoviUea tridentaia and certain cacti,
must get their water during dry seasons from the rotten caliche stratum
and from the more deeply placed cracks in the underlying caliche itself.

During the period in which observations were made on the water content
of the soils of the bajada, October 3, 1907, to April 11, igo8, it was learned
that there was much less water in the soil on the bajada than in the soils
on the Hill. Soil samples were taken at two depths, 10 cm. and 20 cm.
From the beginning of the study until February 11, the water in the soil
at either depth was less than 10 per cent of its dry weight. On February

i8 KDiiT iiAHiTs < n" i)i:si;kt tm.ants.

1 1 the saiii|)k- at :i (Iri)lli i>i k. cm. had i 7. ; ]xr i\iit water, and that from
a (k'ptli of 20 cm. had i,s.() jK-r cciU its dry weight. Ten davs later the
percentages liad fallen to i 1.7 and 14.;,, respectively, for the two depths.
I'ollowinj,' this date the amount of water diminished rapidiv until, four
weeks after the maximum water content, i,t was no more than immediately
before. The readings on March 12 for the depths of 10 and 20 cm. were
7.9 and 9.5 per cent dry weight. This was considerably under the amounts
of water found on Tumamoc at the time and shows the bajada to be the
more arid of the two areas.

The period during which the adobe clay of the bajada at this situation
contained sufficient water to be of use to shallow rooted plants was thus
not more than three weeks of the year in question. This ])criod would
probably be somewhat extended and might be comparable to the Tumamoc
Hill soils in portions of the bajada where the soil contains less rock and
caliche fragments and also has tlie advantage of position near higher areas.
The brevity of the period in which tlie bajada soil contains sufficient water
for the growth of annuals, or of perennials without water-storage organs,
is therefore an important feature of the xerophilous conditions of this
physiographic area and indicates that it is the most intenselv arid of anv
examined.

THE WEST WASH.

Tumamoc Hill, as above noted, is situated apart from the main range of
the Tucson mountains. At both the east and the west bases of the Hill
are washes, of which the former heads in the hill, while the latter receives
drainage also from portions of the main range. The West Wash, therefore,
drains a large area and during seasons of heaviest storms carries a large
amount of water which may overflow' its banks and flood the bordering
plain.

The soil of the wash is coarse sand and allow-s water to percolate through
it very rapidh'. It is possible that there is a water table imderlying the
wash and its flood-plain, but this has not been demonstrated. The sandy
loam of the plain has a water capacity of about 25 per cent its drv weight,
or considerably higher than that of the upper reaches of the bajada.

The soils of the plain onlv have been studied ; the studies were made
from October, T907, to April, 1908. The studies on the loams of the plain
showed that up to Februar}^ 1 1 they* contained less than 10 per cent of
water. On Februarv 11 the soil at a depth of 15 cm. contained 14.9 per
cent. At a depth of 30 cm. the soil during the entire period did not con-
tain over 8.2 per cent water. On March 2 the soil at the lesser depth had
g.p, per cent and that at the deeper location 6.2 per cent water. April 1 1
the percentages of water for the two levels w^ere, respectively, 5.3 and 4.6.
The dryness of the upper soil is due to its sand-loam nature, which permits
the rapid sinking of water and its rapid evaporation. In this place the
soil is over 2 meters deep, and had studies been carried out on the deep

MOISTURE IX THI-; Sdll.. IQ

soil, as, for example, 2 meters beneath the surface, a much higher water
content would probably have been found.

The relation of the roots of the plants of the area will be spoken of later
in the paper, but it may be mentioned here that of non-fleshy forms only
those with deeply penetrating roots are to be found, and that only here
does a deeply penetrating root develop in a thoroughly normal and typical
manner.

FLOOD-PLAIN OF THE SANTA CRUZ RIVER.

The conditions of soil moisture of the river flood-plain are relatively
favorable for plants, perhaps the most so of any area under discussion,
which is partly owing to the character of the soil and partly to the com-
paratively level surface. In earlier times the flood waters covered the
plain, making of it a cienega ; but comparatively recently the river has cut
a channel and the only flood waters that go over the plain, at times not
inconsiderable, are derived from the adjoining bajada. The river channel
carries water only part of the year, and yet during seasons of storms it maj'
be a yellow torrent with great erosive power. The water table of the
flood-plain lies from about 5 meters, near the channel of the river, to about
12 meters at the western edge.

The top soil of the plain is an adobe loam with a water-retaining capacity
of about 38.5 per cent its dry weight, the largest retaining capacity of any
soils of the different habitats so far examined.

The digging of a well on the western side of the flood plain afforded an
opportunity of determining the water content of soil to a depth of 5.25
meters, although the upper adobe stratum did not attain a greater depth
than 5.10 meters. It was found that the water content increased with
depth to the limits of the adobe. Following are the determinations: At
the depths of 0.20, ,3.30, 4.00, and 5. to meters the water contents per dry
weight of soil were 12.9, 19.0, 22.6, and 23.1 per cent respectively. At the
depth of 5.25 meters, where sand was encountered, the water content was
7.1 per cent. It would appear from these determinations, therefore, that
sufficient water is present in the adobe, at whatever depth examined, to be
of use to plants, probably throughout the year. But, as will appear repeat-
edly in later portions of the study, the plants at the present time do not as
a rule actually penetrate to the deeper levels except only close bv the river;
for the most part, the roots are confined to the upper two meters of soil.

Determinations of the water in the plain soils were made from October
3, 1907, to April II, 1908, and revealed the fact that the moisture content
of the soil at both depths taken for study, 15 and 30 cm., was greater most
of the time than 10 per cent its dry weight. The sample taken February
II showed a moisture content of 26.00 per cent at the depth of 15 cm.
From this date until April 11 the water content gradually fell away until
it became 10.9 and 11.6 per cent for the two depths respectively.

TEMPERATURE OF THE SOIL.

The observations on the temperature of the soil have been confined to
two series, namely, to a continuous one by means of thermographs and to
a fairly large numbcrof rcadingsof thermometers. The thermometric read-
ings are for depths of i, 2, and 12 inches. The thermograph records date
from the simimer of 1905 and the spring of the following year, and give the
temperature for one station, close by the laboratory building, on the north
slope of Tumamoc Hill, and for two depths, 15 and 30 cm.

RECORD OF IHERMOGRAPH: 15 CM. DEPTH.

An almost continuous record of the temperature of the soil at tlie 15
cm. depth is at hand for the years 1905, 1906, 1907, 1908, and 1909. An
examination of these temperature graphs shows interesting features, some
of which will be noted.

The record shows an undulating line of which the curve-crests correspond
to the warmest for each day, and the depressions the coldest. The crests
for any record (each one is for 7 days) are remarkably uniform in height, as
also the depressions are uniform in depth. The difference between the
crests and the depressions is about 8°F., with 12° as the greatest variation,
which usually occurs in March and July. Owing to the lagging of the soil
temperatures, as compared with those of the air, the maximum at this
depth is not attained until about 6 p. m. and the minimum about midnight.

The uniformity of the daily temperature range is broken during stormy
periods, particularly in the winter season. In summer, the undulating
curve of daily variation, even during stormy periods, may be identified,
although its amplitude is greatly decreased. The extreme yearly range of
temperature for this depth, for any year since the records began, is 69° F. ;
and the extreme range for the entire period, 1905 to 1909, is 73° F. The
following table gives the greatest range in temperature for each year.

Table 2. — Maximum and Minimum Soil Temperatures at a Depth of 15 cm.

Year.

Maximum.

Minimum. |

Date.

Temp.

Date.

Temp.

1906. . . .

1907

1908. . . .
1909. . • ■

July 18
July 2
July 6
July 12

°F.

98

105

lOI

99

Jan. 3
Jan. 7
Dec. 22
Dec. 25

°F.

34

36

42-5

42

If we follow the course of the temperature record throughout the year,
we shall see that the period of greatest heat is during the latter part of
July, or immediately preceding the rainy season of midsummer. With the
advent of the rains, the temperatures suddenly decline, the decline con-

TEMPERATURE OF THE SOIL. 21

tinning until midwinter. As great a fall as 14° F. has been recorded in a
day. In January the temperature begins to rise, and the rise is gradual
until the last of March, when it becomes accelerated, so that by May the
soil approaches the temperature characteristic of early summer and is prob-
ably as great as that of the germinating period of midsummer. The course
of the temperature for the j^ear is fairly well illustrated by that of the
minima for 1908. From January 4 to March 29 the minimum lay between
50° and 60° F. ; between April 5 and April 26 it was between 60° and 70°;
in May the minimum was between 70° and 80°; in the first half of June
it was between 80° and 90°, and from then until July 1 5 the thermograph
did not record any temperature under 85°. With the coming of the sum-
mer rains the minimum began to fall and was near 80° until October 4;
it fell to about 60° until November 26. The minimum was about 50° until
December 14, and between 40° and 50° the remainder of the year.

RECORD OF THERMOGRAPH: 30 CM. DEPTH.

The soil temperatures for the depth of 30 cm. were recorded imder the
same conditions, except only the greater distance from the surface, as
those of the 15 cm. depth just described. The sensitive bulbs of both
instruments were only a meter apart. It needs but a glance at the records
of the two instruments to reveal their most striking difference : the daily
curve of the more deeply placed instrument has slighter undulations. In
addition, the annual variation of the two and the course of temperature
throughout the year are also markedly dissimilar.

The usual daily range in temperature of the soil at a depth of 30 cm. is
about 2° F. ; the maximum daily range for 1908, which may be accepted
as approximately the maximum for the period under observation, was a
drop of 4° in July. But as this followed as a result of the rains it may be
taken to represent the vmusual rather than the usual temperature variation
for the depth. The maximum temperature for the entire period, 1905-
1909, occurred July 3. 1907, when the thermograph recorded 99° F. ; the
minimum was on January 3. 1907, at which time the temperature was 44°.
The annual extremes in temperature are presented in table 3.

T.^BLE 3. — Maximum and Minimum Soil Temperatures at a Depth of 30 cm.

Year.

Maximum.

Minimum.

Date.

Temp.

Date.

Temp.

1905 ■■■■
igo6. . . .

1907

1908

1909

Aug. 7
July 23
July 3
July 4-7
July 12-13

°F.
97
95
99
96
96

Dec. 23
Jan. 4
Jan. 3
Dec. 21
Dec. 5

°F.

49
46

44
50
50

KiioT iiAnns 111' i)i:si:kt i-i.ants.

The coiirs*.' of tin- liin|Krattiri.- for tlic yi-ar at tin- (k|)tli (if t,o cm. is
somewhat (lilTriviit froiii llial at the ksser depth. This is graphically
shown l)y llie figure below, which was made from an inspection of the
records for the year 1908-1909, and is accurate only in general form, not in
detail, l)iit illustrates the facts. As fig. 1 indicates, the highest temperature
for the \ear is in July, or August, just prior to the rains of midsummer.
^^■ith the coming of the rains the lemiierature falls and continues to decline
until the middle of March, when it begins to rise and (|uickly reaches the
degree characteristic of summer. There is thus a grand maximum, which
precedes the time of the germination of the summer annuals and the active
growth of the summer perennials, and a grand minimum which follows the
most active period of vegetation of the winter season. Broadly speaking,
therefore, when the activities of the two large classes of plants are awakened,
there is seen to be a difference of approximately 30° F. in soil temperature
at the 30 cm. depth.

F
90°

70

60

50

A"ff-

St'p.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

Mi.\'

June

July

^^

"^

/

^

N

/

/

/

/

^-^

y

Fig. 1. — Temperature of the soil, at a depth of 30 cm., for the year 1908-1909.
Desert Laborator>\

SOIL TEMPERATURES : 2.5 CM. DEPTH.

The onlv available records of temperatures of the soil at less depths than
those above given are thermometer readings made at a station less than
50 meters from the location of the thermograph records given above. These
readings extend from March 22 to May 10, 1907, and owing to the rather
brief period covered by the thermometer record a discussion of it pet- se is
precluded. However, it will be of interest to compare the thermograph
record and that of the thermometer for the same days, as shown in table 4.

The most interesting facts revealed by the records are the differences in
temperatures which the soil show^s at anj' moment, and the relatively high
temperatures of the least depth. During the days of which the temper-
ature record is available the greatest difference of temperature between
the depths referred to is shown in the following summary; At the 30 cm.
depth the variation was 3° F. ; at the 15 cm. it was 1 1° ; and 2.5 cm. it was
40°. The greatest difference in maximum temperatures at any moment

TEMPER.\TURE OF THE SOIL.

was observed on April 15, when there was a variation of 2_V5° between the
upper two levels ; unhappily the record for the greatest depth for the time is
missing, or the difTerence would doubtless be much more. PVom these
records it appears that the roots of a plant which reach as deep as 30 cm.
may at one moment in springtime experience a range of temperature as
great as 22.50° F., and inother portionsof the vear probablva much greater
range.

T.\BLE 4. — Soil Temperatures at Different Defiths, March-Mav, 1907.

Date.

2.5 cm.

15 cm.

30 cm.

Thermometer.*

Thermograph.

Thermograph.

Min.

Max.

Min.

Max.

Min.

Max.

March 22

March 25

March 29

April 12

April 15

May I

May 8

May 10

°F.
63
5"
50
71
71
65
65
64

°F.

83.5

90

So
103
103.5
98
97
97-5

°F.

62.5

64

58. 5

75

74

75

76

76

"F.

88.5

75

69 ■ 5

85

80

84.5
84

° F

64
G6 . 5
63.5

75
76.5

77

°F.
67

67-5
64. 5

77

77.5

79

*Maximum and minimum thermometers were employed in the study.

SUMMARY OF ENVIRONMENTAL CONDITIONS.

The habitats, which have been studied in connection with the observa-
tions on the roots of plants inhabiting them, comprise mesa, or bajada,
and include locations on the flood-plain of the Santa Cruz river, the West
Wash (near Tumamoc Hill), and on Tumamoc Hill. The studies on the
soils, the moisture in them and their temperatures, adopted from various
reports, have been limited to Tumamoc Hill, or the bajada and Hood-plain
near by.

The surface soils are an adobe clay with a varying admixture of rock
fragments, either of volcanic origin or of caliche, and the leading apparent
differences in the soil are dependent on the amount of this coarser material
present. The upper soil is usually from 15 to 30 cm. in thickness. Beneath,
to a considerable but varying depth, lies a hardpan caliche. Between
the upper stratum and the hardpan is an intermediate zone in which there
are large fragments of caliche, or other rock, mingled with adobe. The
intermediate stratum is usually not more than 20 cm. in thickness. The
total soil generally available for the roots is thus 50 cm., and frequently
(usually on the bajada) it is much less than this.

The depth of perennial water varies with the habitats, although uu
Tumamoc Hill no water table exists. On the bajada it lies 25 meters or
more beneath the surface, and on the flood-plain of the river the water table
is 5 meters deep, or more.

24 KiioT IlAlilTS HI" DHSIiRT IM.ANTS.

The rainfall on each of the are^, in the vicinity of Tucson, is approxi-
mately the sanu-, l)ut the differences in the mutual relation of the areas,
till- character c)f the soil, and its depth, as well as the surface configuration,
operate to make the water relations of the habitats very unlike.

The flood-plains of the Santa Cruz river and of West Wash, in addition
to the rains which fall on them directly, receive the rmi-off and seepage
waters from the bajada, and the bajada, in turn, those from Tumamoc Hill
or other higher elevations. In former years at Hood-time the Santa Cruz
river overran its banks and covered the bottom lands, but now the river
has cut a deep channel and Hooding from tliis cause never takes place.
The change is probably associated with the cutting of the mesquite forest,
which was once a rather heavy one, and the conversion of the bottoms
into ranches or waste lands, with second-growth mesquite.

The water-retaining capacity of the soils of the river flood-plain is the
highest, below which should be placed the adobe of the bajada, at the north
base of the Hill, with the loamy sand of the flood -plain of West Wash as the
least.

So far as is known from studies on the water content of the superficial
soils, the amount of water present during the dry autuinn and winter on
the bajada north of Tumamoc and on the flood-plain by West Wash is
insufficient to be of direct benefit to the plants whose roots do not penetrate
deep, wliile at the same time the soils of Tumamoc Hill and of the river
flood-plain have moisture present in usable quantity.

The period of optimum moisture content of the upper soil includes the
rainy period and a relatively short time after the beginning of the dry
season. The upper levels dry out soonest, and on the bajada were air-dry
three weeks after the close of the rains, while the soils on the river flood-
plain and Tumamoc Hill remained moist for a period exceeding six weeks.
This places the limit to the growing period of most annuals and the most
active vegetative period of all perennials without water storage capacity,
or deeplv penetrating roots.

The temperature of the soil has been obserA-ed on Tumamoc Hill only,
and chieflv at 13 cm. and 30 cm. The highest temperatures immediately
precede the summer rains. With the advent of the rains the temperature
falls quicklv and continues to decline until January-February, at a depth
of 15 cm., or March-April at the greater depth — 30 cm. The time of the
germination of the summer annuals and of greatest vegetative activity of
the perennials, therefore, is below the maximum heat, as that of the winter-
spring plants is above the minimum.

The dailv fluctuations of temperature for the two depths is unlike : that
at the greater depth is about 2° F., while that at the lesser depth is about
8° F. The yearly range at the 15 cm. depth is about 34°, and that at 30
is about 44° F., with the maxima at 99° and 105° for the two depths
respectively.

CANNON

a. Amaranthus palmeii, Tuinanioc Hill, Aiiji^ust 10, 1909.
b Anoda thurberi. Tumamoc Hill, August 10. 190!).

c. Aster tanacetifolius, Tumamoc Hill, August 2(), 1909.

d. Aster tanacetifolius showing, at left, root-formation at base of lateral of
first order. The figure at right has se.gment of tap-root bearing lateral of first
order with its branches.

ROOT HABITS OF DESERT PLANTS.

In presenting descriptions of the root-systems of the desert plants, it
seems best to be guided by the biological grouping — that is, to bring plants
together which are naturally segregated either in time or in space. Accord-
ingly perennials which are typical of the leading habitats treated are
grouped under the respective habitats, but since the annuals of winter and
of summer occupy the same areas and are separated only by time they are
divided into winter and summer forms. By summer annuals is meant
those which appear in midsummer, and by winter annuals those which
come after the winter rains, although some of them might more trulv be
called spring annuals than those of winter.

SUMMER ANNUALS.

In early July the different habitats show only perennials, and the bare
ground between them shows the dried remains of the annual vegetation of
the preceding rainy season. With the coming of the rains of late July or
August, however, the appearance of the land is suddenly changed. vSeeds
which have lain dormant for nearly 1 1 months promptly germinate, and
hill and plain arc speedily clothed with a growth of evanescent forms which
are in great variety and, for desert forms, of surprising densitv.

The study of the summerannuals was carried on in the following manner.
The roots of typical forms, mostly mature, were carefullv washed out by a
small jet of water, and the entire plant, or as much as was recovered bv this
means, was preserved in weak formaline for subsequent studv. By this
method the entire root-systems were rarely to be had, but enough was
always saved to give the general character. More than one plant as a rule
was preserved of each species so that a control on the observations might be
had. A very serious drawback to this method of study lay in the impos-
sibility of surely determining the extreme depth to which the main root
(or the laterals) penetrated the soil, although in manv cases this feature
was learned with close accurac}'. The following summer annuals were
examined :

Amaranthus palmeri S. Wats. Dysoda ])apposa Lag.

Anoda thurberi Gray, Euphorliia glyptosperma Engelm.

Aster tanacetifolius HBK. Kallstrremia grandiflora Torr.

Boerhaavia sp. Pectis prostrata Cav.

Cladothrix lanuginosa Nutt. Solanum eUtagnifolium Cav.

Datura sp. Trianthema portulacastrum L.
Ditaxis humilis (Engelm. and Gray) Pax. Vicia sp.
(perennial).

Am.\ranthus Palmeri.

The species of Amaranthus studied was growing near the Laboratory
building and was removed from the soil on August lo, at which time the
shoot was 21 cm. long. The plant was not in flower.

The root-system consists of a tap root and several large laterals which
bear filamentous branches. The roots do not intergrade in size, certainly

26 KodT IIAIIITS i)K UKSKRT PLANTS.

not ill (liaiiKlir. Only id cm. dI the main root was recovered, but it inav
have been much longer than that, since, where it was broken, it was 0.5 mm.
in diameter. The root was 4 mm. in diameter at the crown. I'rom the
main root 5 leading laterals take their origin. These are coarse and over 25
cm. long. The laterals of the first order bear numerous long delicate roots,
which in turn arc branched. The ultimate roots are long and filamentous,
but rudiments, mere root tips, are also very numerous.

The root-system of Amarajitlius thus is characterized by two classes of
roots, coarse and fine, without intcr^jradation. and by the presence of rudi-
ments (plate 2).

AnOD.V TlIfRBKRI.

The specimen of .4)!()</(7 which was studied was procured August 10; it
was an unbranched shoot 22 cm. in height and bore several large leaves.
The root-system is characterized by a prominent main root and several deli-
cate secondary ones. The tap root was over 18 cm. long and was forked.
From the tap root arise numerous laterals, mostly within 4 cm. of the
surface of the ground, of which the largest is over 6 cm. in length. The
laterals of the second order bear filamentous branches. Xo rudiments are
present. As a rule the roots arise singly, although a few of the ultimate
ones are in groups (plate 2).

Aster Taxacetifolu's.

On August 26 the roots of Aster were removed from the soil. The plant
has a dense habit of growth and bears many leaves. The shoot was 25 cm.
high. The tap root is especially well developed. It is 4 mm. in diameter
at the crown, and over 15 cm. was recovered. The laterals form a dense
tuft 3 to 6 cm. beneath the surface of the ground and are uniformly slender.
There are no rudiments.

A peculiarity, not observed in other species, was the place of origin of the
laterals of the second order. All of the laterals of the first order, which
were in the dense tuft referred to, were short — that is, less than 3 cm. long —
and the tips were dead, but they- all bore long and delicate branches, as
shown in plate 2 . The cause of this was not definitely ascertained , although
laterals with branches near the somewhat enlarged base were seen with
the tips yet living, so that the condition noted mav be the usual occurrence
in the species. The ultimate roots of Aster are of the fourth order.

Below the tuft of absorbing roots just described are scattered slender
laterals arising from the main root. The root habit, with the relation of
the laterals to the main root, is shown in plate 2.

BOERHAAVIA SP.

The specimens of Boerhaavia examined were from Tumamoc Hill near the
Laboratory building and were washed out August 10. The shoot of the
plant to be described, which was in bud, was r 2 cm. high and bore numerous
large leaves.

ROOT HABITS OF DESERT PLANTS. 27

The main characters of the root-system are the relatively large size of the
laterals of the first order and the paucity of filamentous roots. The ulti-
mate roots are of two sorts, a longer and a shorter kind. The latter are
rudiments and are borne in the axils of the former; they are shown, but
imperfectly so, in plate 3. The plant thus shows a tendency to produce
roots in groups.

Cladothrix Lanuginosa.

The specimens of Cladothrix studied were removed from the soil on August
19, and were mature plants with large leaves and shoots about 19 cm. in
length. The main root was over 2 mm. in diameter at the crown and was
6 cm. in length with the tip atrophied, but it was continued 1 7 cm. farther by
two successive laterals, so that the depth attained was over 2,3 cm. The
laterals are numerous and rather fine and for the most part arise within 4
cm. of the surface of the ground. They bear laterals of the second order,
which, in turn, are branched. The ultimate roots are represented in very
numerous cases by rudiments.

As a whole, the root-system of Clailotltn'x is characterized by the slender-
ness of its roots, a large number of which are filamentous. Root hairs are
present in great abundance. The absorption faculty, therefore, is a feature
strongly developed.

Datura sp.

The specimen of Datura studied was collected on August 10 and was
immature. The shoot was 15 cm. in length and bore several large leaves.
The root-system is characterized by the very large number of fine roots,
which are usually long. At the crown the main root is 4.5 mm. in diameter,
but tapers rapidly and is under 10 cm. in length. At a point 8 cm. beneath
the surface of the earth it was less than 0.5 mm. in diameter. The laterals
are all long and of two diameters, a few are noticeably heavy, and many
are very slender. All of the laterals of the first order bear filamentous
branches which are long and branch but little. The roots of the third order
are especiallv fine. Plate 3 shows only imperfectly the density of the
absorption system of Datura.

Ditaxis Humii.is.

The species of Ditaxis examined is a perennial herb, which was in fiower
when collected, August 10. The shoot is leafy and over 12 cm. long. The
roots reach deeper than those of the other herbaceous forms studied and
the absorption system is poorly developed. The main root is relatively
slender; it is over 23 cm. in length and is only 2.5 mm. in diameter at the
crown. Four large and numerous small laterals of the first order arise
within 8 cm. of the surface of the ground. Very few laterals are given off
from the main root below this. The laterals may extend 1 1 cm. and over,
and mostlv are in groups of three or four. The laterals of the second order
are filamentous and rarelv branch. There are no rudimentary roots.

28 R(J()T IIAIIITS OF DliSIiKT ri.ANTS.

Dysoda Papposa.
Specimens of Dysodii in llowir were procured August 26. The shoots
were 12 to 15 cm. in length, nuich branched, and well covered with leaves.
The tap root dominates the root system. This evidently reaches deep,
although unfortunately but 10 cm. of the root was uncovered. The laterals
are filamentous and are given off from the main root somewhat farther
from the surface of the ground than is usually the case in summer annuals.
The laterals of the first order bear branches and these also are branched,
but the entire extent of the filamentous roots is relatively slight (plate 4).

EuPHORBI.\ Gl.VPTOSPERMA.

Euphorbia glyptospcima is most abundant on Tumamoc Hill, where the
specimens examined were collected August 26, but, like most of the other
annuals it also occurs in the other habitats in the vicinity of the I.aboratorv.
The specimens studied were about 7 cm. high, much branched, with a large
leaf surface, and in flower. The roots grade in diameter from tlie main root,
which is somewhat over i mm. in diameter at the crown, to the ultimate
filamentous ones, although as a whole the roots are relativelv coarse. The
laterals of the first order arise for the most part within 3 cm. of the surface
of the ground. They occur singly ; no rudimentary roots were obser\-ed.

Kallstrojmi.v Graxdiflor.\.

Kallstrcemia is one of the less abundant but most striking of the summer
annuals. It is of a semiprostrate habit, which varies with its stage of
growth or with the conditions under which it has developed. If the summer
rains are slight, so that the period of growth of the plant is short, its habit
is nearly upright, but should the stormy season be prolonged, or should
rains occur after those of early summer have passed, and before the plant
dies, growth is renewed if the latter is the case, or in the former case growth
is so active that the plant becomes vine-like and prostrate. Under such
favorable circumstances it may cover as much as 16 square meters of
surface, and perhaps even more than this. The plants whose roots were
removed either were young or had attained the first stage of growth de-
scribed above, that is, they were upright and mature.

The plant to be described was collected on Tumamoc Hill on August 10.
The shoot consisted of two branches and several compound leaves and more
numerous flower buds. It was 27 cm. long.

The root-svstem consists of a main root, with a relatively stout crown,
and one long and one short branch, together with slender laterals, which
are mainlv close to the surface of the ground, and their proper branches.
The longest lateral is over 21 cm. in length and bears two branches over 10
cm. each. ]Most of the laterals of the first order and all of the higher orders
are filamentous. The main root penetrated the ground more than 22 cm.

The leading features of the root-system, which is a rather extensive one,
are shown in plate 4.

a. Boerhaavia sp. b. Cladothrix lanuginosa, c. Datura sp. d. Ditaxis humilis.

CANNON

a. Dysoda papposa, flood-plain of Santa Cruz. b. Euphorbia glyptos]ievma,
Tumamoc Hill. c. Kallstroemia grandiflora, Tumamoc Hill. d. Pectis
prostrata, Tumamoc Hill. e. Solanura elseagnifolium, Tumamoc Hill.

root habits of desert plants. 29

Pectis Prostrata.
The specimens of Pectis studied were growing on Tumamoc Hill near the
Laboratory building. They were from 7 to 12 cm. in length and in flower.
The general habit of the plant is shown in plate 4. The main root is very
delicate and bears several long laterals within 3 cm. of the surface of the
ground. The longest of these were over 13 cm. and bore numerous branches,
which also were branched. The root-system as a whole may be character-
ized as being little branched, the laterals of all orders are relatively few,
and composed of delicate roots only.

SOI.ANUM El^AGNIFOLIUM.

Immature plants of Solannm were collected August 22, of which one had
a shoot 6 cm. high and bore several large leaves. A tap root dominated the
root-system ; it was traced over 15 cm. Laterals were given off at occasional
intervals along as much of the main root as was examined, but they were
mainly confined to the upper 4 cm. The laterals of the first order are fine
and bear filamentous branches. There are also roots of the third order
present. The roots arise singly; there are no rudiments. The leading char-
acteristics of the root-system are the rather stout tap root and the delicate
laterals, with no roots of intermediate diameter between (plate 4).

Trianthema Portulacastrum.
The specimens of Trianthema which were studied were collected on the
western edge of the Santa Cruz flood-plain, August 26. The shoot of the
plant to be described was about 1 7 cm. high and bore 9 branches and num-
erous large leaves, and therefore possessed a large transpiration surface.
The root-system is characterized by a preponderance of coarse roots.
The tap root is 3 mm. in diameter at the crown, is over 15 cm. long, and
bears (within 4 to 5 cm. of the surface of the ground) 5 prominent laterals,
the longest of which was more than 17 cm. At intervals of about i cm. the
laterals of the first order bore short filamentous roots and rudiments of
roots. The number of rudiments is large, a feature which distinguishes
this plant from nearly every other one examined. The rudiments appear
as faint dots along the laterals in plate 5.

ViCIA SP.

The specimen of Vicia whose roots were examined was growing near the
Laboratory and was collected August 10. The shoot of the plant was
mature and bore fruit and flowers. The specimen studied bore 7 branches
and numerous linear leaves. The root-sj-stem is characterized by a prom-
inent tap root, which was 2 mm. in diameter at the crown and over 20 cm.
long, and large laterals arising rather far beneath the surface of the ground.
A portion of the laterals of the first order are in .groups of 2 or 3, although
they mainly arise singly. The laterals branch but little and leave the
impression that the absorption system of the plant is a rather meager one.
A few tubercles are borne on the larger laterals.

,%<) Kill IT iiAiin^ 111' iii>i:k'T plants.

WINTER ANNUALS.

lietwccii tluinniulis of November and .Marcli, andoccasionallyonem<JiiUi
before and one month after that period, if the moisture conditions are
favorable, a large number of annuals make their appearance in the different
habitats considered in this study. These are for the most ])art of different
genera and species from those which occur in stunmer, and like the latter
have to a large degree a generalized type of distribution. Often the same
species of winter annuals are to be found wherever the soil and the water
conditions are suitable irrespective of the habitat, whether Tumamoc Hill
or the lower-lying bajada, but they may be present in greatest profusion on
the lower slopes of the Hill. The following winter annuals were examined :

.\nisinckia spcctahilis Fischer and Meyer. Hordcum murimim L.

Astragalus nullallianus DC. Malva borealis Walliii.

Bowlesia loliata R. and P. lIalacolhri.\ clevelandi Gray.

Brodia>a capitata Benth. Medicapo denticulata Willd.

Daucus pusillus Michx. Mentzelia alliicaulis Dougl.

Krilrichiuni pterocaryum Torr. Microseris linearifolia Gray.

Erodium cicutarium (L.) 1,'Her. Monolepis chenopodioides Moq.

Festuca octoflora Walt. Orthocarpus i)urpurascens Benth.
Gilia bigelowii Gray. var. palmeri Gray.

Harpagonclla palmeri Gray. Parietaria debilis Forst.

Amsinckia Spectabilis.

Aiiisinckia spectabilis is one of the most abundant of the winter annuals
and, as far as its local distribution is concerned, one of the most generally
distributed. It also is one of the most drought-resistant, a condition
which is illustrated b}- the following note on its behavior. In the spring of
1907 one of the staff of the Desert Laboratorv was studying the invasion of
plants into a denuded area. On March 19 the storms of winter were over
and the arid conditions were rapidlv becoming marked ; on that date the
temperature out-of-doors in the shade was 94° F. with a relative humidity
of 20 per cent. The winter annuals were for the most part already dead.
On the periphery of the denuded plot mentioned there had been a heavy
growth of Sisymbriitvi reflexion, Bou^lesia lobata, Aiiisinckia spectabilis, and
Phacelia tanacetifolia. Of these annuals the first two were nearly all dead
and were shedding their seed , while the last two were apparently unaffected
by the drought, though all of these plants were apparently under similar
conditions.

Amsinckia has a deeplv penetrating main root and numerous laterals,
all (if which, especially in the young plants, are slender and branch but
little. The root-system is extensive rather than intensive.* As the plant
becomes older the tap root becomes heavier, and the difference in diameter
between it and the laterals is somewhat accentuated.

As in the summer annuals, the length of the main root as given in this
paper is usually somewhat less than the actual length, owing to the difR-

*\\'here roots are thus characterized it is not intended to classify them after Busgen,
unless so stated, but the words are merely used in a descriptive sense.

ROOT HABITS OF DESKRT I'l.AXTS. 3I

culty in removing the root-system and keeping it intact. However, great
care was always taken to obtain as much of the roots as possible. A shoot
of a mature plant of Amsinckia 13.5 cm. in length had a tap root over 8 cm.
long; a plant with a shoot 18 cm. long had a tap root more than 12.5 cm.
in length. Thus, although the greatest penetration was not learned, it was
sufficient to enable the plant to reach and to tap a relatively good water
supply at a time when the upper layers of the soil were too dry to provide
a sufficient amount of water for plants whose roots were entirelv restricted
to the surface soil la vers (plate 5).

Astragalus Nuttalliaxus.

In this vicinity Astragalus grows under a large variety of conditions as
regards exposure, kinds of soil, and relations to other plants. It is practi-
cally cosmopolitan in its local distribution. A mature plant with a shoot
6 cm. long, together with young specimens, was preserved for examination.
The leading character of its root-system was the well-developed tap root,
13 cm. in length. The laterals were few and coarse, and little branched.
Tubercles were present in considerable abundance.

BOWLESIA LOBATA.

Bowlesia is to be found on Tumamoc Hill and on the lower detrital slopes,
and usually occurs under the protection of a shrub or tree or in other situ-
ations where the water relations are relatively favorable. The leaves are
large and the plant has the appearance of being ill-adapted to successfully
withstand very severe arid conditions. The root-system is not an extensive
one, and is characterized byhavingamain root and lateralsof the first order
of about the same diameter and by the relative coarseness of all the roots.
The main root does not go straight down, but zigzags downward in a ^•erv
irregular fashion. From the main root there arise, at intervals of about a
centimeter, rather coarse laterals which branch sparsely. It seems prob-
able, although not proved by actual experiments, that the restricted dis-
tribution of Bowlesia may be directly associated with the shallowlv placed
and poorly developed root-system of the plant.

Brodi/Ea Capitata.

Brodiixa is restricted in its distribution to Tumamoc Hill and to the up]3er
bajada slope, and does not occur in the other habitats, namelv, vSanta Cruz
flood-plain or that by West Wash, or on the bajada as a whole. It is a
bulbous perennial with the vegetative and flowering periods in the winter
season. Its bulbs are somewhat over i cm. in diameter, and are 5 cm.,
more or less, beneath thesurfaceof the ground. Therootsare adventitious,
coarse, and unbranched. They are relatively few and are usually more
than 5 cm. in length. The depth to which the roots of the plant penetrate,
therefore, is approximately 10 cm.

32 ROOT I1AHITS OK DICSICKT PLANTS.

Xo forms of roots otlur than that just sketched were seen in i\w Ikld, hut
dimorphic roots aiJiH-and in Knenhmise cultures. Although this will be
referred to later, it will he well to diserihe these roots in this place. The
conditions under which the unusual form of roots occurred were as follows:
Among preliminary experiments, looking to a study of the aeration of
roots, was one wliich was arranged with two kinds of soils, namely, sand
from a wash and line adobe clay (sifted) from the river bottoms. The bulbs
were planted, or were intended to be planted, on the line, which was a verti-
cal one, separating the two kinds of soil, but on removing the plants at the
end of the experiment one bulb was found to have been jjlanted in sand
and the other in adobe, about i cm. in each instance from the opposite kind
of soil. The culture was running during a portion of November (1008)
and all of December, which was a rather cold period and unusually cloudy.
The green-house was not heated.

When removed from the soil, the plant of the adobe side of the culture
had two leaves, 25 cm. long. The bulb had nearly or entirely disappeared
and in its place was the fleshy base of a stout root from the upper end of
which, or from the portion of the bulb remaining, was springing the usual
type of roots. The relation of the two is shown in plate 23. Of the
ordinary type of roots there were 13, which ranged from 2 to 1 1 cm. in
length and were wholly confined to the adobe side of the culture. The
central fleshy root was about 10 cm. long. It was i.i cm. in diameter at its
crown and narrowed rapidlyas it approached the tip, whichwas attenuated.
The position occupied in the soil by the root was peculiar. In place of
going straight down — as did the root of the sand-grown plant — or of taking
a horizontal position, like the other secondary roots, it went downward but
at the same time inclined sharply away from the source of water supply,
and ended in a curl as shown by a figure of the plate.

The plant on the sand side of the culture in certain particulars behaved in
a manner somewhat different from that described for its adobe-grown fellow.
The 2 leaves were 22.5 cm. long. The bulb was not resorbed, and from
it there sprang two sorts of roots, the relations of which are indicated in
plate 23. There were 7 rather coarse absorption roots, from i to 7 cm.
long, and a single fleshy root. All were of secondary origin ; the fleshy root,
however, as in the case of the plant described above, sprang from a point
on the bulb very near its base and went straight down from the bulb ; it
was 7.2 cm. long and 0.4 cm. in diameter at the crown. None of the roots
entered the adobe side of the culture.

An explanation of the transformation of fibrous into fleshy roots of
BrodicBa will not be attempted in this place. It may, however, be associated
with the unusually large amount of water available to the two plants rather
than to unfavorable conditions of aeration. A parallel instance, referred to
later, was observed in cultures of several arborescent cacti, inwhich the for-
mation of tuberous roots in species which do not normally produce tuberous
roots was induced in cultures conducted much as that just described.

CANNON

K

a

a. Trianthema portulacastrum, flood-plain of Santa Cruz. b. Vicia sp.
c. Amsinckia spectabilis. d. Daucus pusillus.

CANNON

a. Festuca octoflora, March, 1908. b. Gilia bigelowii, March, 1907. c. Malva
borealis, showing development of root-system, February, 1907. d. Medicago
denticulata, February, 1907. e. Microseris liuearifolia, March, 1907.

ROOT HABITS OF DESERT PLANTS.

Daucus Pusillus.

Daticus l?usillus, one of the most generallydistributed of the winter annu-
als, is abundant on both Tumamoc Hill and the upper portion of the bajada
north of the Hill. Its root-system can be characterized as being extensive
rather than intensive. The main root dominates the root-system, although
it is never fleshy, or even very coarse, and the laterals are relatively few
and branch but little.

Young as well as mature plants were studied. The tap root of a plant
whose shoot was i8 cm. long and which was in flower was over 15 cm. long.
Just beneath the surface of the ground, where the tap root was sHghtly
over I mm. in diameter, several fine roots, but not filamentous, were given
off' about 3 cm. from the surface of the ground, and 3 to 4 cm. deeper a prom-
inent lateral, 7 cm. in length, had its origin. Roots of the second order were
few in luimber. As plate 5 shows, there is no great difTerence in diameter
between the main root and the laterals, or between the laterals of the first
and the second order.

The leading feature which marks the mature plant is characteristic of the
young plant as well. Thus a seedling with seed leaves and only one leaf of
the adult type had an unbranchcd tap root over 5 cm. long, while another
of about the same age had a tap root over 6 cm. in length, which every 3 to
12 mm. bore laterals about 2 mm. long. This type of root-system was
seen also in a somewhat older plant. It may be concluded, therefore, that
Daucus not only penetrates the ground deeply (how deep was not deter-
mined), but also quickly, and is thus admirably adapted to endure after
more shallow-rooted annuals have perished .

Eritrichium Pterocaryum and Harpagonei.la Palmeri.

Eriinc/! »()» and H(7;7'a(/oKc//a, two boraginaceous winter annuals, although
relatively small, usually considerably under 15 cm. in height, and thus
inconspicuous as to individuals, occur in such great numbers that they
constitute an important element in the plant covering of their favorite
habitats. These are common on Tumamoc Hill as well as on the upper
portion of the bajada to the north of the Hill. Plants of difi"erent ages
and from diff'erent localities were examined. A mature specimen of Eri-
trichimn, with the shoot 13 cm. long, had a filamentous tap root which
penetrated over 8 cm. The laterals were borne in the region between 3 and
6 cm. from the surface of the ground, and were 4.5 cm., and less, in length.

The root-system of Harpagonellawas somewhat better developed ; plants
from dry and moist situations were stvidied. Those from the drier locations
had unbranched shoots 7, g, and 10 cm. in length. The roots of the smallest
plant penetrated over 1 1 cm. The laterals were given oft" from the main
root from i to 4 cm. beneath the surface of the ground. The specimen of
Harpagondla from the more moist soil, with a shoot 19 cm. long, had a tap
root which was followed over 15 cm. The laterals, like those of the plant
from the drier situation, were unbranched.

34 KDdT iiAnns hf dicshkt plants.

IvKliDir.M ClCl"TAKIl-M.

luodium cicutaiium, wliicli is an iiilroduci-d species, lias alrcadv a wider
distribution in this vicinity than most native plants and promises to become
one of the most successful of the winter annuals (.Spalding, Distribution and
Movements of Desert Plants. Carnegie Institution of Washington. Publi-
cation 1 1,",, 1909, p. 27). Although its root-system was not studied closely,
there are some points of interest that may be referred to. The dominant
features of the root-system are the prominent tap root and the yjoor devel-
opment of the laterals. How deep the tap root penetrated was not accu-
rately learned, but its length is believed to equal that attained by the roots
of uiost perennials occupying the same habitats. The laterals were most
numerous about 4 cm. beneath the surface of the soil, and in the specimens
examined averaged only 1.7 cm. in length.

FESTUCA OcTOFLORA AXD HoRDEUM MrRINTM.

Two biologic typesof grasses, ainuials and perennials, occur on Tumamoc,
each of which appears to have its characteristic distribution. The annual
grasses, both those of summer and those of winter, are generally distributed,
but the perennial forms occur where the water relations are especially
favorable. Of the grasses only two winter-growing ones were examined.
The type of the root-system of the grasses is well known so that there is
no need of presenting a description of it in detail ; but it will be instructive
to compare the extent of the root-system of the grasses with that of the other
annuals which appear at the same season.

In the mature plants the roots are entirely of secondarv origin, and
there is no main root to dominate the system. The length of the roots
is variable but considerable. In a plant with a shoot 17 cm. long there
were 3 large roots, each about 5 cm. in length, with numerous branches 2
cm. in length, more or less. Another plant (both were of the genus Fcsluca)
19cm. high had 4 roots over 12 cm. long. The laterals borne on these roots
were filamentous and varied greatly in length (some being 10 cm. or more),
and bore in turn long and short filamentous branches (plate 6) .

The root system of Hoideum was similar in character and approached in
extent that of Fcstuca. A plant with a shoot 22 cm. in length had 4 large
roots, all of which were over 14 cm. long; and another specimen with a shoot
30 cm. long had 7 large roots, of which the longest was over 17 cm. All
bore numerous filamentous branches. The depth of penetration of the
laterals was not learned. It may be safely assumed, however, from other
observations as well as from those just presented, that the roots reach at
least 15 cm.; perhaps still deeper in the largest specimens.

GiLIA BiGELOWII.

Gilia bigclowii, one of the smallest winter annuals, is generally distributed
on Tumamoc Hill. The roots of Gilia are admirably adapted to enable the
plant to reach relatively deep, while at the same time it takes advantage

ROOT HABITS OF DESERT PLANTS. 35

of available moisture near the surface of the ground. The leading feature
of the root-system is the long and slender tap root which carries numerous
rather short filamentous laterals. The general character of the root-system
is shown in plate 6. The length of the tap root varies with the age of the
plant from 5 cm. in one 4 cm. high to over 9.5 cm. in a plant with a shoot 8.5
cm. long. The laterals of the first order are usually less than 2 cm. long,
and branch infrequently, so that as a whole the root-system corresponds
very well with the "extensive" type of Biisgen* (plate 6).

Malv.\ Borealis.

Maha, one of the most common of the winter annuals, was studied on
Tumamoc Hill only, although it occurs on the flood-plain of the Santa Cruz
also, where it is perhaps most abundant along the irrigating ditches. The
root-S}^stem of the mature plant is characterized by the prominence of the
laterals, which are coarse and relatively long. Several characteristic changes
should be noted, leading to the formation of the mature root-system as
shown in plate 6.

In the development of the root-system of Malva there are two fairly well
differentiated stages, of which the immature is very different from the
mature. The seedling plant sends down a tap root which penetrates the
soil to a depth of about 8 cm. before laterals become prominent, or indeed
appear in numbers. The prominent tap root, therefore, is the leading
feature of the seedling. As the plant increases in size the tap root strikes
down quickly and strongly until a condition shown in plate 6 is reached.
After this the laterals begin to grow and from this time they constitute the
chief feature of the system. At first the laterals arise singly, and close to
the surface of the ground, but in time they become of importance by their
increase in length, by the formation of branch roots of the second order, and
by the formation secondarily of laterals of the first order along the main root
axis. But the tap root is always an important feature, since in the mature
plant it may attain a length exceeding 16 cm.

The secondary formation of laterals of the first order, by which groups of
three or more roots are formed, which are of unequal age, has also been
noticed in several other annuals and probably also occurs in many peren-
nials. This not uncommon character of plants must greatly increase the
efficiency of the root-system by adding much to the absorbing surface
without greatly increasing the distance of water transport or of bringing the
roots into more active competition with those of neighboring forms.

Medicago Denticulata.

Mcdicago, one of the introduced forms which have their period of greatest
activity in winter, is abundant on the flood-plain of the Santa Cruz. The
plant examined, however, was growing on Tumamoc Hill in the vicinity of
the Desert Ivaboratory. Mcdicago has a generalized type of root-system,

♦Compare von Alten, Wurzelstudien, Bot. Zeit., vol. O7, p. 192, igog.

^f> KDor lIAIins oK UICSHRT I'LANTS.

characterized by few roots wliicli are ratlier coarse and wliicli braiicli
infrequently. In the young plant the tap root predominates, but as it
matures the laterals become important; the behavior of the root-system in
its development recalls that of Malva. Tubercles were not present on
the plants examined. Plate 6, slightly less than life-size, shows the main
features of the root-system so that further description in this place is not
necessary.

Mentzelia Albicaulis.

MentzeUa alhicauUs occurs on Tumamoc Hill and on the upper bajada-
slopes. The leading character of its root system is the prominent lap root.
The root-system of the plant was studied mainly in young individuals, and
the penetration of the roots of the fully developed plant was not learned.
A young plant of Mentzelia with a shoot 2.3 cm. in length had a tap root
1 1 .5 cm. long. The laterals in young plants are numerous, unbranched and
occur singly. In older plants the laterals are in groups, as in Malva and
other annuals.

MiCROSERIS LiNEARIFOLIA.

Microseris is abundant on Tumamoc Hill and on the slopes of the bajada
to the north. The specimens examined were growing near the Desert
Laboratory on Tumamoc Hill. The root-system, as shown in plate 6, is
characterized by the prominence of the tap root and by the slendemess of
the laterals of the first order. The root-SA'stem of a mature plant, with a
shoot 20 cm. in length, consisted of a tap root, 2 mm. in diameter at the
crown and over 11 cm. long, and a few filamentous laterals which were
borne for the most part within 5 cm. of the surface of the ground. The
laterals branched sparingly. Microseris is among the most deeply rooted
of the winter annuals.

MoNOLEPis Chenopodioides.

Monolcpis chenopodioides was collected for study in February from the
immediate vicinitj' of the Desert Laborators^ where it was growing in
abundance. Its tap root, as plate 7 shows, is the most prominent feature
of its root-system, although laterals are early developed. The laterals of
the first order, although small as compared with the tap root, are, however,
rather coarse, and branch but little. Thus the root-s\'stem of Monolepis
should be classified as extensive.

OrTHOCARPUS PtmPURASCENS.

Orthocarpus occurs on the upper slopes of the bajada especially, and
onlj' appears in abundance if, as happened in the spring of 1908, when the
root-sj-stem of the annual was studied, the rains have been copious. It is
a low annual, seldom above 15 cm. in height, of a gray -green color, and has
lately been found to be an habitual root parasite.*

*Cannon: The Parasitism of Orthocarpus purpurasccns Benth., Plant World, vol. 11,
1909, p. 259.

a. Monolepis cheiiopodioides, February, 11)07. 1). Orthocarpus purpnrascens
parasitic on Astragalus sp. c. Orthocarpus purpurascens on Lupinus sp., March,
1907. d. Parietaria debilis, mature plants from favorable and unfavorable moisture
conditions, e. Phacelia tanacetifolia, March, 1907. f. Plantago fastigiata, Marcli,
1906.

CANNON

a. Sisymbrium canescens. b. Streptanthus californiciis.

ROOT HABITS OF DESERT PLANTS. 37

Parasitism of the plant has been observed on the following species :

Astragalus nuttallianus. Eschscholtzia mexicana. Pectocarya linearis.

Bigelowia hartwegii. Festuca octoflora. Phacelia tanacetifolium.

Bowlesia lobata. Gilia bigelowii. Plantago fastigiata.

Daucus pusillus. Lesquerella gordoni, Silene antirrhinum.

Delphinium scaposum. Lupinus sp. Streptanthus californicus.

Erilrichium pterocaryum. Mentzelia albicaulis. vSisymbrium canescens.

The typical root-system of Orthocarpus may be described as consisting
of well-developed tap root and few laterals, but greater variation in the
character of the root-system of this species was seen than was observed
in that of any other annual. Three leading modifications of the roots
were seen. These were: (i) roots with short and coarse laterals; (2) those
in which the laterals were filamentous and cither long or short; (3) root-
systems in which the tap root is broken up into three or more forks.

While the immediate causes of these variations were not investigated,
it is thought that they may be connected with the nature of the root-
svstem of the host plant, as will seem probable from the following.

In association with Astragalus, Orthocarpus has a prominent tap root
with both long and short laterals which are rather coarse. The long lat-
erals are independent of the host plant, but the short ones bear haustoria.
In the case of the length of the laterals, it is presumably directly dependent
on the distance by which the parasite and the host are separated, which,
however, would not account for the coarseness of the roots of the parasite.

When growing in association with Festuca octoflora as the host, the roots
of Orthocarpus are filamentous and the root-system is poorlv developed.
On Eritricliiuin the roots of the parasite are also filamentous, but very
numerous. In connection with Silcnc. the main root of Orthocarpus is
broken up into several relatively large roots and the laterals are very few.

Whatever may be found to be the chief causes in bringing about the
modifications in the character of the root-systemof Orthocarpus, as described
above, it is of interest to note that the soil conditions were as uniform as
one would expect to find in a single habitat, and that the root-svstems of
the host plants had specific differences which the roots of the parasite to a
degree reflected. Thus the roots of Astragalus are coarse and those of
Eritricliiuin are filamentous, and the roots of Orthocarpus when attached to
these plants are coarse or fine, as the case may be. Whether Orthocarpus
can five independently is not known, so what its root-system would be
as an independent plant can not at present be told. However greatly
Orthocarpus changes the character of its root-system with differences in
the roots of its hosts, it does not modify its life-cycle if that of the host
chances not to coincide with that of its own. When associated with
annuals, the plant would naturally not be influenced to prolong its period
of existence, and when connected with a perennial, as with Bigelowia, it
appears not to do so (plate 7).

T,& KiHiT llAHITS iiK DKSEkT PLANTS.

Pakiktakia Dehii.is.

Paiiitaiia occurs almost exclusively in the shade of rocks, or elsewhere
where it enjoys protection cithir against excessive light or severe aridity.
The plant closeh- resembks the eastern clearwced (Spalding: Distribution
and MoNcnienls o{ Desurt Plants, Carnegie Institution of Washington,
Publication ii_^, 1909, p. 20) and has a very high water content; though
growing in the desert it has no characteristics which ally it to typical desert
plants. Parictaiia varies greatly in size and appearance, a feature depend-
ing on water relations more than on any other environmental condition.
This may be illustrated by the following observation. In March, 1906,
many annuals were seen to be of very unequal size ; the difference was so
great in man\- instances as to make sure recognition of well-known species
difficult. Among forms exhibiting this condition, none showed so extreme
modification as Paihtaiia, mature specimens of which, growing under favor-
able and unfavorable conditions as regards water relations or protection by
neighboring plants or rocks, were found whose shoots were from 8 mm. to
39 cm. long; that is, there was a difference of i to 49 in stature (plate 7).

The root-system of Parieiaria is an extremely meager one. It does not
penetrate far either horizontally or vertically. The main root of the
plant with the longest shoot seen was over 4.3 cm., or about one-ninth the
length of the shoot. The length of the main root of the smallest specimens
was about 5 times the length of the shoot. The relation of the length of
root and of shoot in these instances, together with the general character of
the root-system, is shown in plate 7.

Phacelia Taxacetifolia.
Phacclia is one of the most common of the winter annuals on Tumamoc
Hill, and one of the most generally distributed, although it may be most
abundant and attain greatest development on the northern slope and in
the protection of larger plants and of rocks. A prominent tap root is the
chief characteristic of its root-system. A mature plant with a shoot 15
cm. high had a tap root 2 mm. in diameter at the crown and over 18 cm.
long. The laterals of the first order are fine, usually short, and arise singlv.
Under certain circumstances, however, the laterals become a more pro-
nounced feature of the root-system, as, in a younger plant than that above
described, laterals 6 cm. in length were seen within 3 cm. of the surface of
the ground. With its prominent tap root and fine laterals, Phacclia is to
be classed among the plants with an extensive root-system, like the most
of the annuals examined (plate 7.)

Plaxtago Fastigiata.

Plantaqo is abundant on Tumamoc Hill and on the upper bajada at its
northern base. It grows everywhere in the open where it is exposed to the
sun and is among the most drought-resistant of the winter annuals. The

ROOT HAKITS OF DESERT PLANTS. .^9

leading characteristics of its root-sj-stem are the long tap root and the small
development of laterals. The tap root of the young plant may reach as
deep as 13 cm. before laterals appear to any extent (compare plate 7), and
even in the mature form there are few laterals, and these branch but little.
The extreme depth to which the tap root of the mature plant reached was
not learned, but it may safely be considered among the most deeply placed
of the roots of any annual.

Rafinesouia Neo-mexicana.

Rafincsquia occurs on the bajada at the northern base of Tumaraoc Hill
and on the hill itself. Its root-system is characterized by a prominent tap
root which bears filamentous laterals. The tap root is 2.5 mm. in diameter
at the crown. The laterals of the first order are 5 cm. long, more or less, and
branch infrequently; laterals arise singly and cultures (to be mentioned
below), in which attempt was made to induce the formation of laterals
secondarily and in groups, were not successful. In Rafincsquia, therefore,
the appearance of laterals singly should be considered a fixed character.

Streptanthus Arizonicus, Sisymbrium Caxescens, and Sisymbriu.m

Refi.exum.

The three cruciferous annuals whose root-systems were studied occur
both on Tumamoc Hill and the upper reaches of the bajada at its northern
base. Sticptdiithiis, however, appears to be limited to the Hill. Slirptau-
thus and Sisymbrium have similar root-systems. The tap root is a prom-
inent feature, and the laterals are fine and long, and branch but little. The
roots thus are extensive rather than intensive, enabling the plant to reach
out as well as to penetrate the ground a considerable distance. Plate S
shows well the general character of the voung and of the mature plant of
StrepianthAis and does not need further comment.

A specimen of Sisymbiium with the shoot 15 cm. long and fruiting was
examined. The tap root was relatively heavy and was traced over 16 cm.
into the soil. The laterals of the first order were either fine or filamentous
and were 10 cm. or more in length, and were borne for the most part within
2 to 6 cm. of the surface of the ground.

Both in Stieptanthus and in Sisynib) iiiiu the laterals arise singly; there
are no groups of laterals and no rudiments.

GENERAL CONCLUSIONS AND SUMMARY OF STUDIES ON

ANNUALS.

On Tuiiiamoc Hill, and that portion of the bajada and the ll<jud-plain of
the Santa Cruz river within the Laboratorj' domain, as well as by the West
Wash, 22,'? species of annuals are reported. Of these, 1 22 are winter annuals
and 44 appear in summer. Representative species both of winter annuals
and of simimer annuals, as well as one or two perennials and long-lived
annuals, were examined in connection with the present study. In looking
over the results of observations on root-systems, several facts were brought
to light, of which the following appear at present to be the most important.

A feature of several species of both winter and summer annuals is the
presence of rudimentary roots in certain species and their consistent absence
in others. Among the plants with rudimentary roots are Amsinckia, Eritri-
chinm, Hai pagonella, Peciocarya, Erodium, and Malva, among the winter
annuals, and Amaiantlius, Boerhaavia, Cladotlnix. and Trianlhciiia among
those of summer. Frequently associated with the presence of the root
rudiments, perhaps always so, is the appearance of laterals, particularlv
those of the first order, in groups of three or four; another feature is the pre-
cocity of rudimentary-root formation in several species, particularly in
A»!sin<:kia and its relatives. With the desire to throw some light on the
causes underlying the appearance of the rudiments the following experi-
ments, which are regarded as preliminar>', were set up.

On February 14 two pots were planted with seeds of Amsinckia spedabilis.
At the time of the sowing of the seeds the pots were thoroughh- watered,
after which one was not given water again and the other was watered
frequentlv. About April 6 the young plants in the dry pot were seen to
be in a wilting condition and were removed from the pot and their roots
examined. It was found that the laterals of the first order were borne
singly on the main root and that at the base of each lateral there were one
or two rudiments. The rudiments of the smallest plants in the pot were
best developed. The root-systems of the plants growing in the well- watered
pot had the following characters : the laterals of the first order were mostlv
in pairs, and at the base of each group there was at least one rudimentary
root. It would appear, therefore, that the organization of the rudiments
in .4 nisinckia is not dependent on an improvement of the water conditions,
although their subsequent development is. and that they are to be con-
sidered a constant character in the root-system of the species.

In many of the plants no rudiments were seen at any time ; experiments
were set up, consequently, to learn whether the formation of rudimentarv
roots might be induced; the experiments were confined to Rafinesquia neo-
mexicana and the conclusions are not assumed to be applicable to any
other species. In December a pot of suitable earth was prepared and was
thoroughlv soaked with water, and seeds of Rafinesquia were sown. After

40

GENERAL CONCLUSIONS AND SUMMARY OK STUDIES. 4 1

the seedlings appeared no more water was given the pot until February 2,
when by wilting in the da}i:ime the plants showed that more was required.
On February 2 the earth of the pot was well watered. Very shortly after
the watering, the plants recovered and began growing vigorously, and on
February 9 they were removed from the pot and their roots were examined.
It was learned that the main root bore laterals of the first order singly ; no
rudimentary roots were present. It would therefore appear that in Rafin-
esquia not only are rudimentary roots not present in the natural condition,
but their formation may not be induced by improved water relations.

The root-svstems of the winter annuals are usually easily distinguishable
from those of the summer annuals. The most striking characteristics of the
roots of the former are the prominently developed tap root and the meager
development of the laterals, which are generally filamentous or at least thin.
The annuals of summer, on the other hand, have root-systems which are
frequently of a more generalized form; that is, the laterals are developed
well and are frequently rather coarse, and the main root is often forked.
The absorbing surface of the summer annuals appears to be greater than that
of the winter forms, although the depth of penetration of the roots of the two
classes is apparently about the same.

The causes leading to the differentiation in the root-systems of the win-
ter and the summer annuals are not surely known, but on the probability
that they lie in the nature of the species as well as in the difference in the
environment of the two classes of plants the following hypothesis, as a basis
for subsequent experimentation, is offered. The facts in addition to those
presented in the foregoing paragraph are as follows. The rains of summer,
in addition to thoroughlv wetting the soil, serve to cool it as soon as they
come. The air temperature also immediately falls, and the relative humid-
ity at once becomes high. In brief, the conditions for a tropical luxuri-
ance of growth are at hand, and the shoots of the plants which appear at
this season bear a noticeably large number of large leaves ; the transpiration
surface is relatively great.

In the winter season, however, the general character of the annuals, and
that of their environment, are strikingly different from those just described
as obtaining in summer. As was shown in the discussion on soil temper-
atures, the temperature of the soil begins to fall with the coming of the
summer rains and continues to decline until March-April. Therefore, the
rains of the winter season do not materially change the course of the cur\'e,
as is the case in summer. The air temperature at the time of the winter
rains is relatively high, but much lower than in summer. There are also
occasional periods of really high temperatures and drying winds. The
winter plants, so far as I have observed, usually do not grow as quickly
and do not have as large leaf surface as the summer annuals.

Given a sufiicient amount of water both in winter and in summer, it is
possible that the difference in the relative temperature relations of soil and

42 Ri«)T IIAHITS ii|- l)i:si:K'r IM.AN'rs.

of air of the two seasons is mainly the operating cause in bringing about the
difference in the root-systems. The leading factors in tliis process would be
as follows. In summer the temperatures both of soil and of air ])ermit, on
the one hand, very rapid growth, and on the other the best conditions for
water-absorption, with the result that the root-system of a plant is well
developed. In winter, on the other hand, the soil at the 20 to 30 cm. depth
is in daytime colder than the air, which operates directly to depress the rate
of water-absorption, to limit the development of the roots, and thus to make
the conditions unfavoraVjle for the fullest growth of the shoot, with the
result that in turn the demands on the root-system are relatively low.
Could the winter soil be warmer than the air, the growth of the shoot would
probably be much more vigorous and the root-systems of the plant much
more extensive and of a different character than they actually are.

From the foregoing brief statement of the leading general differences
between the annuals of summer and those of winter, it will appear that the
summer season may be conceived as favoring hygrophily more than the
winter season ; so that the root-systems of the summer annuals can be said
to tend toward hygrophily and those of the winter forms toward xerophily.
With these differences in mind, it is of interest to classify those of the
winter annuals which have a generalized type of root-system, or one which
approaches the type of the simmier ]5lants. As a fact, we find that Bowlesia
Paridaria, and Malva, the only plants examined with a marked general-
ized type of root-system, are limited in their normal distribution to situa-
tions where they enjoy relatively favorable water relations. The winter
annuals with specialized or winter type of root-system, that is, deeply
penetrating, have the widest distribution.

Although no attempt has been made in this study to group plants accord-
ing to the various types of root-systems, or to describe the roots from the
standpoint of classification, such, for example, as has been done by Freiden-
feldt (Studien iiber die wurzeln krautiger Pflanzen. I. Ueber die Formbild-
ung der Wurzel vom biologischen Gesichtspunkte, Flora, 91, 1902), it has
been noted that each species examined had its peculiar form of roots. So
striking is the individuality of the root-svstems that it may be possible, and
probabl}' would be possible, to determine the form solely from the character
of its root.

The most striking roots of autotrophic annuals examined were those of
Aster tanacdifolius. although those of Amsinckia and its relatives were also
very individual. The root-system of the parasite Or/Ztocar/'WJ- purpurascens,
which was found in connection with 18 hosts, was more diversified than that
of any other annual. The roots were either coarse or fibrous, with a single
main root, or broken up into several main roots. Whatever may have been
the determining cause of this diversity, it was observed that the roots of the
parasite reflected in a degree the characters of the roots of the host plants.

ROOT-SYSTEMS OF PERENNIALS.
The problem of studying the roots of the larger plants in the field, and of
recording the results, is naturally quite different from that of examining the
root-systems of annuals. The environment to which the roots of perennials
are exposed and to which they should be related, is likewise much more
varied than that of the smaller plants of briefer life span. In the case of
the annuals the entire root-system might by appropriate means be brought
under the eve, photographed, measured, and examined microscopically; but
such procedure is impracticable in perennials, and choice had to be made
arbitrarily between minute examination of ultimate roots, as Biisgen studied
the roots of Javanese and German trees, and a study of the more permanent
portions of the root-systems. Since the survival means successful resistance
to desert conditions, and, further, as the study was primarily one in which
the plant was to be compared to its environment, the latter method was
decided on.

Accordinglv, in this report on the results of the investigation, the mature
root-systems are characterized and the more obvious relations to tempera-
ture and water are presented.

The perennials selected for study comprise the most striking ones only,
and probably do not include many which will be found worthy of subsequent
examination. The list includes trees and shrubs, mainly the latter, and
among these is a great diversity in habits and habitats. There are fleshy
as well as non-fleshv forms; plants with shallow and those with deeply
placed roots ; plants deciduous as well as evergreen ; those which come into
leaf in summer only and those which form leaves whenever there is adequate
amount of water whatever may be the time of the year, and finally there
is one parasite {Kiamciia).

Following is a list of the species examined, together with the habitats in
which they were studied :
TuMAMOc Hill: Echinocactm ic'islizcni Engelrn., Encelia jan'nosa Gray, J^iliopfia cardio-

phylla (Torr.) Muell. Arg., Opunlia discala Griftiths, Opuniia Icplocaulis DC,

Opuntia versicolor Engelm.
The Bajada: Carnegiea gigantca (Engelm.) Britt. and Rose, Covtllea tridcntata (DC)

Vail, Fouquieria splendens Engelm., Franseria deltoidea Torr., Kramen'a

canesccns Gray, Krameria glandulosa Rose, Opuntia arbuscula (?) Engelm.,

Opuntia julgida Engelm., Riddellia cooperi Gray.
The Bajada, Rincon Mountains: Dasylirion tcxannm .Scheele, Mmtonui scahrclla

Gray, Yucca sp., Yucca radiosa Engelm.
Flood-Plain of Santa Cruz and West W.\sh: Condalia \patliiilata Gray, Ephedra

trijiirca Torr., Ka'rlicrlinia spinosa Zucc, Opuntia vivipara Rose, Zizyphus

parryi Torr.

Besides these forms some observations were made also on Pivsopis
-velutina Wooton, Lyciiim sp., Olncya tesota Gra\-, Parkinsonia ndcropkylla
Torr., and a few other forms which will be given incidentally in the account
of the root-systems below.

44

ROOT HAIUTS OP DUSURT I'l.ANTS.

PLANTS FROM TUMAMOC HILL.
Of the areas under consideration in this paper, Tiimamoc Hill is the richest
in the number of species of perennials and the bajada in annuals. The same
species frequently occur on all of the habitats, but to this there are numerous
exceptions. Of the plants studied on Tumamoc Hill the following do not
grow in the other areas : Jatropha cardiophylla. Opuniia discata, and Opiintia
versicolor. On the other hand, some forms which are most typical of Tuma-
moc Hill, such as Carnegica gigantea, but which are in other areas also, were
studied where the soil conditions were most favorable for excavating, and
not necessarily on the Hill exclusively.

ECHINOCACTUS WiSUZENI.

Two specimens of Echinocactns were selected for special study, one of
wliich was growing by West Wash and the other on the north shoulder of
Tumamoc Hill about 60 meters north of the Laboratorv building.

^

^O"

Fig. 2. — Root-system of Echinocactus wislizeni, horizontal and vertical extension. Intruders
are represented: by broken lines, Acacia constricta; by large dots, Menodora scabra; and
Opuntia discaia by dotted lines.

CANNON

I

ir^v?

.0,: 6

c<.'

Root-system of Echinocactus Wislizeni.

A. Showing anchoring and absorbing roots in natural position, from West Wash.

B. Bird's-eye view of roots from which overlying soil has been removed.

fr - -^ &

ft' ir^

'-■ d

a. Root and shoot habit of Opuntia leptocaulis which was growing under the
protection of Acacia constricta, Tumamoc Hill.

b. Opuntia versicolor showing habit of plant and character of environment.
A small specimen of Echinocactus wislizeni is to be seen at base of main stem.

c. Bird's-eye view of a portion of the central part of the root-system of the
plant shown in b, with the overlying soil removed, leaving roots as far as
possible in their natural placing.

d. This and figures on plate 11 are of ime main lateral and some branches of
the plant shown above.

CANNON

^•'Jw' .<:S!

■SKv' .. >',

1^

. » »

ti^''^'^^-* u'^-JV-

fit . -("T

^

'^•v.

'.s^^fe^s^

*&v^ iMM

^ M&mw^f^

Continuation of plate 10. Bird's-eye view of root-system of Opuntia versicolor.

ROOT-SYSTEMS OF PERENNIALS. 45

The general soil conditions of Tumamoc Hill have already been described
and need not be repeated here. It will only be necessary to add that, at the
precise spot where the specimen of Echinocadus was growing, the bed rock
was so deep that it was not encountered in the course of the excavation of
the plant's roots. The roots were wholly confined to the upper soil layer
or the malpais, which here was relatively heavy. In West Wash (the soil
conditions of which are also described above) the soil is somewhat lighter,
owing to an admixture of sand, and exceeds 2 meters in depth. The water
and the temperature relations of the habitats are unlike, and as far as
studied are also characterized in the section above referred to.

The Echinocactits examined on Tumamoc Hill, April, 1907, was 38 cm.
high and of about the samediameter. Asplates 9 and 10 and fig. 2 indicate,
the root-system of the plant has a very meager anchoring portion and a very
extensive absorbing portion. The main root went straight down 20 cm.
and gave off several short laterals between its tip and its crown, which
latter was 10 cm. beneath the surface. The most deeply placed of the short
laterals extended 12 cm. away from the main root axis and attained a depth
of 25 cm.

The main absorbing system consisted of three laterals of the first order
which left the tap root at its crown and extended in two directions, namely,
uphill and downhill. The larger portion was that which extended up the
hill. The uphill portion originated from a single lateral, as shown in the
figure, and consisted of numerous roots of the second, third, and fourth
orders, which were so disposed as to cover the ground included by them
fairly thoroughly

The roots of the main absorbing portion of the system were of a light
brown color, and were rope-like and tough ; they were slender throughout
their length. The depth of the roots may be learned from the following
measurements, which were made on the portion which was placed uphill
from the main axis of the plant. At a distance of 40 cm. the lateral was
3 cm. deep ; 3 meters distance it was 3 cm. ; and at 3.5 meters the roots were
1.5 cm. beneath the surface of the soil. Thus the roots of the absorbing
portion of the system are all extremely shallow, so shallow that they are
often exposed by the eroding effects of severe storms.

In order to observe the possible range of variation of the root-system of
Echinocadus, other specimens were examined, one of which was growing
by West Wash. The leading points in the study of this plant may be sum-
marized as follows : The horizontal system was sharply differentiated from
the vertical system, both of which, particularly the latter, were densely
branched. The horizontal portion of the root-system lay for the most part
within 5 cm. of the surface. It was apparent that the anchoring roots were
also very actively engaged in absorption, and that the absorbing roots were
relatively not so important as the corresponding roots in the specimen from
Tumamoc Hill.

4'' KMuT IIAltlTS i)I" DKSUKT I'l.ANTS.

1 he relation of the specimen of Echinocactus from Tumamoc Mill to other
jjerennials included within the radius of its horizontal roots was as follows:
One specimen of Acacia constricta was placed i meter from the main axis of
the plant ; one specimen of (^punlia discata was slightly farther than Acacia ;
and ID s]3ecimens of \huodora scahra, a shrubby form, occupied various
positions nearer than the other species named. The neighboring forms,
however, held very unlike relations to the Echinocactus, a fact, in this case,
dependent on the character of the root-systems. The roots of Mcnodora
penetrated deeply and there was little development of the laterals corre-
sponding to the horizontal or absorbing system of Echinocactus. Onlva few
of the roots oi Acacia were encountered while exca\-ating for the root-system
of Echinocactus, but two long and a few short roots of Opunlia were seen
to lie in close proximity to the downhill roots of Echinocactus. From the
positions occupied by the roots of these plants it may be concluded that
only the species of cactus enter into active competition with one another
for water. Competition between plants whose roots occupy horizons 20 to
30 cm. apart, as between the cacti and other plants, is not to be con-
sidered as direct.

The story of the developmental changes which take place in the root-
system of Echinocactus was not learned, although it is undoubtedly difTerent
Irom that of such a fleshy form as the sahuaro {Carnegica ijigantca) in which
the horizontal portion of the root-system becomes modified secondarily to
meet the demands of the plant for mechanical support, as will be described
below. The root-system of Echinocactus is not suited to enable the epigeal
portion to withstand severe lateral strains, as is the case with the sahuaro,
for which reason the plants are often found uprooted, and are usualh"
easily overthrown by a well-directed push. It is a matter of common
observation that in falhng, or in leaning, the plant generally leans or falls
in a southerly direction. It thus may be called the compass-plant of the
desert. The consistence of this position is without doubt founded on
normal developmental changes to which the plant is subjected, such as the
greater growth of tissues on either the north or the south side, as in the
sahuaro (E. S. vSpalding: Mechanical adjustment of the sahuaro, Cereus
qiganteus, to varving quantities of stored water. Bull. Torr. Bot. Club,
vol. 32, page 64, 1905), by which an asymmetrical distribution of weight
takes place.

ExcELi.v Farinos.v.

Encelia has a very decided habitat preference. Not only is the species
limited to Tumamoc Hill, but it is most abundant on the southern face of
the hill, though occurring also on the west and east, and to a limited extent
on the north side. As the most typical habitat was not favorable for exam-
ination of the roots of the plant, on account of the rock exposures and the
steep gradient, it was studied on the northern slope where the soil is
deeper and the roots more easily removed.

ROOT-SVSTEMS OF PERENNIALS. 47

Encclia is a very striking shrub, less than i meter high, with a close and
rounded contour. The ends of the rather sparinglj- branched shoots, which
are of a yellow-green color, bear large grayish leaves during the more moist
seasons, and drop all but the terminal small ones when the arid conditions set
in. Both leaves and flowers are formed whenever the water relations are
favorable.

At the place where the specimen of Encclia was studied the malpais,
about 15 cm. in thickness, was underlaid by hard caliche of about the same
thickness, with a stratum of rotten caliche between, and this in turn was
resting on a layer of sandy malpais of undetermined thickness.

The specimen selected for study was 90 cm. in height and was composed
of 12 shoots, all of which bore large leaves. The plant was mature and was
a very vigorous one. Its root-system was composed of a rather stout tap
root, running directly downward, and numerous laterals which arose com-
paratively far from the surface of the ground. The main root went straight
down through the malpais to the rotten caliche, where it narrowed abruptlv
and ran through the rotten caliche to the hard caliche, through a crack in
which it made its way to the more deep-lying sandv caliche beneath. When
the latter was reached it turned abruptly and took a horizontal course for a
distance exceeding 50 cm. It was followed to a depth of 55 cm. beneath
the surface of the soil.

The laterals of the first order, with no noteworthv exception, were given
off in the malpais stratum, that is, within 20 cm. of the surface, and were
placed between 15 and 30 cm. deep. There were about 5 main laterals;
they branched but little. The diameter, length, and position of the laterals
may be learned from measurements on one of the typical roots. The lateral
in question left the main root 15 cm. from the surface ; 15 cm. distant it was
r. mm. in diameter; at a distance of 60 cm. it was 2 mm. in diameter and
lay 24 cm. deep; i meter from the main root axis it was 24 cm. deep, and
at the tip, i .5 meters distant, it lay 22 cm. beneath the surface. In addition
to the larger laterals there are about 6 shorter ones of the first order which
are wholly confined to the malpais.

All of the laterals bore groups of filamentous ro(.)ts, recalling those of
Aster tanacctijolins, 5 cm. more or less in length, and about i cm. apart.
At the time the root-system of Encclia was studied, February 11, these
rootlets were no longer living, and the time of their origin is not known, but
is supposed to be much later than the formation of the mother laterals that
bear them. To judge from their appearance onlv,* one would suppose
them to have been in part produced during the preceding year, probably
summer, and to constitute an important element in the absorbing system of
the plant. Further comment on this type of roots, which were observed on
several species, will be given below.

*On January 4, 1910, after rains had moistened the ground well, the roots of Encelia
were seen to bear youns and growing rootlets of the kind descril>ed in the above
paragraph.

4S

KtioT IIAlinS or DHSHRT IM.ANTS.

Figure 3 shows the horizontal and vertical extensions of the root-system
of Encelia faiiiiosa.

The specimen of Encelia studied had very little competition from its
perennial neighbors. No other perennial grew within reach of its roots,
but the roots of two plants, Ofyuntid ttiscala and Pdikinsonia microphylla,
reached into or passed through the area occupied. The roots of the former
were more shallow than those of Encelia, while those of Parkiiuonia ran
beneath the roots of Encelia. It should not be concluded, however, that
the plant is without competitors for ground water since during two seasons,
covering three months or more, annuals with deeply penetrating roots
occur in large numbers and probablv extract more moisture from the soil
than Encelia itself does. The relative thickness of the 3 upper soil layers,
adobe, rotten caliche, hard caliche, are shown diagrammatically in fig. 36.

Fig. 3. — Encelia jarinosa.

Opuntia Discata.

Several types of flat-stemmed opuntias are native on Tumamoc Hill, and
certain of them, as far as the habitats considered in connection with the
present paper are concerned, are restricted to the Hill. Among these is
Opuntia discata, a plant of varying habit, with prostrate branches when
young, and with branches free of the ground when older. Two specimens
of this plant were selected for study, one mature and the other young.
The latter was growing on the north face of Tumamoc and the former on
the west side. The soil conditions where the cacti were placed were appar-
ently alike ; the upper soil to a depth of 1 5 to 20 cm. is malpais, with rotten
caliche beneath, all lying on the bed rock.

The plant from the northern exposure had two shoots composed of two
and three "joints " each. Both branches were lying on the ground, so that
the main stem bore none of their weight. The root-system consisted of
several main roots, which either ran straight downward from the base of
the main axis of the plant, or at an acute angle, and also about 8 laterals,

ROOT-SYSTEMS OF PERENXIALS. 49

which arose from these vertical roots and extended in a more or less horizon-
tal direction for various distances not far beneath the surface. There was
no well-defined tap root, or supporting roots, all roots functioning mainly
as absorbing roots.

PiQ 4 — Opunlia discata. Ilnrizontal and vertical extension of root-system.

The main points are shown in fig. 4, which represents the horizontal as
well as the vertical extension of the root-system. For the most part the
roots are confined within a radius of 50 cm. from the central axis, but two
laterals, the one going uphill and the other down, were approximately 1.5
mm. in length. The root reaching southward maintained a depth of 20 cm.
for a distance of 20 cm. and then ascended rapidly until it lay at the 10 cm.
level, about which it remained during the rest of its course. The roots
extending east were approximately 15 cm. deep. Those to the west ran
along the face of a boulder at a depth of 15cm., and after leaving this protec-
tion quickly rose and occupied a position near the surface. The roots of the
Opuntia were slender throughout their course and branched very little.

The roots were seen frequently to be in intimate association with boulders
whose presence and position greatly affect the branching and probably
other habits of the root-system. Where the rocks are shallow the roots run
beneath, but where they are as deep as 25 cm., for example, they run along-
side. The roots which are placed under rocks are closely appressed to them
and form uitricate patterns as they there branch very densely ; such roots
also remain functional for a longer period than those of the same plant less
advantageously situated. These features are clearly to be associated with
the better water relations that are obtained beneath the rocks which serve
to retain as well as to conduct water better than the bare soil. As illus-
trating this statement the following observation on the penetration of water
may be noted. Soon after a rainfall of half an inch, by which the soil would
usually be moistened to a depth approximately 8 to 1 2 cm., that underneath

5f) UcMiT IIAIMTS i)I' DIvSIvKT ri.AN'l'S.

a bdukki \va> pLixvptibly wcl In a. ckplh ul J5 cm., uliilc ks^ lliaii i iiicler
distant the soil, free of large rocks, was air-dry at this depth.

The root -system of the older ])lant, which grew on the western side of
Tumamoc was different in some particulars from that of the plant just
described. The shoot of the plant consisted of two main branches and
three secondary ones, with a stout central axis, and the weight of all of the
branches was born by the stem. There was no main or tap root, as in
other cacti, but several roots, constituting a brush, left the base of the stem
and jienetrated the ground at an acute angle. Of these roots, the longest
attained a depth of 15 cm. in a horizontal direction. From the central
group there arose 8 leading roots which formed the superficial portion of
the system: these varied in length from so cm. to 1.;, m. : the longest were
those extending uphill. The superficial roots varied greatly in depth, some
being as close to the surface as 2 cm. and others penetrating as deep as 20
cm., but the deeply placed ones uniformly ran underneath boulders, while
the more shallow roots were situated where the soil was free from large
rocks. The bases of the superficial roots were somewhat enlarged ; for
example, a root 50 cm. long was 1.5 cm. in diameter at its base, but the
roots as a whole were very slender. The enlargement of the bases of the
roots probably represents a regulatory response on the part of the roots
by which the weight of the shoot is borne and the strains incident to winds
successfully withstood. Thus roots functioning primarily as absorbing
organs become, in the absence of a well-developed ta]) root, the most
important mechanical support of the heavy shoot.

The superficial roots of the cactus favor absorption of water even when
the rains have been slight, as maybe illustrated byone or two instances. On
October 1 7 half an inch of rain was recorded as having fallen at the Desert
Laboratory, ^^'ithin 3 days the joints of Opiiutia discata lost the shriveled
appearance which they had during the dry season and became plump. On
another occasion the response was noted within 24 hours after a rain of 0.54
inch; also, 24hours after irrigating a certain plant it showed by its increased
turgidity that it had taken up water. In none of these instances had the
water penetrated sufficiently to be of benefit to plants the absorbing system
of whose roots was 10 cm. below the surface.

Opuxtia Leptocaulis.

( >[>ii)ilia hptocaiilis grows mainly and was studied only on Tumamoc Hill ;
it has very slender shoots, as is indicated by the specific name, and has
certain biological relations, probably associated with the character of the
shoot, which are of much interest.

As a rule the cacti in the vicinity of the Desert Laboratory do not hold
close and fixed relations with other plants ; they grow near or remote from
other plants, apparently wholly according to chance. But the species
leptocaulis is a very striking exception to this, in that it rarely occurs alone
or remote from other and larger forms. The shoot habits of Opuntia vary

ROOT-SYSTEMS OF PERENNIALS.

51

with the position wliich it occupies relative to other plants. When it is
isolated, the shoot is rounded and much-branched; where the plant is pro-
tected by another plant, the branches are few and may be much elongated,
and we shall see that the roots also of the plant vary with its position.

I'iG. 5. — Root-systems of Opuntia h'l>lncaxilis. a, plant growing at the base of Acacia con-
stricta. l\ plant growing apart from other species. Both figures are of mature plants.

Another character of the species is the presence of short branches, shown
in plate 10, which constitute the chief chlorophyll-bearing organs. Under
favorable water conditions, these short branches may be very numerous,
but where the moisture conditions are otherwise, they are few. The
branches really function as leaves, in that they are formed at times of active
growth and that their subsequent history varies with the moisture relations ;
if favorable, they are long retained; if unfavorable, they early fall away.
This effect can be quickly achieved by uprooting a plant and hanging it in a
dry place, under which treatment the short branches to a large extent soon
drop off. The exfoliated branches do not commonly, if ever, act as propa-
gative bodies, as is the case in otherspecies, notably in O. vhi para .2iSvi\\\ be
shown below.

The root-systems of two specimens of Icplocauliswx-K studied inl'ebruary,
1907; one was growing at the foot of Acacia constricta (fig. 5a) and the
other independently (fig. 56). Both plants were on the northern slope of
Tumamoc Hill tmder the same conditions of soil and moisture as described
above for Encclia. The protected plant was 80 cm. high, and the sub-aerial
portion consisted of a central axis with a few long slender branches that
carried many short ultimate ones of the character spoken of in a preceding

52 ROOT HABITS OF DESKKT I'l.ANTS.

paragraph. The entire habit of the plant was symmetrical and loose.
Figfiire5and plate lo show the extension of the roots in the ground and their
appearance when removed. There was a tap root, which penetrated 1 5 cm.
and which bore a few laterals not over 10 cm. long, and about 4 main laterals
3 of which extended from the main axis as far as 30 cm. and varied in depth
from 5 mm. to 5 cm. All of the laterals were slender and bore almost no
branches. Thus the root-system was very meagerly developed.

The specimen which was growing apart from other plants was 50 cm.
high, of a compact growth-habit, and was much branched. The root-
system of this plant may be briefly described as follows. There was no well-
defined tap root, but there was a brush of slender roots varying much in
length. The roots radiated out from 60 cm. to 2.8 meters and the longest
was less than i cm. in diameter at its base. The depth attained by the
longest roots ranged from 4 cm. to 14 cm., with the usual depth between
5 and 8 cm. The roots branched hardly at all.

In contrasting the root-systems of the two specimens it will be seen that
that of the protected plant is composed of fewer and shorter roots than that
of the unprotected one, and that the roots of the former are also nearer the
surface of the soil. In either instance it is probable that the character of
the root-system, at least as far as regards the number and the length of the
roots, is immediatelv connected with the development of the shoot, and
that the character of shoot development and the vertical position of the
roots are intimately associated with the protected or unprotected condi-
tion of the plant.

Opuntia Versicolor.

Opuntia versicolor, one of the cylindro-arborescent forms, is one of the
best represented species on Tumamoc Hill, although in no place does it
occur in large enough numbers to constitute the dominant plant type. The
habitat where the plants studied were growing was in the main similar to
that described for Encelia farinosa, although it differs in certain details.
The upper malpais soil is relatively deep and carries, on its surface and
embedded in it, stones and boulders in some abundance. The water con-
ditions are better than where Encelia grew, owing to the nearly level char-
acter of the ground, the rather deep soil, and also to the fact that the place
receives considerable water as superficial run-off and as seepage from higher
portions of the hill.

Two specimens of versicolor were studied, of which one was young and the
other mature. The younger plant was less than 50 cm. high and had few
branches. There were two well-defined root types in the plant, namely,
those shallowly placed and extending horizontally, and those extending
vertically and reaching rather deep. The latter, the anchoring or sup-
porting system primarily, consisted of a tap root, with short and long lat-
erals, which penetrated to a depth of 20 cm. One branch of the tap root
attained a depth of 30 cm. The superficial portion of the root-system was

«

/,

b

a, Vertical and, b, horizontal extensions of root-system of Opuntia versicolor, Tumamoc
Hill, views of which are shown in plates 10 and 11. One-fortieth natural size. The root-area
of the cactus contained other perennials of which one specimen of Echinocactus wislizeni, at the
base of the cactus studied, is shown in yellow, one specimen of Jatropha cardiophylla in purple,
and the tips of the roots of a neighboring Opuntia discata, on margin of plate, are in yellow.

CANNON

yi^f' ^-

PLATE 13

O .

.;:*^:i

Root-habit of Jatropha cardiophylla, of which the horizontal extension
of the root-system was shown in plate 12.

ROOT-SYSTEMS OF PERENNIALS. 53

composed of two laterals which left the main root just beneath the surface of
the soil and ran in opposite directions each for about a meter. These roots
were little branched and lay within 5 cm. of the surface. In addition to the
difference in the position of the two portions of the root-system there was a
further diiference in size : the deeper part being uniformly of greater diam-
eter than the more shallow horizontal portion.

The older plant whose roots were examined was situated about 10 meters
south of the one just described. It was i meter high and bore several
branches, and although mature was somewhat under the average size for
the species. The habit of the plant is shown in plate 10.

In the immediate neighborhood of the cactus, or growing within the reach
of its roots, were numerous perennials, Echinocacius wislizeni, Mcnodora
scabra, Jatiopha cardiophylla, Cassia bauhinoides, Acacia constricta, and Par-
kinsoniamicrophylla. There also were several species of annuals. The posi-
tion of specimens of Mcnodora and the census of the species within the
T;emco/or area are as follows : (i) 50 cm. northeast; (2) 1.4 meters north;
(3) 2 meters north; (4) 2.2 meters southwest; (5) 2.6 meters west; (6) 50
cm. south; (7) 60 cm. southwest; (8) 1.5 meters west; (9) 1.6 meters east;
(10) 2.1 meters east; (11) 1.2 meters southeast. Of these plants, only one
was placed so that the roots of the cactus did not go near it. The situation
of Jatropha and Echinocacius are indicated in plate 12. Of other perennials
growing in the area, two specimens of Cassia were 2.8 meters southeast
and 2.2 meters south respectively, one specimen of Acacia was 2.7 meters
northwest, and one Parkinsonia was 2.2 meters southeast. Among the
herbaceous iorms Astragalus nuttallianus , Daucus pusillus, Brodicea capitata,
Harpagonclla pabncri, and Hilaria sp., all of which except Astragalus and
Daucus were in flower at the time the roots of the cactus were examined.
No record was to be had of the number or kind of the summer annuals which
belonged to the area. In addition to the perennials and annuals, both of
summer and of winter, which were growing within reach of the cactus
roots, the roots of a neighboring specimen of Opuntia discata invaded the
area. Thus within a space of little more than 20 square meters there were
16 perennials belonging to 6 genera, and numerous herbaceous forms, of
which those appearing in winter belonged to 5 genera. This area was the
richest in variety as well as in number of plants, both perennials and
annuals, of any observed during the course of this investigation.

The root-system of the older cactus was an extensive one, the leading
characters of which are shown in plates 10 and 1 1 and require little further
description. The root-system is composed of a widely extending horizontal
portion and a rather deeply penetrating portion. The former system was
not seen to attain a depth greater than 5 cm., but the latter reached as
deep as 25 cm. The horizontal roots of the first order are slender except
at the bases, which, as the plates show, are rather heavy. The laterals
branch but little except where they run imderneath stones, where, as in

54 KOI IT IIAIilTS ()K DICSI'KT PLANTS.

Ofyiiiilni ilisciiht. llicy art- iiiiich branched. IBesidcs Ihc- more prominent
roots, such as those just referred to, there are filamentous ones which occur
in Kroujis and which are usually about i cm. long. Such filamentous roots
arc to be seen thronu'iout tlu- inovt of (he course of the su])erficial larger
laterals.

Growing' within the compass of the roots of the cactus, as described in an
earlier paragraph, there were several perennials whose position would indi-
cate, if other conditions were favorable, that they competed with the cactus
for water. An examination of the roots of these plants indicated that this
might be the case in some species, but that it probabh- was not true in
others. Menodoia, for example, has a tap root which penetrates deep
but has little extension of laterals, and the roots of Parkinsoiia also pene-
trate the ground more deeply than those of the cactus. These i)lants, there-
fore, did not enter into close competition witli the cactus.

Of the other aliens, Echinocacliis would be supposed to have superficial
roots which would be placed similarly to the corresponding roots of rcrsi-
coloy, but owing to the fact tliat the specimen was young, the anchoring
roots, and not the widelv extending ones, were developed, and competition
between the two species of cacti had not yet begun. The case was different,
however, with the specimen ol Jatropha and the neighboring (?/>!( H/j'a discata.
The distal roots of one lateral of the discata. as the plate indicates, occupied
the same horizon as those of versicolor, and were mingled with the roots of
the latter cactus. There must have been active competition for water,
therefore, between the roots of the two cacti. The root-system of Jatrof>ha
was of the sort that, had the plant been more fortunately placed, active
competition with it must also have occurred.

The root-system of Jatropha is entirely superficial and placed fairly close
to the surface of the ground. As shown in plates 12 and i,^, the root svs-
tem of the plant was composed of a long and fleshy main root, which ran
horizontallv, and several shorter roots, some of which were fleshy also.
There were also two relatively short fleshy roots which arose from the
enlarged shoot-base and ran horizontally. At a point 10 cm. from its base,
the main root was 2.5 cm. in diameter. The terminal rootlets of Jatropha
w-ere placed at about the depth of the small superficial roots of the cactus,
and the plant would have competed wdth the cactus actively had the roots
of the two species chanced to occupy the same territory. Thus it seems
that despite the large population of the area the roots of Opuniia versicolor
were in close relation with those of but one alien, and that one with its
central axis relativelv remote. However, this statement would undoubtedly
be modified if consideration were given to the relation of the roots of the
cactus and those of the numerous summer and winter annuals that also
occupy the area, as the roots of many of them must be in close relation, if
not in ]3hysical contact, with those of the cactiis.

ROUT-SVSTEMS OF PKKIiWIAI.S. 55

THE BAJADA.

The- plants of the bajada which were studied cither were peculiar to or
characteristic of the bajada although not confined exclusively to that habi-
tat, or, being characteristic of other formations, occurred nevertheless on
the bajada where for different reasons they were found most convenient for
studv. CoviUca tiidcntata is found in greatest numbers on this habitat,
where it may in places produce almost a pure growth, but the species is to
be found on the other areas also and in fact attains its largest development
on the flood-plain or close by streamways. Riddcllia coopcri occurs mainly
on the bajada, and Opuiiiia arbitscula and O. jiilgida grow only there. Of
other species whose root-svstems were examined, Carnegiea gigantca, Fou-
quieria splcndcns. Franscria dcUoidea, Kvamcria canesccns, and A.', glandii-
losa are found on Tumamoc Hill also, and occasionally, on the flood-plain
save Fouquien'a which was not seen on the last named formation.

Two species of Yucca, an Agave, and a Dasyliymn from the bajada to the
west of the Rincon mountains, about 20 miles east of Tucson, were also
studied.

Carnegie.v Gig.\xtea.

The root-svstems of several specimens of the sahuaro have been observed,
but the studv has been carried on mainh" on plants from the bajada imme-
diateh- west of West Wash, and from the flood -plain of the wash itself. The
soil conditions comprise A'ery shallow as well as fairly deep surface soils, as
a result of which the extreme variabilit\- of the roots might be expected.
The water relations, also, were very diverse, and probably the temperature
conditions as well. However, it should be stated that this cactus not only
avoids the habitats where the soils are shallowest, but also the opposite
extreme, where they are the deepest : or, briefly, it does not occur as a
general rule on the bajada or the flood-plain, but rather on rocky areas,
chieflv on the southern, the western, and the eastern exposures.

The bajada habitat is at the eastern end of the long slope which leads
awav from the main range of the Tucson mountains. The soil at the place
where the cactus was studied is of a dark color, closely resembling the
malpais of Tumamoc Hill, and is rather thicklv strewn with large and small
stones. A section of the soil shows the following leading characters: The
uppermost soil, about 30 cm. in thickness, is of the adobe-malpais referred
to above ; beneath the superficial stratum is a thinner one of caliche which
rests on bed rock. (>See figure to the left, plate 14.) Numerous stones and
boulders are embedded both in the adobe and in the caliche layers. The
contour of the ground at the place is such that little if any water comes to
it by seepage from higher ground, or bv superficial run-oft", the plants
obtaining all of the water supply directlv from the rains.

The root-system of the plant is differentiated into an anchoring portion,
which is the more deeply placed, and a superficial portion, which is pre-
eminently the part engaged in absorption, although it has other functions.

56 ROOT iiAnns of dhsijrt i'uants.

Of the spcciiiuns of saluiaru studied, the lirst to be described was a liealthy
and very vigorous plant 1.2 meters high and 35 cm. in diameter at the largest
part. The anchoring system consisted of a stout tap root which went
straight down to a depth of 30 cm., or to the caliche, and a few laterals, of
which one ran horizontally about 25 cm. The superficial system arose from
the main root close to the surface of the ground and was composed of about
6 main laterals which radiated from the main axis so as to divide fairly
equally the available area between them. They extended from 1.5 to 5
meters from the central axis, and branched but little. As an instance of
the latter characteristic it may be stated that four of the laterals did not
give off branches during 1.5 meters of their course, and one root, which
branched freely at the tip and was 3 meters long, bore onlv4 branches. The
superficial roots end in a tuft of fine rootlets, nearly all of which die with
the advent of the drier season, and groups of delicate rootlets are borne by
the superficial roots the greater part of their length. It is probable that
these fine rootlets are produced each year with the coming on of favorable
conditions of growth and water absorption, as in Fraiiseria deltoidea and
other desert plants. The plant is thus enabled to exploit thoroughlv more
of the root area than would otherwise be possible.

The superficial portions of the root-system penetrated the ground to a
depth which was fairly uniform, although in no case verv great. For
example, one root left the main root at the surface of the ground, and both it
and its three main branches, with little deviation, ran 7 cm. from the surface.
Another root ran from 3 to 40 cm. from the surface, but gave off a branch
which went straight down to a depth of 1 7 cm. Others of the superficial
system varied between 5 and 15 cm. in depth, and one dipped under a large
boulder, whose lower surface was 30 cm. from the general ground-level, after
which it ascended to about the depth characteristic of the older parts.

Where the larger of the superficial roots left the tap root they were rela-
tively heavy, but they tapered rapidly until a small diameter was reached
which was maintained with little change for a long distance. A few meas-
urements will show the point. One of the larger laterals was 1.6 cm. in
diameter at its base; another was 7 mm. in diameter; another root, 45 cm.
from its base, was i cm. in diameter, and it extended 2.5 meters beyond the
point of measurement. Roots not above 50 cm. long were about 3 mm. in
diameter at the base. A root 5.5. meters in length was 2 cm. in diameter
30 cm. from its base.

In a sahuaro of the size of the one under discussion, it is probable, there-
fore, that the tap root constitutes the main support by which the plant
successfully withstands the pressure of the wind, which at times is heavy,
and that the superficial roots function mainly as absorbing organs. With
the growth of the plant, however, the need of anchorage is greatly increased
and the tap root no longer suffices as thesole stay, so thatthe laterals assume
the added role of mechanical supports and become much changed in size,
perhaps in other regards also, to meet the new demands. This develop-

Horizontal and vertical extensions of root-systems of Carnegiea gigantea, Covillea
tridentata, and Parkinsonia microphylla, from the bajada west of West Wash,
Dec-ember, 1906. The Carnegiea is represented l.>y black, the Covillea b}' yellow, and
the Parkinsonia by green.

CANNON

.--or

A. Anchoring roots and bases of some lateral roots of Carnegiea gigan-
tea, 0.8 m. high, from the bajada near West Wash.

B. Tap-root and secondary roots making up the anchoring system nnd
the bases of some of the superficial roots of a specimen of Carnegiea, 1.8 m.
high, from same habitat as the cactus shown in A.

C. Tap-root and bases of laterals of Parkinsonia microphylla which was
growing near Carnegiea shown in B, and which appears in plate 14.

ROOT-SYSTEMS OF PERENNIALS. 57

mental modification can Vje shown by the main characters of the root-system
of a larger specimen. A sahuaro 6.8 meters high and bearing 4 branches,
from 40 cm. to i meter in length, and growing in a situation apparently
similar to that of the cactus above described, had a relatively heavy root-
system. The roots of the anchoring system were sharply differentiated
from the superficial roots and were composed of a cluster of 10 main roots
with their branches which ran directly downward about 77 cm., and ended
abruptl}- on reaching the bed rock. Of the superficial system three leading
laterals were uncovered, one of which ran southeast over 9.7 meters, one
northeast over 4 meters, and one west over 4.5 meters. All of these roots
were very heavy, especially at the base, as plate 15 indicates. In this, and
in other specimems examined, it is clear that the increased diameter of the
superficial roots is connected with the large size of the sub-aerial portion of
the plant and its need of better support than would be afforded by the
"anchoring" roots alone. A proof of the importance of the laterals as
mechanical supports of the stem is often afforded by the spectacle of
uprooted plants, whose laterals have become diseased and no longer func-
tion as efficient props or stays.

A specimen of sahuaro growing on the flood-plain of West Wash was also
examined. The root-system did not differ materially from either of those
above described ; that is, there was an anchoring portion and a superficial
portion, which in the plant seen (which was less than 2 meters high) were
not as yet sharply differentiated either in function or in position. The main
root did not penetrate deep, although the soil was somewhat deeper than
at either of the other habitats mentioned. This was the leading point of
interest : the root-sj^stem of the sahuaro is essentially superficial even if the
soil conditions are such as to permit deep penetration.

From the brief discussion of the root-system of the sahuaro it %\ ill be seen
that there is no sharp distinction in function between the superficial roots
and the anchoring roots, as is to be found in Echinocactiis especially, but
that as the plant increases in size the superficial system gradually assumes
the role of supporting the sub-aerial portion in addition to its chief role
in the absorption of water.

In the immediate vicinity of the cacti growing on the bajada were also
the following perennials: Echinoccrcus jcndlcri, Covillca tridcntata. Encclia
farinosa, Kramcria cancsccns, Parkinsonia miciophylla, and some other
plants, especially species of cacti.

The relation of the roots of sahuaro to those of other perennials was exam-
ined closely only in connection with the study of the sahuaro which was 1.2
meters high. In this case one specimen of Parkinsonia and one of Covillea
were situated within the proper root-area of the sahuaro, as plate 14 shows.
The Parkiiisonia was growing 1.3 meters distant from the stem of sahuaro.
The following were the leading points in the character of its roots. There
was a tap root which went straight down to the caliche, where it terminated

58 ROOT iiAinrs nv dicsickt plants.

abruptly. Several slender roots lel't the main root Ixiween 5 and 10 cm.
from the surface of the ground and either extended fairly horizontallv or
dipped downward to a depth of 45 cm. and even deeper, where they pene-
trated the caliche. The root-system of Covillca. so far as its jjosition was
concerned, was similar to that of Parkinsonia; that is. it extended down-
ward through the nialpais and the caliche, and ran on the surface of the
bed rock.

The relation of the root-systems of tlii,- three species is shown graphically
in the cuts of the horizontal and vertical extension. It will be seen that,
although growing in close jjroximity to one another (the Covillca was only
50 cm. from the cactus), the roots are not in physical contact anywhere, so
that comjietition for water in the soil is probably not keen in this instance,
notwithstanding the ]jroximity of the plants.

Covn.i,E.\ Tride.nt.\t.\.

Perhaps the most widely distributed perennial in the \icinity of the
Desert Laboratory is Covillea tridentata, which occurs not only on the bajada,
where it is the most characteristic shrub, but on the flood plain of the
Santa Cruz river and West Wash, and on Tumamoc Hill. It attains its
most luxuriant growth on the flood-plain, or elsewhere where the soil is
deep and the water conditions relatively favorable. The specimens spe-
cially studied were situated on the flood plain of West Wash and on the
bajada at the northern base of Tumamoc Hill. In these locations the
extremes of soil conditions, as far as concerns its depth, were met, together
with the greatest dift'erences in water relations to be found in the four
habitats under consideration.

The upper soil at the bajada habitat is of adobe clay, with an admixture
of rock fragments and larger stones, about 20 cm. in thickness, below which
is a la^'er of rotten caliche, also about 20 cm. thick, and the latter is under-
laid bv hard caliche of undetermined depth. A section of similar soil con-
ditions, made from a photograph of a cut not farfrom the habitat inquestion,
is shown in plate i, and illustrates the conditions which obtain here.

The larger plants of the vicinity were all of the genus Coiillca. of which
those nearest the specimen were i .5 meters northwest, 2.6 meters southwest,
and ,vi meters south. The roots of each of these plants, and probably the
roots of others also, invaded the root-area of the specimen speciallv studied.

The Coiillca examined was 95 cm. high and was composed of numerous
branches. It was a typical and vigorous plant. The leading characters
of its root-svstem were found to be as follows : A main root went down
to the hard caliche, about 35 cm., where it forked. One branch was traced
about ; meters northwest and lay from ;;5 to 53 cm. beneath the surface
of the ground. The other branch was followed about i meter and lay from
35 to 45 cm. deep. These were the longest roots of the plant and
extended to pockets in the hard caliche.

KOOT-SVSTEMS OF PEREXXIAI.S.

59

Fig. 6. — Root-systems of Covillea Iridentaia, from various locations on the domain of tl:c

Desert LaboratoO'.
I! and h. Horizontal extensions of root-system of one plant, separated for sake of cleanicss.

Dotted lines indicate the intruding roots of neighboring Covilleas encoimtered on the

same level, or nearly so, as the roots of the plant studied,
c. Vertical extension of bajada-grown plant.
d and e, Vertical extensions of root-systems of two plants from the flond-plain of West Wash,

Entire length of laterals in c is not shown.

About lo large and numerous small laterals were given off from the
main root within 15 cm. of the surface of the ground. These roots were
from 5 mm. to i.i cm. in diameter at the base, and the longest extended
as far as 4 meters from the central root axis. This, the longest lateral,
branched from the main root ;, cm. from the surface, and lay 2r, ;,9, ;,i,
and 28 cm. from the surface of the ground at various distances from its
base. The position of this root with relation to the surface is fairly illus-
trative of all of the rest. The mutual relations of the principal roots,

6o RdOi llAIUTS OF DESERT I'l.ANTS.

together with vertical extension of two plants from West Wash, are shown
inlig.6. The general character of thebascsand tlKniimberof smaller laterals
are adequately shown in plate 1 6. Besides the larger branches of the main
laterals, which, as fig. 6 shows, were relatively few, there were groups of
filamentous roots occurring along their course as in Franscria and other
plants. Although not specially studied in Covillea, these rootlets were of
limited growth, serving the plant for a short period only, as in the species
mentioned.

The soil of the flood-plain by West Wash, where the other specimen of
Corillca studied was growing, was of adobe and sand to a depth exceeding
2.1 meters. The perennials of the habitat, both as regards kinds and nun;-
bers, were more numerous than on the bajada. Near the specimen of
Covillea from the flood-plain, whose roots were examined, were Acacia
grcggii, Lycium andeisonii, Parkinsonia ioireyana, Prosopis lelutina, and
other forms, mainly shrubs.

The Covillea studied by West Wash was 1.65 meters high, or somewhat
larger than the bajada specimen examined, and, from the size and number
of the leaves and the character of the branching, it gave evidence of having
a better water supply than the bajada-grown plants. An examination of
the root-system of this plant showed that in certain ways it was very dif-
ferent from that of the plant from the bajada. The points of difference
appear well in comparing the vertical extension of the roots of the two
plants, as shown in the figures. The tap root was traced to its end at a
depth of 30 cm., although in another specimen growing near by the tap root
went down 1.7 meters, indicating that in the specimen under consideration
some calamity had brought about the untimely end of the main root. The
laterals were, in part, given off from the bases of the shoots and, after
leaving the main plant axis, had little uniformity in position. Some lay
20 to 45 cm. from the surface, others were still deeper. Such roots took
a more or less horizontal position. From these laterals branches were
given off, one of which went straight downward as far as 2 . i meters.

Although the laterals of the first (?) order ran fairly horizontally as a
whole, they were not as long as the corresponding roots of the bajada-
grown plant; the radius included within the root-area did not exceed 2.2
meters, as opposed to nearly twice this figure in the other form studied. In
addition to the larger laterals just described, many smaller ones arose from
the bases of the shoots or from the bases of the larger laterals. As plate 16
shows, there were in the plant from the flood-plain many more small adven-
titious roots than in the plant from the bajada. The laterals bore relatively
few branches.

The root-system of the specimen of Covillea from the flood-plain may
be characterized, therefore, as having a deeply' penetrating main root and
laterals which run in a fairly horizontal direction, although they may lie
as deep as 53 cm. and may give off branches that go straight downward.

ROOT-SYSTEMS OF PERENNIALS. 6l

The number of small laterals at the bases of the plant is especially large.
The roots are mainly either horizontal or vertical, without being at any
intermediate angle, a character so strong in the root-system of Prosopis,
to be described later, and of so wide occurrence among the larger plants,
where the soil conditions are favorable, that it forms one of the most striking
of the general root characters of desert plants.

The lateral extension of the roots of the Covillca from the flood-plain is
considerably less than that of the plant from the bajada. Those of the
latter reach out over 4 meters, while the roots of the former, as mentioned
above, extend hardly half as far. The root-area, or the area included
within the reach of the roots of the bajada-grown Covillea, was about 50
square meters, while that of the plant from the ffood-plain was only one-
fourth as much. These differences make it imperative that fewer plants
occupy a given space on the bajada than on the flood-plain. It is of interest,
however, to note that the mass of earth compassed by the root-system of
the two specimens of Covillea was approximately equal.

With the characteristic differences in root development and root posi-
tion of the bajada and flood-plain plants are associated differences in the
relations of these plants to their neighbors. In excavating the roots of the
specimen from the flood -plain no roots of other woody plants were encoun-
tered within the root-area of the plant studied. But the condition of the
bajada-grown Covillea was quite otherwise. No fewer than 60 roots of
neighboring Covilleas were encountered at the same level, and frequently
in physical contact with the roots of the plant studied, and ver}^ many
more were met between the levels of these roots and the surface of the
ground. Therefore the competition between neighboring Covilleas on the
bajada, for soil water, is presumably keen, while, on the other hand, com-
petition between neighboring Covilleas on the flood-plain, for soil water, is
at best indirect, and may be so slight as to be negligible.

FououiERiA Splendens.

Foitquieria occurs most abundantly on the northern slope of Tumamoc
Hill, on its lower drainage slope, or the upper portion of the bajada, and
on the bajada near West Wash. It was not found on the flood-plain of
West Wash or of the Santa Cruz, that is, in other words, the species in the
vicinity of the Desert Laboratory does not grow where the soil has consider-
able depth, so that the "normal" behavior of its roots is not known.

The soil conditions where the specimens of Fouqtiieria specially studied
were growing, the bajada to the north of Tumamoc Hill, are nearly the
same as those of the Covillca habitat above described, except that the super-
ficial adobe clay is somewhat thicker and there may be a greater admixture
of small stones. The superficial soil is underlaid by rotten caliche and the
latter by hard cahche, as in the habitat referred to.

6-'

K(j<(T n.\iin> uF i)i:si:kT i'i..\\ts.

Fig. 7. — Root and slioot liabit of Fciitqtticyia -^pleflJetts.

ROOT-SYSTEMS OF PEKEXXIALS.

63

The othor woody plants growing near I-'oiiguiciia were mainly Covillca
iiidcntala. and an occasional specimen of ParkiiiiOHiii intcrophyUa.

Two specimens of Fouquicria were specially studied, of which one was
young and the other mature. The younger plant had a shoot 24 cm. high,
which had a central main axis and two branches 3 cm. and 15 cm. long.
The habit of the plant is well shown in fig. 7.

The roots of Fouquicria splcndciis are coarse and brittle, and bear rela-
tively few branches. The tap root was large and penetrated the ground
8 cm., where it forked, one branch extending horizontally 13 cm. and the
other going down 18 cm. before branching. Each of the main forks of the
main root branched onlv two or three times.

Fio. 8, — HorizonUiI extension of the root-system of Foiiqitieria splciijeus from the bajada at
the north base f)f Tumamoc Hill. Dots represent position of CoviUea tridenlata. \\hii:h
occurred within the root-area of FuuquUria, and dotted lines such roots of CoviUea as
were found on the same horizon as the Fottqiiieria roots.

Above the forking of the tap root, about 3 to 4 cm. from the surface of
the ground, two laterals arose, which, as far as traced, ran in a nearlv hori-
zontal direction. One of these bore 3 branches and was over 22 cm. long.
At the base the laterals were less than i cm. in diameter, and, where broken
off, they were slightly more than i mm. in diameter. Thus the roots of
Fouquicria are so heavy that they may perhaps be termed fleshy.

64 ROOT HAniTS OF DliSHKT I'l.ANTS.

A character of the smaller roots of the plant was the bcarinfj of Rroiips of
filamentous, adventitious rootlets about i cm. in lenj(th. At the time the
study was made, January, these rootlets were dead.

The only roots of neighboring plants which intruded on the root-area of
Fouquicria were two of Parkinsonia micro phylla, which lay 3 and 1.5 cm.
from the surface of the ground.

The larger specimen of Foitq^iieiia examined was 1.75 meters high and
bore numerous long branches. In February, when the study was made, the
plant was without leaves.

The following w-cre the main characters of the root-system of the larger
plant. A tap root penetrated the ground until it met a mass of caliche
which lay immediately below the base of the plant. The depth attained by
the root was about 1,5 cm. Five laterals arose immediately below the sur-
face of the ground ; they were 5 cm., more or less, in diameter, at the base.
It is a characteristic of Foiiqiiieiia that the roots, although relatively heavy
at the base, taper rapidly; for example, one 5 cm. in diameter at its base
had a diameterof i.i cm. about 50 cm. away. After the diameter of i.o cm.
is reached, the root maintains this very closely for a considerable distance.

The depth usually attained by the laterals may be illustrated by a few
examples. Taking a typical root, w-e find that while it leaves the tap root
immediately beneath the surface of the ground, at a distance of 40 cm. it
is 25 cm. deep ; at 50 cm. it is 16 cm. deep ; at a distance of i meter it is 23
cm. beneath the surface; and at a point 1.75 meters from the main axis,
where it is 2 mm. in diameter, the root is 21 cm. deep. A branch of this
root was traced downward to a depth of 37.5 cm., but at that point it was
no longer living.

Groups of filamentous roots were not seen in the larger specimen. There
were several specimens of Covillea in the area included within the scope of
the roots of Fouquieria. One was about 25 cm. distant and its roots occu-
pied much the same area as those of the Fouquicria, but they were deeper
and, although the roots of Fouquieria were exposed with great care, only
four instances were noted where roots of the neighboring Covilleas were
found in the same horizon as those of the plant examined. Competition
between the tw-o species of shrubs, therefore, is probably not very active.

Fr.\nsERIA Deltoidea.

Although perhaps as widely distributed locally as Covillea, the character-
istic habitat of Franseria deltoidea on the domain of the Desert Laboratory-
is the bajada, just to the west of West Wash. In this habitat the popula-
tion of Fratiscria for any given area is greater than that of any other shrub
in its characteristic habitat in this vicinity. In fact, the species at the
place in question fairly completely conceals the ground.

In the neighborhood of the Franseria examined, the following w'oody
plants also occurred : Covillea tridentata, Kramcria canescens, Krameria

^-.

A B

A. Covillea tridentata from bajada at north base of Tumamoc Hill. Horizon-
tal and vertical extensions of roots of this plant shown in fig. (i.

B. Covillea from flood-plain near West Wash, showing the large number of
slender adventitious roots springing from upper part of roots and long tap-root, of
which only a portion appears.

C. Fouquieria splendens from bajada near where Covillea shown in A was
growing. Horizontal extension of roots of this plant shown in fig. S.

a. Franseria deltoidea from flood-plain, FL-bruary, 190i), showing
part of tap-root. b. Lateral of the first order bearing filamentous roots
in groups, which, at time photograph was made, were no longer
functional. c. Krameria canescens from flood-plain near West
Wash, February, 1909, showing essential superficial placing of roots,
d. Lycium andersonii from near West Wash.

ROOT-SYSTEMS OF PERENNIALS.

65

parvijolia, Opuntia discata, Opuntia versicolor, other species of Opuntia, and
Parkinsonia micro phylla.

The soil of the habitat is much hke that of the portion of the bajada at
the northern base of Tumamoc Hill, where CoviUca was studied, adobe clay
constituting the upper soil to about 20 cm. depth, under which is rotten
caliche, the latter resting on hard caliche. The situation is probably the
most severely arid of any habitat studied.

Fig. 9. — Vertical extension of the root-system of Franscria dcltoidca from
dilTerent habitats, a. root-system of a bajada-grown plant. Upper
surface of caliche hardpan indicated by dotted line, b, root-system of
a plant from the flood-plain by West Wash to show depth to which
tap root penetrated, 1.8 m.

Two specimens of Franseria from the bajada habitat and one from the
flood-plain of West Wash were examined. Therefore, as regards soil con-
ditions and water relations, the two habitats present the greatest contrast
to be found on the domain.

66 KDoT II.\III1>> iiK DICSICUT I'l.AN'TS.

The sliool ol llif larger sjK-ciiiR-n of Iiiiii.sciiii from llu- hajaila was 50
cm. high and was composed of numerous slender branches. The species
is well covered with leaves, so that, compared with most of the local shrubs,
it has a large transpiration surface. The leading characters of the shoot
are shown, but rather imperfectly, in plate 17.

The root-system consisted of a tap root bearing several laterals and of
numerous adventitious roots originating from the bases of the shoot. The
taprootwent straight down 29cm., where itforked,one root penetrating the
rotten caliche to a depth of .^8 cm. from the surface of the ground. About
12 cm. from the surface, the main root gave off several laterals which
reached as far as 1.6 meters from the main axis and lay 12 to 30 cm. deep.
Thus the roots are confined to soil abo\c the hard caliche. The roots are
dark brown, very brittle, and are ridged longitudinally with cork. The\-
are always slender; the largest root, the tap root, 2 cm. from the crown,
■\vas only 8 mm. in diameter.

The younger specimen of l-'iaiisciin sludii.(l was ;, meters distant. Its
shoot was 14 cm. high and composed of four separate branches. Upon dig-
ging, it was learned that the surface soil was relatively deep, and that over
a small area the hard caliche had given place to adobe, a condition similar
to that shown in plate i .

A main root of the young plant was traced down i meter, and evidently
penetrated farther. The main root gave off one lateral 3 cm. long, another
less than 10 cm. long, and a third which, shortly after leaving the main
root, penetrated to a depth of 60 cm. The vertical extension of the root-
system is shown in fig. 9. No roots of neighboring plants were seen in the
root-areas of this plant.

For comparative purposes, the roots of a Fianscria growing on the flood-
plain not far to the east of the proper habitat of the species were also
examined. The soil is a sandy loam, sand and adobe, of a depth exceeding
2 meters. No excavations have been made below this level. The place is
occasionally flooded and the water-table is variable as to its depth, but
probably the ground at a depth reached by roots is moist all of the year.

The leading woody plants of the flood-plain in the immediate neighbor-
hood of the specimen of Fiansciia studied were Acacia grcggii, Covillea
tridentata, Ephedra tvijiirca, Lyciiim andersonii, Parkinsonia torrcyana, and
Prosopis veluiina. The flood-plain is the characteristic habitat of all of
these except Covillca and Franseria.

The leading points of interest in the root-system of Fianscria from the
flood-plain were as follows : A tap root which went straight down toadepth
of 1.8 meters was only 5 mm. in diameter at its crown. For the most part
the laterals arose within 15 cm. of the surface, and one of these extended
1.4 meters in a fairly horizontal direction (see fig. 9) ; another took a down-
ward course at an angle approximating 45° until a depth of 45 cm. was
reached, after which it was approximately horizontal : a third root, after

ROOT-SVSTEMS (IF TEREXXIALS. 67

reaching out about 20 cm. from its place of origin, turned directly down-
ward to a depth of 78 cm.

Besides roots arising on the main root, and of primary origin, there were
over 50 slender adventitious roots, about 3 mm. in diameter, which arose
from the shoot bases and from the crown of the main root. For the most
part, these roots took a horizontal direction, but one of them ran downward
and was traced to its end at a depth of 1.3 meters. Such laterals as lay
about 15 cm. from the surface of the ground bore groups of filamentous
roots (plate 17), which were dead when the study was made. The rootlets
were 2 cm., more or less, in length and were six or so in a group ; the groups
were from 5 mm. to 2 cm. apart.

Filamentous roots were seen also in the bajada-grown plants, and in
others from Tumamoc Hill. In January, 1910, after the soil had been
moistened some days by rains, freshly formed rootlets were seen on many
roots of Franseria. These roots, therefore, are formed in winter, and
probabh- in summer as well, and are very short-lived. Xo study has yet
been made of the conditions under which the rootlets are developed, their
period of activity, or whether, if conditions remained constantly favorable,
they would be active indefinitely.

Kr.vmeri.v C.\XESCENS.

One of the most generally distributed species in the vicinity of the
Desert Laboratory is Krameria canescens, occurring as it does on each of the
physiographic areas treated in the present paper. It attains its largest size
where the water relations are most favorable, as on the flood-plain of the
wash, on the bajada near it, and on Tumamoc Hill. As has been shown
elsewhere* Krameria is an habitual parasite and this is probably the lead-
ing cause for the wide distribution noted.

The first specimen of Krameria, of which the root-system was examined,
was an isolated plant on the flood-plain of West Wash. The soil condi-
tions of the place have already been noted ; in brief they were : a sandy clay
over 2 meters deep and relatively favorable water relations, as evidenced
by the occurrence of the most hygrophilous of the native species.

The specimen studied had a much-branched shoot, about 60 cm. high,
which was leafless in February. The general character of the shoot is shown
in plate 17. The root-system consisted of a short tap root, no longer living,
and six, or more, laterals which extended as far as 2 meters from the main
axis (fig. 10). Besides the main laterals there were about an equal number
of more slender ones, less than 2 mm. in diameter and about 20 cm. long,
which ran in a horizontal direction. The longer roots, from 4 mm. to i cm.
in diameter at the base, lay from 13 to 18 cm. beneath the surface. In
addition to these roots, several were dead and decaying. A very striking

*Conditions of Parasitism in Plants, by D. T. MacDougal and W. A. Cannon. Publi-
cation No. 129, Carnegie Institution of Washington, loio.

68

ROOT HABITS OF DESKRT PLANTS.

Fig. 10. — Root-system of Krameria canescens.
The large laterals d, d, d were dead.

feature of the root-system of the plant was the ahiiost total absence of
branches, nor were the filamentous roots which occur in Fianseria, Encelia,
and in other perennials, observed in Krameria.

Only two foreijjn roots
(shown as dotted lines, fig.
lo) of an unknown species
were encountered during
the excavation of roots of
Krameria .

The presence of so many
dead roots made it advis-
able to examine another
specimen of equal age in
order that a perfectly "nor-
mal" root-system should
be described. Accordingly,
another plant, also growing
on the flood-plain, was
selected for study. The
results of the second excava-
tion substantiated those of
the first, namely, that the tap root had reached a rather large size before
dying, that there were many dead laterals, and that those yet living were
near the surface, although the soil at the place was of considerable depth.
In examining another plant, under circumstances which allowed the obser-
vation of the relation of the roots of the species to those of its neighbors,
the fact of the parasitic habit of the plant was revealed.

After the discovery of parasitism in Krameria a very large number of
specimens growing under widely difierent conditions were examined and
root attachment to the following plants was demonstrated: Acacia con-
stricta, Covillca tridenlata, Encelia jarinosa, Ephedra trifiirca, Franseria
deltoidea, Lycium andersonii, Menodora scabra, Opuntia sp., Parkinsonia
viicrophylla, Prosopis vehitina, and Zizyphus parryi. Subsequentlv para-
sitism of the plant on Parkinsonia acidcata, a species indigenous within 75
miles of Tucson, was induced in cultures.

The attachment to the host is usually by means of small roots and only
rarely does one find the association of old roots of host and parasite. Thus
the connection is probably made during the growing seasons, and persists
if the conditions continue favorable during the subsequent drj- periods.

The hosts already found are only trees and shrubs, and it is not at present
known whether the parasite is strictly limited to such, or whether it may
not derive temporary sustenance from annuals, some of which are very
resistant to drought and endure for considerable time after the rainy seasons
have closed.

^^^v%-'-^

^pt

.^,-

V

.,w^^:k

■Va

Bird's-eye view of superficial roots of Opuntia arbuscula, from bajada,
a mile east of the Santa Cruz, showing their fleshy character.

ROOT-SYSTEMS OF PERENNIALS. 69

The root-system of the seedling has a pronounced tap root with relatively
few and short laterals which do not bear haustoria if the plant is growing
alone, but develop them if the roots of another form, such as Parkinsonia
aciileata, are in close relation with its own. The root-system of the seed-
ling, however, differs from that of autotrophic plants in the fact that it does
not form root-hairs. As the plant develops, the tap root continues to be a
leading feature, since in the old plants remains of the tap root have always
been encountered. When or why this organ dies is not known. In the
mature plant the superficial laterals become the most prominent feature of
the root-system and include a large area within their compass.

In associating character of root-system with habit in any species, it has
repeatedly been shown above that an autotrophic plant without water
reserve is always provided with a deeply placed root-system, or as deeply
as the soil permits, but that a plant with water-storage facility has a root-
system which lies near the surface of the soil. In Krameria the roots are
shallow, although the form is not a water-storing species — suggestive of
unusual life-habit of the species.

Opuntia Arbuscula.

Among the species of cacti limited to or characteristic of the bajada are
Opuntia arbuscula and 0. fulgida, though the latter only occurs on the
domain of the Desert Laboratory The habit of the two arborescent cylin-
dro-opuntias is unlike. The species arbuscula occurs in groups of few
individuals usually, of common descent, as will be shown, while fulgida at its
best forms fairly dense growths, to the exclusion of other woody plants.

The specimen of Opimtia arbuscula chiefly studied was one of a small
group growing on the bajada about a mile eastof the Santa Cruz river, April,
1907. The upper soil is of adobe clay to a thickness approximating 20 cm.
with a layer of rotten caliche beneath, of variable thickness, which rests on
the hardpan.

In the vicinity of the cactus were Acacia constricia, Bigclowia hartwegii,
Covillea tridentata, Krameria cancsccns, Opuntia fulgida, and O. spinosior.
Of these species, 3 plants of Covillea and one of Bigclowia occurred within
the root-area of the cactus. The Covilleas were 20 cm. west, 70 cm. south,
and 2 meters northeast, all mature shrubs of large size.

The Opuntia whose roots were examined was a much branched and old
plant about a meter high, apparently in perfect health and in every way a
normal specimen. Its root-system was composed of an anchoring portion
and a widely reaching horizontal portion (fig. 11).

The anchoring roots were not specially studied. The horizontal sj'Stem
consisted of 4 main members arising from the crown of the tap root just
below the surface of the ground. They radiated from the main axis in such
fashion that the ground around the base of the plant was fairly equally
divided between them. These main laterals, as distinguished from the cor-

70

RoiiT MAllITS i>K HKSICkT PLANTS.

respondinfj roots of all oiIrt opuntias seen, branched repeatedly through-
out their course, in places forming a network closely covering the ground.
The laterals extended for about t, meters and lay so near the surface of
the ground thai the tips could be lifted out with a walking stick, and thev
could be torn out of the ground their entire length. More exactly stated,
the roots lie from 2 cm., and even less, to 8 cm. beneath the surface.

I'iG. 1 1. — Horizontal extension of root-system of Opuntta arbusciila. Black dots indicate position
of CLnilU-a Irideittalii which had invaded the root-area of the cactus.

As compared with the roots of other opuntias, those of aibuscula are
large (plate 18). This will be appreciated from the following measure-
ments. A meter from its base one of the larger roots had a diameter of
2.5 cm., 2 meters distant it was 2 cm., 3 meters distant it was 1.5 cm. The
tips of these roots, excluding the most recent growth, were not less than
I cm. in diameter. From the size and tuber-like appearance of the roots it
was concluded that they function as water-storing organs. This conclusion
was strengthened by the obser^-ation that inside of 48 hours after the roots
of this plant ^vere removed from the soil they were rapidly losing their
plump aspect and had begun to slnivel strikingly. A noteworthy pecuU-
arity of the larger roots was their frequent abrupt ending through the death
of the more distal portion.

The lleshv roots of aibi<sciila. in addition to functioning as water-storing
organs, a rare root-character among mature opuntias, further ser\'^e as prop-
agating organs, and to this function the occurrence of the species in masses
is due. The procedure, in brief, is this. One or more shoots may arise
from a single lateral while the latter retains its connection with the jiarent.

KO(iT-SVSTEMS OF riiREN'.N'IAI.S. 7 I

The shoots put down roots, the- connection with the parent plant is prob-
ably early cut off (when was not learned), and the daughter plant is inde-
pendent. The result is that arbu.scula frequently forms clumps several
meters in diameter, to the total exclusion of other species.

From the previous description of the root-system of Covillca it is perhaps
hardly necessary to point out that the roots of this species growing within
the root-area of the cactus are much deeper than the roots of the latter
plant, and hence are not in actixe competition with the cactus roots for
their water su]5ply.

Opun'Tia Fulgida.

On the domain nf the Desert Laboratory, Opiiutia fHl<;ula occurs only
on the bajada near West Wash, where in places it forms fairlv dense growths
constituting the most prevalent species. This is an unusual habit for anv
cactus in this vicinity, since usually the cacti are either scattered in an
apparently chance fashion, unless controlled by such external factors as
temperature or other physical environmental conditions, or at best but
few individuals of a species are congregated.

The soil in the habitat of jiilgida is a sandy clay with an admixture of
rocks of various sizes which appeared to be in stages of active disintegra-
tion. No caliche was found to a depth of 60 cm. The habitat, therefore,
presents certain striking differences from that of Fransoia. not far distant.
In the immediate vicinity of the Opiuitia studied were Acacia constricia,
Echinoccrcus jcndlcvi, Covillca lyidcntata.Opiintia jiilgida, ( ^puniia versicolor,
Parkinsonia microphylla, and Riddcllia coopcri, and of these plants only one
example of Acacia and two of Riddcllia occurred within the root-area of the
cactus. vSince, as will appear directly, the roots of julqida extended as
far as 3 meters from the main plant axis, the perennial jiopulation of the
habitat is relatively light.

The specimen selected for study was growing somewhat a])art from the
t\pical fulgida habitat, but the plant appeared to represent thoroughlv the
typical and mature species. The root-system, like that of Opuntia vcrsi^
color, has a somewhat deeply penetrating portion, the anchoring roots, and
a widely reaching horizontal portion not far from the surface of the ground.
The horizontal and \ertical extensions of the root-svstem are shown in
fig- 12-

The anchoring system consisted of about six stout roots which ran down
at an acute angle to a depth of 35 cm. Of these the largest was 2.5 cm. at
the crown ; it tapered rapidly and gave off several branches which ran out-
ward for a distance of 5 to 10 cm. The anchoring roots did not reach hard-
pan or bed-rock, or other material which might prevent deeper penetration.
The superficial root-system consisted of 6 main laterals, of which 4 ran 3
meters, more or less, in an easterly direction (down-hill), and the others
took a fairly well defined up-hill direction. The roots branched but little;
for example, one ;, meters long gave off no important branches, and a some-

ROOT IIAIIITS uK I)i;sivRT I'l.ANTS.

wliat larger ouv hraiiclictl onh- twice. Tlic roots of no cactus studied, and
in fact no perennial plant except the parasite Kiamciid, branched less than
this one.

The roots were slender, as the following representative measmements
will indicate. One of the superficial roots which was 3 meters in length
was 6 nun. in diameter 35 cm. from the base; another long lateral, 1.2
meters from its base, was 3 nmi. in diameter; and a third root, 1.5 meters
from the point of origin on the main root, was 4 mm. in diameter. This

Fig. 12. — Root-system of O/jhjj/i.i Ixdoida. In the sketch of the horizoulal extension of O. iulgida,
.\, two specimens of Riddellia cooperi (a) and one Acacia cojistricia (6) are shown. The
vertical extension of the roots of O. fulgida is given in b, and of Acacia consiricla in c.

system was also shallow, the main lateral running southwest, at a point 35
cm. from the central axis, was 4 cm. deep, and at a point 1.2 meters from
the main plant axis it was 6 cm. beneath the surface of the ground. The
other laterals also were found to lie 5 to 6 cm. below the surface, although,
as was pointed out above, the soil at the place was relatively deep and
would have offered no obstacles to deeper root penetration.

The roots of the 3 specimens of RiddcUia cooperi, growing within the
root-area of the cactus, were also examined. They each sent a tap root
straight down, and in each root a few laterals were given ofi" which ran in
a more or less horizontal direction as far as 10 cm. How deep the tap
root of RiddcUia penetrated was not learned. At no place, however, did
the root-svstems of RiddcUia appear to occupy the same horizon as the

CANNON

-ju v-i'.

A. Riddellia cooperi, from sandy loam east of West Wasli, showing the large num-
ber of slender laterals arising from crown of tap-root and the branching feature of
the bases of the main laterals. B. Riddellia from red clay soil near West Wash, to
illustrate lack of slender roots at crown of tap-root.

C. Dasylirion texanum, from bajada near Rincon mountains, with root-system
partly exposed.

D. Shoot and root habit of Yucca sp., from bajada near Rincons. Plant supported
by two props, one at side and one below, which should not be mistaken for roots.

E. Root character of Yucca radiosa from bajada near the Rincons.

ROOT-SYSTEMS OF PERENNIALS. 73

roots of the cactus, so that active competition between the roots of the two
species could hardly have taken place.

The specimen of Acacia, which was about a meter distant from the
main axis of the cactus, had a generalized type of root-system. It con-
sisted of 2 main roots and about 12 smaller ones. Of the larger roots, one
ran north of the main axis of the cactus and extended beyond the east
boundary of its root-area, as fig. 12 indicates, and lay from 9 cm. to 40 cm.,
where it was left, beneath the surface. The other roots descended rapidly
and evidently penetrated deep, although they were not traced more than
60 cm. from the surface. The disparity in the position of roots of O. fulgida
and of Acacia was so great that there could be no active competition
between the two species for ground water.

RlDDELLI.\ COOPERI.

Riddellia coopcri is one of the widely distributed shrubs in the vicinity
of the Desert Laboratory, occurring not only on Tumamoc Hill and on the
bajada, but on the flood-plains both of the Santa Cruz river and of West
Wash. It was studied on a slope separating the bajada west of West Wash
from the flood-plain of the wash and also on the flood-plain east of thewash.
The soil west of the washisred claycontaining coarse particles of sand, with
the caliche hardpan at a depth of 55 cm.; the soil to the east of the wash
is a sandy loam, apparently homogeneous to a depth exceeding 1.4 meters.

Riddellia is about 30 cm. high and consists of numerous shoots, the ter-
minal portion only, equal to about half, bearing flowers, and the proximal
portion onlv bearing leaves. This should be borne in mind when consider-
ing the stature of the plant as presented in the succeeding measurements,
since, for various reasons, the measurements were made to include both
the flowering and the non-flowering portions of the shoots.

The specimens situated west of the wash had a generalized type of root-
system. There was a tap root, the lower portion of which was dead, which
gave off a branch that descended gradually until it attained a depth of 48
cm. where its tip was 50 cm. distant from a line dropped straight down
from the crown of the tap root. In addition to this root, 5 laterals arose
within 20 cm. of the surface of the ground and extended in a fairly hori-
zontal direction for a distance of i meter from the main plant axis. These
laterals bore few branches and scarcely any filamentous roots, such as are
a feature of the laterals in Franscria, Encclia, and other species, and the
filamentous roots were not borne in groups, but occurred singly.

The roots of Riddellia extended so short a distance in a lateral direction
that the only root found within its root-area was a single one of Acacia
const) icla (fig. 13).

The specimen of Riddellia growing east of the wash had a shoot 32 cm.
high, or the same length of the plant just described, but, as will appear
directlv, in several particulars its root-system was unlike that of the other

74

KiKiT IIAHITS iiK 1>1-:SI:KT PLANTS.

specimen. A lap rool was traced down 40 cm. and gave ofT, within 15 cm.
of the snrfacc of the ground, 6 main laterals. These reached out less than i
meter from the main root axis, and certain of them were inclined down-
ward at a rather sharp angle. The laterals gave off frequent branches of
two sorts, filamentous as described for the other specimen, and relatively
large ones. In addition to the main laterals and their branches, there
were very numerous slender roots close to the surface and taking their
origin also from the tap root. The latter type of roots was entirely lacking
in the first plant examined. The contrast between the two plants in this
regard is shown, but not adequately, in plate 19.

Fig. 13. — Root-systems of RiddeUia coopcri.
a. Horizontal extension of root-system which was growing in the sandy loam cast of West Wash..

showing the most prominent laterals but none of the large number of slender ones which were

at base of shoot,
fc. Vertical extension of plant shown in a. Tap root penetrated 1.44 m. beneath surface,
c. Horizontal extension of root-system of a specimen which was growing west of the wash in clay

soil. Dotted line shows position of an intruding root of a neighboring Acacia cotistricta.

In considering the form of the root-systems of the two plants, both hav-
ing a relativelv short tap root, though the systems were such as to lead one
to expect a deeply penetrating root, it was decided to excavate the roots of
another plant with the view of observing especially the behavior of its tap
root. Accordinglv, a third plant was selected, growing east of the wash
and close by the first specimen studied. The soil conditions where the sec-
ond and third specimens examined were growing were apparently the same.
The shoot of the last plant studied was 38 cm. high. The root-system of
this specimen was dominated b}- a tap root, which was traced to its end,

KOOT-SVSTEMS OF I'ERKXMAl.S.

75

1.44 meters deep. Like the root-systems of the other plants, this one also
comprised several laterals which were borne about 15 cm. from the surface
of the soil and which reached about 90 cm., but then, turning sharply, ran
directly downward to an undetermined depth. The slender roots, so
numerous on the last plant examined, were also a feature of this one.

Dasylirion Texaxum, Yucca Radiosa, Yucca sp., and Agave sp.

Up to this point the plants of the bajada, which have been described,
grew in the immediate neighborhood of the Desert lyaboratory, or at most
not more than a mile distant, but the plants selected for the purpose of
examining tvpes other than had been seen heretofore, were found in the
region west of the Rincon mountains, about 20 miles east of Tucson. The
bajada at that point was perhaps 4 miles from the base of the main range
of the mountains and at an elevation approximating 3,600 feet, or about
1,300 feet higher than Tucson. The greater altitude means lower tempera-
tures and greater rainfall, although, as there are no records of the locality,
the extent of the difference from Tucson in these regards is not known.

The soil conditions
varied where the plants
studied were growing.
At the Yucca sp. habitat
the upper soil, for more
than a meter, was sand
and red clay with caliche
underlying, while in the
habitat of \'iicca ladiosa
and Dasylirion texamaii
the upper soil, although
of like character, was
only about 20 cm. in
thickness. The two
plants first named were
50 meters apart, and
were about 200 meters
from Uasylirion. The Agave sp. was foimd on the upper bajada slope near
the base of an outlying range approximately 2,000 meters south of the
other plants. The soil was similar to that above described, except that
there was a larger amount of sand and rocks.

In the immediate vicinity of the Agave studied were observed CaUiamlia
sp., Encelia jarinosa. I'ouqukria splcndens, Hy/ylis sp., Opuutiii blakcaiia.
and Paikinsonia iiiicinf^hylla. Of the two specimens of .1 (/<;;'(■ examined,
one was about 20 cm. high and the other somewhat larger. The roots of
young agaves, and of the older ones as well, were borne in a cluster at the
base of the shoot. All of the roots were of about equal length and verv

Fig. 14. — ti. \'crtical CKteiision of root-system of \ need
radnna from the bajad:i near Rincon mountains. Dotted
line indicates boundary between adobe upper soil and
underlyinii rotten caliche.
/), Vertical extension of root-system of ^'iicc<i sp. from the
Ijajada near the Rincons.

76 ROOT HABITS OF DKSIvRT PLANTS.

coarse. The older ones wcR'shrivckd, and of a dark red color, but the voung
roots, which were put out from the base of the plant and not as branches of
already existing roots, were stout and pinkish. The young roots ended
abruptly and very evidently functioned more as storage than as absorbing
organs. No tap root was seen in any specimen of Agave examined.

The bases of the leaves and the axis of the stem of Agave are fleshy and
ser\-e the purpose of storing both water and food-stnfTs against the flower-
ing season or other periods of need. The other plant studied at this time,
which has storage facilities above the ground, Dasylirion iexanum, has, as
will appear at once, a similar root-system.

Turning now to consider Dasylirion and Yucca and the habitats in which
they were growing, we shall find, as revealed by the character of the vege-
tation as well as shown by soil differences, that the bajada farther from the
mountain base does not have uniform conditions. For example, in the
vicinity of Dasylirion there are Acacia constricta, Bigelowia hartwegii, Opun-
tia blakcana, Ridddlia coopcri, Yucca sp. Besides these species, in the
vicinity of the Yucca sp. studied, the following occurred: Parkinsonia
microphylla, Parkinsonia torreyana, Prosopis velutina, and Zizyphiis parryi.
At lower altitudes, as at Tucson, these species do not occur together, but
are characteristic of the dilTerent well-defined physiographic areas; for
example, Parkinsonia torreyana grows only in the vicinity of washes, or
where the water conditions are relatively favorable, while P. microphylla
occurs on the bajada above the wash. Both Zizyphus and Prosopis are
chiefly found on the flood-plain, occurring not at all, or as dwarfed speci-
mens onlv. in the other and more arid areas.

In addition to the differences in the two habitats last considered, as
revealed bv the difference in the kinds of woody plants growing in them, the
Yucca sp. habitat has far the larger population. Without further investi-
gation it is assumed that this difference is wholly due to the fact that the
latter habitat has deeper soil, and hence more water and for a longer time,
than the alternate one. The root-systems of the plants growing in the first
habitat, Dasylirion and Yucca radiosa, will be first described.

The specimen of Dasylirion examined was a meter high. The roots
W'ere exposed on the southern side only and had the following character-
istics: A large number of roots, each about 5 mm. in diameter, formed a
cluster at the base of the stem. The roots ran downward at an acute angle
and also extended out into the soil in a more or less horizontal direction.
The roots were all coarse ; the most shallow were 1 5 cm. beneath the surface
and the deepest about 36 cm. deep. The longest roots were the most super-
ficial ones, of which some measured 2.25 meters, though they were mostly
shorter than this (plate 19c).

.Another specimen of Dasylirion, with the shoot about 2 meters high,
possessed a root-system essentially like the smaller plant just described,
although the soil at the place was somewhat deeper than that in which the

ROOT-SYSTEMS OF PERENNIALS. 77

smaller plant was growing. It therefore appears that the root-system of
Dasylirion is essentially a comparatively superficial one.

The shoot of Dasylirion functions as an important organ for food and
water storage, the storage apparatus being made up of the large central
axis and the enlarged leaf bases. The entire organ, in the larger plant
studied, was 25 X 30 cm. In another specimen, of which the stem axis
was 2 meters long, the water capacity would be very considerable, easily
comparable to that of the saliuaro.

The root-S3^stems of two specimens of Yucca were studied. Both of
these plants, although separated by some little distance, were growing
under practically identical soil conditions. The old specimens of Yucca, of
the type examined, have the habit of growing in colonies, to the exclusion
of plants of other species, but the younger forms occurred singly.

The shoot of Yucca sp. was 70 cm. high and bore leaves 60 cm. in length.
The subterranean portion of the plant consisted of a branched tuber-like
root, from which arose several roots of the type characteristic of Dasylirion
and Agave, that is, coarse and little branched. The tuber measured 8 X
25 cm., and was richly supplied with sap. The branches of the tuber were
also fleshy, and three or four of them bore buds destined to produce shoots.
From the tuber 26 roots took their origin, wholly from the lower surface,
which were of two sorts, namely, immature, which did not function as
absorbing organs, and mature roots. The former were of a light pink color,
1.5 cm. in diameter and about 8 cm. in length, and were well filled with sap.
The older roots were dark red and generally shriveled ; they were unbranched
and were devoid of groups of filamentous roots, a characteristic of many
perennials of other families which were studied. The roots were variously
placed. The one penetrating deepest was traced to its end 1.23 meters
beneath the surface. Another, 1.22 meters in length, ran more or less hori-
zontally and reached 8, 12, and 22 cm. beneath the surface at different
points. It terminated abruptly As compared with the root-system of
Dasylirion, that of Yucca sp. can be said to be poorly developed.

The specimen of Yucca radiosa studied had a shoot 64 cm. high and was
situated about 50 meters from the other species of Yucca just described.
It had a stout main root from which arose numerous laterals. The main
root was 5 cm. in diameter about 10 cm. beneath the surface of the ground
and expanded rapidly to a diameter of about 14 cm., which it maintained
almost to its end, 56 cm. below. The tip of the root was expanded and
divided into short forks. The entire root was thus 66 cm. in length and,
as it was well supplied with sap, it must be considered an important water
and food storing organ (plate 19, d, e).

Laterals were given off from the main root from a point about 20 cm.
from the surface to the end of the root. They were all old and in appear-
ance were like those of the other species of Yucca. As in the other species,
they were comparatively close to the surface of the ground.

78 k(ii>r IIAlilTS I>1-- DMSIIKT I'l.ANTS.

THE FLOOD-PLAIN.

TllL- bottom lan(l> wliicli conic- into this sludv coniprisu not onl\ the- llats
near West Wash and tliose along tlic .Santa Cruz river. Ijut also a branch
of the latter, the mouth of a wash having no well-delined channel, about
t) miles down the river, west from Tucson. The first two physiographic
areas both have a dee]) surface soil, and in both cases, in former years
before the Santa Cruz had cut itself a channel, flood waters at times spread
over the bottoms. The water table of the two areas is at different depths :
that of the flood plain of the river is from 5 to 12 meters from the surface,
while that of the wash is much deeper and probably fluctuates with the
seasons or the years. While a certain amount of excavating has been done,
reliance has also been put on authenticated reports and use has been made
of natural excavations, as the caving of the banks of the river and the
exposing of roots by other severe ;vashouts. On the river flood-plain have
been examined the rootsof Kivihciliniaspinosa, Prosopis vclutina.a.ndZizy'
pints panyi; b\- the Wash those of Pcnioceicus greggii, Ephedra tiijurca,
and I.ycium andirsoiiii; Opiintia 'civipara was studied in its habitat near
the Nine-mile Water-hole. The root-systems of some other trees or shrubs,
as Olnrya icsota, Parkinsonia /oncyana. Condalid spatliulata were seen in
I)art f)r incidentallv in connection with other i)lants.

THE FLOOD-PLAIN OF THE SANTA CRUZ
K(ERBEKI.INI.\ SpiNOSA.

Kaerberlinia is one of the most striking of the desert plants of the\'icinity
of Tucson. It is without leaves throughout its existence and the branches
are reduced to rather short, stout spines. The plants are usually under
a meter in height and occur in colonies which are frequently circular in
form and 4 meters more or less in diameter. Where the species has formed
such a thicket it is secure from injury by grazing animals and constitutes a
safe retreat for small rodents. The distribution of the plant in this vicinity
is somewhat limited. It docs not occur on Tumamoc Hill or by W^est Wash,
but on the flood-plain of the Santa Cruz, by the edge of the plain as well as
nearer the river, and on the bajada where soil conditions are favorable.

The habitat of the specimen oiKirrbcrlinia studied is on the line between
the bajada and the flood-plain of the river and on the lower slopes of the
bajada itself. The top soil, to a depth of 30 cm., is adobe clay with sand
or gravel admixture and under this is the caliche hardpan. On the flood-
plain just below, where the most perfect specimens of the plant were grow-
ing, the soil had a depth characteristic of the plain. The character of the
specimens, as well as the greater depth of soil, made it impracticable to
study the roots of the better-developed plants of the flood-plain.

The root-systems of three individuals, or groups, were examined with
the purpose of learning not only the type of the roots but also the manner
of development of the colonial character, a prevailing habit of the species.

ROnT'SYSTp:i\IS OF PEREXXIAI.S. 79

The first plant studied was a separate individual with a shoot under 50
cm. in height, and extremely scragg}' and thorny. It possessed a stout tap
root which went down 75 cm. and then turning sharply ran for an unde-
termined distance horizontally. At its crown the root was elliptical in
section and 2 to 3 cm. in diameter. It gave off a single branch about 1 2 cm.
beneath the surface of the ground, and a single adventitious root was found
which took its origin at the crown of the tap root. This root was 5 mm.
in diameter and ran straight downward. Both the tap root and the adven-
titious root penetrated through the top soil, and the main root went to the
caliche before turning from its vertical course. (See plate 20.)

The second study was made on a group of two plants of unequal size
and with indications that they had had a common origin. The tap root
of the larger plant went straight down, after the manner of the root of the
plant examined earlier, and bore two main laterals, of which one had been
partlv exposed bv the erosion of the soil surface, and one lay about 4 cm.
beneath the surface. The latter root kept its distance from the surface
fairly constant for the distance it was followed, i meter, and was seen to
give off frequent branches. The function of this root was evidently that of
absorption. The more superficial lateral was about 50 cm. long and along
its upper surface bore numerous small shoots, and on its lower surface many
short, slender rootlets. Xo deeply penetrating roots were found beside
the taj) root of the oldest, really parent, plant. It appears, therefore, in
these two plants, that the younger one depended on the parent for its con-
stant water supply, and that during the wet seasons this could be supple-
mented bv what was obtained bv small adventitious rootlets growing along
the lower surface of the superficial root.

The third plant group studied consisted of two plants of unequal size,
but both relativelv small. The larger of the plants, the parent plant, had
a pronounced tap root which gave oft" a single prominent lateral on which
was borne, 15 cm. from its place of origin, the daughter plant. Midway
between the two plants the connecting root was observed to be dead and
partly decayed. An examination of the daughter jjlant showed that it had
several slender and short absorptive roots, but that in addition there was
one of larger diameter which ran straight down and functioned as a tap
root (plate 20). The daughter plant, springing as a shoot from the sucker-
like root of the larger form, had finally become independent and had devel-
oped a deeply penetrating root-system of its own.

Other plants were examined in which the daughter shoots were large and
were borne on stout superficial laterals of the parent plant but which had
not developed tap roots. In the largest of the shoot-bearing laterals no
absorptive adventitious roots w^re seen, but such rootlets are present in
large numbers on such laterals as the one shown.

It would appear from observations on the distribution and root habits
of Kccrbcrlinia . that it must be in continuous connection with a water
supply. This is eft'ected cither by each plant developing a tap root of its

8o K()i)T IIAllITS ol" DKSIvKT IT. ANTS.

own, or l)y nuiiuUiining its union willi the parent anil lU-riving llie necdwl
supply through the parent root. It is this requirement that confines the
species, a])parently of the extreme xerojihytic type, to places where the
soil is of considerable depth and the water relations relatively favorable.

Prosopis Vei,utina.

Piosopis shows a \er>- marked power of accommodation to varied con-
ditions of soil and water, esjiecially the former, and when grown under
extreme conditions, exhibits differences in growth habit so great as nearly
to defy recognition. For instance, on Tumamoc Hill and on the bajada, the
species occurring to a limited extent in both places, Prosopis assumes the
form of an irregular bush, but on the flood-plain, its proper habitat, it may
become a tree 15 meters or more in height, with a well-defined bole. Fre-
quently, however, along the river-bottoms the species takes the form of
the cultivated eastern apple. By an earlierobserver (Havard, The Mezquit,
American Naturalist, vol. 18, page 450, 1884) the variation in the size of
Prosopis was taken as an index of the depth of the water table : if the tree
was large, the water lay close to the surface ; if it was small, the water table
was very deep. That the species is an indication of the presence of peren-
nial water is said to be the belief among native ranchers of southern Arizona,
who, it is said, may even follow- the roots of mesquite in digging wells — a
"water-witch" which points unerringly to subterranean water. Although
these beliefs are largely fanciful, they nevertheless have some foundation ;
where the mesquite grows large the perennial supply of water is relativelv
close to the surface, and where it is small the w-ater-supply is limited but
probably confined to the surface water.

How deep the roots of Prosopis may penetrate the soil is difficult to
learn, but it is conditioned on the character of the soil, the depth of the water
table, and the penetration of the rains. WTiere a substratum, as hard caliche
and possibly fine-grained adobe clay, makes difficult or prc\ ents deep perco-
lation of flood waters, roots of plants will not strike deep ; but where it is
such as to permit the deep sinking of the rains, or the rise ol water from
the perennial water supply, the plant roots may also penetrate to great
depth. The most deeply placed roots of Prosopis known to the writer are
those of plants growing by the Santa Cruz, which penetrate at least 5 meters,
but I have been informed by a reliable observer that the roots of mesquite
growing by a tributary of the Santa Cruz have been seen to reach to a depth
of 8 meters. As one leaves the river and goes toward the sides of the flood-
plain, Prosopis becomes smaller until, at the edges of the plain, it is little
more than a large bush. The water table also is deeper at the sides of the
flood-plain than near the river, and it is believed, although not actually
demonstrated, that the roots of the mesquite reach perennial water only
where the water table is relatively close to the surface.

WTiere the surface soils do not permit the deep penetration of the roots of
Prosopis, as on Tumamoc Hill and the bajada, the plant derives all of its

il'-T-K

"T^^": ".■■T'W^i^^-''

,:ifv :.ti?'s."

§fe^.

''•S^-Ty^W

t-ia

,,^g^

.s«S«P^

RocjT Habit ny Koekheri.inia Smnijsa.

A. Isolated plant with prominent tap-root.

B. Several shoots arising from a single sucker, which still keeps connection with
parent tap-root.

C. Adventitious absorption roots arising from a sucker which has not formed an
independent tap-root.

L). Secondary formation of tap-root in a group with connecting sucker root no
longer living.

CANNON

a. Peniocereus greggii from Hood-plain near West Wash, showing rteshy main
root and most important laterals.

b. Condalia spathulata from flood-plain of the Santa Cruz.

c. Root-system of Prosopis velutina partly exposed by the caving bank of the
Santa Cruz. Vertical distance from surface of ground to water is approximately 5 m.

d. Young plant of Prosopis. The squares are centimeters.

e. f, g. Cuttings of Opuntia arbuscula (Tucson), O. arbuscula (Sacaton), and O.
vivipara, all grown under similar conditions and with a large water supply.

ROOT-SYSTEMS OF PERE.NN'IAI.S. 8 1

water supply from the rains directly, and also, even where the roots may
penetrate deep and actually do so, it appears from the abundance of
superficial roots that the species growing imder such conditions also derive
their water supply mostly from surface waters. Thus, although it is char-
acteristic of the young plant that a strong tap root is developed (plate 21),
ni the mature form many of the laterals remain near the surface of the
ground and, with little change of level, may run 15 meters from the central
axis under especially favorable conditions, as by an irrigating ditch. This
observation was verbally communicated by Prof. R. H. Forbes, of the Ari-
zona Experiment Station.

CONDALIA SpATHULATA AND ZlZYPHUS PaRRYI.

Condalia and Zizyphus are both dwellers on the flood-plain of the Santa
Cruz and of West Wash, but the characteristic habitat of both forms is on
the river bottom. In this vicinity Zizyphus reaches its largest size in the
forest of Prosopis vduiina near the old San Xavier mission, 9 miles south
of Tucson. Both species, however, were studied near the western edge of
the Santa Cruz flood -plain about 6 miles west of the domain of the Desert
Laboratory.

In the neighborhood of the plants examined the leading perennials, in
addition to Condalia and Zizyphus which were fairly abundant, were Acacia
constricta, Bigelowia hartwegii, and occasionally Kacrhcrlinia spinosa, with
many arborescent specimens of Prosopis velutina.

Soil conditions where Condalia and Zizyphus were studied are character-
istic of the edge of the flood-plain, that is, adobe clay with particles of rock
fragments to a considerable depth. Condalia is evergreen, but Zizyphus is
deciduous, dropping its leaves soon after the beginning of the arid autumn.
The specimen of Condalia studied was 74 cm. in height. The root-system
of the plant consisted mainly of a tap root with numerous small and a few
large laterals. The tap root was traced directly downward 1.3 meters and
where left it was 8 mm. in diameter, so that it would not be possible to state
the depth attained, but without question it was much more than actually
demonstrated. At itscrown the tap root was 2.5 cm. in cross-section. The
leading laterals arose as follows: three, 5 mm. in diameter, originated just
below the surface of the ground, three more were given off at a depth of
36 cm., and one arose 90 cm. beneath the surface. Throughout the first
meter of the tap root there were, between the larger laterals, very many
smaller ones about i mm. in diameter, but these were relatively long. The
length of the larger laterals was not learned. Filamentous roots were not
seen on the laterals.

The shoot of the specimen of Zizyphus examined was 1.4 meters high and
bore numerous branches. At the time its roots were examined (April 22)
it was not in leaf. The root-system closely resembled that of Condalia;
it was characterized by a stout tap root from which arose several laterals;
the main root was 3.5 cm. in diameter at the crown and was traced straight

S- Kixif iiAniTs I'l" i>i>i:kt im.ants.

down to its (.-ikI, i . ;6 inctcrs (k-tp; i.S laterals, i c-in. or k-ss in dianicler,
were given off from the tap root within i 5 cm. of the surface of the ground ;
at a depth of 1 meter a larger lateral was given olT. Xone of the laterals
bore fdamentous roots along their course, as dirl those of I-^ucrhd. I'niinciid.
and other plants (plate 21/)).

From this study of the root-systems of Comlnln: and Zizv(>htis it will be
seen that, more than in any other plants examined, unless perhaps it is
Kacbaiinia, a well-developed ta]> root appears to be an essential character,
and it is due to this fact that these species are confined to jilaees where the
soil has considerable de]itli.

Iu'iii;uu.\ 'rKii-rKC.\.

Ephedra, on the doinain of the Desert Laboratory, occurs exclusi\el\- on
the flood-plain, especially that by West Wash, where it attains large size.
It is there associated with plants characteristic of the wash. Acacia qicggii,
Piosopis vilittiii ;. I'tnkinxonia tonrya)i(i. and a few specimens of Poiiocercus
greggii. shown in jilate j 1 , which is sonu what rare in the \icinit\' of Tucson.

Fig. 5. — Root syslcin of Kfhctlra Irijiirca showing position of tap root and typiial latiTals.

The Ephedra whose roots were examined was about i meter high and the
shoot was composed of numerous slender branches. The root-system was
characterized by a stout tap root and few relatively slender laterals. The
main root was 13 cm. in diameter at the crown, and was traced 75 cm. to
the place where it forked ; one fork was followed 25 cm. deeper, a total
depth exceeding i meter, and the end \vas not found. The laterals were of
two sorts, a larger and a smaller kind. The latter were confined to the
crown and resembled the slender roots in an analogous situation inthcjjlants
of Franseria. growing b>- the Wash. The larger laterals originated from
15 to 50 cm. Ix'neatli the surface of the ground, and, as is usual with the
roots of plants which grow where the soil is deep, either went straight
down or fairly horizontally. The depths and the lengths of two of the lat-
erals may be taken as being representative of all of the rest; one arose 15
cm. from the surface and kept this distance very closely to end, 1.8 meters
from the main root ; the other lateral, which arose at about the same depth,
after running straight out 10 cm., tiu-ned downward at a sharp angle to a
depth of 57 cm., when it took a horizontal course for a short distance.

ROOT-SYSTEMS OF PERENNIALS.

83

Opuntia \'lVn'AKA.

In the vicinity (if 'i'ucson the cacti usually occup)- the more arid situations
as the bajada or Tuinamoc Hill, and avoid the ilood-plain, but among the
exceptions to this condition should be included Pcnioccrcus gicggii and
Ofiuntia vivipara. The former, owing to the lleshy main root, requires con-
siderable earth-room for its full development, and usually occurs
where the soil is relatively deep. ('/>;()(/('i; i'n'Z/'cra is known only

from a locality not far from the
mouth of a wash which debouches on
the tlood-plain of the Santa Cruz near
the Nine mile Water-hole.

(Ipuniiii vivipara is of the cylind-
rical tj'pe and is remarkable esjjecially for the great
number of young plants which take their origin from
fallen "joints" and which cover the ground, beneath
everv large individual of the species, with a fairly
dense growth. The manner of origin of most of the
plants is indicated by the figure in plate 21, which
also shows something of the fleshy nature of the
young roots.

The root-systems of sev-eral plants were examined
and what follows is merely a resume of the observa-
tions made on them. In most regards the root-
system of the mature plant of vivipara is similar to
other arborescent opuntias, e.g.. that of (^. versicolor,
as described in a foregoing paragraph : that is, there
is an anchoring svstem and an absorbing system, both well differ-
entiated. But in a certain particular the roots of this species are unlike
those of any other cactus examined : they^ are usually slender, but occasion-
ally one is found which is fleshy, although the more distal portion as well
as the proximal portion mav be of the usual type. The departure from
the usual root-type will appear in the following measurements. A lateral
of the usual form, that is, 3 to 4 mm. in diameter up to a point 50 cm. from
its place of origin, may then become 1.5 cm. in diameter and retain this
larger size for 25 cm., while nearer as well as farther from the main root the
lateral will be of the slender type. The peculiarity of the root-system of
the species, in addition to the fact that fleshy roots are present, is that both
slender and fleshy roots occur on the same plant.

It was shown in the case of Opuntia arbnscida that the fleshy roots of that
species gave rise to plants as a regular habit, and it was learned that the
fleshy roots of O. vivipara are capable of doing the same thing, although
the characteristic has not been observed in natme.

Fig. 16. — Horizontal e.xlcn-
sion of rool-systcm of
Oiyiintia vivipara. All
laterals in this speciiiifii
were slender.

EXPERIMENTAL CULTURES.

Although the primary object of the present research was to learn the
most striking facts regarding the root-system of mature desert plants, as
a necessary preparation to later experimental work, problems arose dur-
ing the course of the study on which experiments were carried out intended
to be suggestive rather than necessarily conclusive. The experiments
naturally centered around the relation of the roots to water supply: (i)
as regards formation of adventitious and temporary rootlets; (2) the direc-
tion of growth or position of the roots; (3) as regards the quality of fleshi-
ness in the young as well as mature roots of certain opuntias.

The root-systems of many species of perennials and a few annuals are
provided with filamentous roots in groups of about 6 each. In perennials
these roots appear during favoring seasons and disappear when such sea-
sons have passed. In annuals their behavior is not so clear but their pres-
ence is probably also associated with an increase of the water supply. The
leading experiments undertaken along this line may be briefly stated.
Among the perennials, Franseria deltoidea and Enceliajarinosa form tempo-
rary rootlets in midwinter, if the ground is moistened, but other species
such as CoviUca tiidcntata, Fouquieria splcmlcns, Lycium andcrsonh, and
Opuntia discaia, are not provided at this season with newly developed
rootlets. Therefore, in certain of the species named, besides an improve-
ment of the water relations, a condition has to be fulfilled before the tempo-
rary absorbing roots may be formed, and this very clearly is higher tem-
perature, since freshly formed rootlets are to be found on these plants in
early autumn. However, such a plant as Opuntia discata can absorb water
in the winter season without the development of recognizably new rootlets,
as a series of experiments in the winter season well shows.

On November 11, 1908, after several weeks of drought, a small specimen
of Opuntia discata was well watered, and the watering was repeated the
following day. Six measurements on the thickness of the flat joints were
made on as many diff'erent places; 31 hours after the water was applied
the joints had begun to increase in thickness; the increase in thickness
continued for three days, after which the maximum diameter was main-
tained for an unknown period. On the sixth day after irrigating, roots
of the plant were carefully removed from the soil and examined. No newly
formed rootlets were present. In summer, however, such roots are formed
on this species, and the vegetative activityof the plant is very great, as shown
by new growth, the formation of leaves, and a high rate of transpiration.
Among the annuals the behavior of the roots as regards the formation
of adventitious rootlets also appears to be varied, although probably con-
stant for any species. A large proportion of the annuals studied were seen

84

&1^ ^

a. Cultures of Fouquieria splendens and of garden watermelon showing differences
in character and development of extreme types of root-systems.

b. Shoot of Opuntia vivipara springing from a fleshy root; natural size.

EXPERIMENTAL CULTURES.

85

to possess them, but they were not to be found in a few, and in one species
their formation could not be induced experimental!}-. In Amsinckia spec-
tabilis, these rootlets are present as rudiments, whatever may have been
the water conditions, but are only brought to full development under favor-
able moisture relations; while in Rafincsquia, as stated above, no adventi-
tious rootlets were seen in nature and none could be induced to form in
experiments. The cultures were carried on in the plant house where the
temperatures were higher than out of doors. (See page 40).

In December-February, 1907-8, a culture was set for the purpose of seeing
the relative extent of the roots of a typical desert seedling and those of
a pronounced mesophyte. The seeds planted were of the garden water-
melon and of Fouqiiicria splcndens. When the plants were taken up the

shoots of each species were about of a
height, although the transpiring sur-
face of the water-melon, naturally,
was much greater. The root-system
of the seedlings of the two species was
found to be very unlike; that of the
water-melon was three to four times
as long as the shoot and bore very
many laterals of the first order, while
that of the Fouqiiicria was only about
as long as the shoot and bore few lat-
erals. The relative development of
the two is imperfectly shown in plate
22. The behavior of the roots of Foti-
quieria brought about the starting of
another culture for the purpose of
learning the direction which the tap
root of a typical desert plant would take if provided with an abundance of
water. Water poles of porous clay tubes were placed in opposite ends of
a box and, after the soil moisture coming from the poles had reached an
equilibrum throughout the box, seeds of water-melon and of Parkinsonia
aculeata were sown at different distances from the centers of water supply
(fig. 17). The root of the water-melon went straight down, while that of
Parkinsonia inclined away from the water pole in a marked manner. The
difference in reaction of the roots of the two plants is thus a striking one,
whether the immediate cause is too much water or too little oxygen, or
other causes not now suspected.

This form of culture was later repeated using bulbs of Brodicea capifata
in place of the other plants previously used, with the distribution of water
from porous clay cups, of the kind employed by Livingston, arranged as
before, but with fine and coarse earth arranged in alternating vertical col-
umns. Bulbs were placed both in the fine soil and in the coarse, and on

Fig. 17. — Culture of water-melon (1) and
Fotiqtiicria splctnleti^ !2}in a soil kept con-
stantly moist by a porous clay cup shown
at left.

86 KDiir ii.Murs m- I)i:si:kt I'i.a.ms.

the line sc-paratinj; llie two. TIr- nsiilts of this culture have already Ik'ch
given above and need only be summarized here.

The plant on the adobe-clay side of the culture had develojied two sorts
of roots: a lleshy tap root which inclined away from the water jK)le and
ended in a curl, and several of llie usual sort (plate 23). The bulb had
entirely disappeared. The plant on the sand side of the box (both were
very close to the opposite kind of soil) had developed two kinds of roots
also, a large tap root and a few laterals. The tap root went straight down
without regard to the water pole (plate 2,^). In the latter plant the bulb
had not wholly disappeared.

In connection with the behavior of the Ihodiaa, whatever may be the
immediate underlying causes, it is of interest to review the reaction of
certain cacti to analogous natural and cultural conditions. In the course of
observations on the roots of the cacti it was learned that frequently the
seedlings of the arborescent forms were provided with fleshy roots, and also
that in the root-systems of the mature plants of Ofyiintia vh'ipara some
of the roots were slender and some fleshy, and also that all of the laterals
of Opuntia arbuscula were fleshy. In the latter species the description
applies onlv to the plants from near Tucson: those seen near Sacaton
are slender throughout. Cultures of cuttings from the Sacaton form of
arbuscula, the Tucson representative of the same species, and of Opuntia
vivipara, were grown in the experiment house during the spring and sum-
mer. The soil in which the plants were placed was sand and adobe, about
equal parts. The culture was frequently watered throughout the entire
period. On November 21 the plants were carefully removed from the soil
and it was noted that all of the larger roots of O. vivipara were fleshy; the
longer roots of the arbuscula from Sacaton, the species which does not pos
sess fleshv roots in its natural surroimdings, were fleshy also, as were those
of the specimens from Tucson. Bv:t the fleshiness of the last was the least
marked of anv (plate 21). It would appear, therefore, that the character
of fleshiness in these cacti and in Brodicra is to be associated with an
abundant water supply.

Besides ser\^ing as water-storage organs the fleshy roots of Opuntia
arbuscula. from Tucson, also propagate the species, as mentioned above
and described bv Preston (Non-sexual propagation of Opuntia, Bot. Gaz.,
p. 128, vol. 31, 1901). This appears to be a very common way of increas-
ing the number of individuals. To learn whether similar conditions
might obtain in those roots of 0. vivipara which were tieshy, one was brought
to the experimental plantation and given water freely for some months.
From this root a single well-developed shoot with roots appeared and con-
tinued to grow vigorously (plate 21 "I.

PHYSIOLOGICAL FEATURES OF ROOT-SYSTEMS.
•CHARACTER' IN ROOTS.

The roots of desert plants, possibly to a greater extent than the roots
of plants of the more humid regions, are remarkable for their individuality.
The roots of each genus, often perhaps of each species, possess peculiarities
of form, of branching habits, of color, of texture, of position in the ground,
or of more subtle physiological reactions. In how far these features can
be used for systematic purposes is yet to be shown, but there is no question
of their importance in the field of plant biology, particularly in ecology.

It has been found convenient to group the types of root-systems into
generalized and specialized forms with a physiological rather than a sys-
tematic bearing. Bv a generalized root-system is meant one that has both
the tap root and the laterals well developed. ,Such roots penetrate the
ground deeply and reach out widely. To this type belong most of the
desert perennials, of which I'^iaKsciia deltoidea and Prosopis veluiina, among
the shrubs and trees, may be taken as representative. Nearly all the
annuals, also, belong to this type. There are two forms of specialized
roots, those with the tap root the chief feature, as in Ephedra tnfurca and
Kacrhcrlinia spinosa. and those with the laterals, which are placed near the
surface of the ground, especially well developed. Oi the latter, the cacti
are almost the sole representatives. So far as desert plants are con-
cerned, it is probably true that generalization in root-systems looks toward
mesophilv, and specialization toward xerophily.

The specialized root-systems of either form are changed little with envi-
ronment, but the generalized roots a e often extremely variable, ranging
from a pronounced ta]i root to a marked development of the laterals, depend-
ent on soil characters and water relations. Thus it will appear clear that
rigidity of plasticity of root-system may be an important factor in the
local distribution of a species, a feature to be discussed below.

One of the most striking characters of the root-systems of perennials is
the variation in the branching habit, although as the habit is a matter of
degree rather than of kind it is not possible to express the difference in
exact terms. Under parallel conditions, the roots of any species act con-
sistentlv, and where the conditions are much changed, as from the bajada
to the flood-plain, and modification of the branching habit results, still the
induced variation is superimposed on the specific and familiar habit in such
a way that the proper habit is easily recognized. The roots of Covillca
branch repeatedly, wherever the plant is growing, but those of Foiiquieria
are little branched. The most richly branched root-system observed was
of Opuiilia arhusculci, which covered the ground so completely that it would

88 ROOT HABITS OV DUSERT PLANTS.

have been impossible for any other planl, perennial or annual, to gain a
foothold without encountering the roots of this species. Kramcria can-
esccns, the parasite, however, has a root-system that is scarcely branched
beyond the first order. Among the annuals it was observed that the root-
system of those appearing in summer was more richly branched than the
roots of the winter forms, and that the annuals both of summer and of
winter varied greatly, but consistently in the species, so that genera at least
coidd be distinguished merely by the branching habits of the roots.

The more superficial laterals of the generalized root-systems of perennials
and a few roots of the cactus type of the specialized root-systems are pro-
vided with filamentous roots, borne in groups, of limited period of activity.
These temporary absorption roots, referred to in this paper as deciduous
roots, are formed always during the rainy season, either of summer or
winter, and persist until the soil about them becomes unbearably dry.
The length of this period is not known, but probably is not far from the
time which limits the growth of annuals, from three to six weeks, or even
more. Similar roots are to be seea in some of the annuals, particularlv in
Ainsinckia among those of winter, and Trianthciiia of those of summer, but
are regidarly absent from certain other species.

THE RELATION OF ROOTS TO SOIL TEMPERATURE.

The relation of the roots of annuals and of perennials to the temperature
of the soil can be presented briefly. As appeared in an earlier section of
this paper, the grand course of heat movement finds its maximum just
prior to the summer rains, and its minimum in late winter or early spring.
With the coming of the rains in summer the soil temperature falls immedi-
ately and considerably, but the rains of winter do not appear to have so
marked effect on the temperatures of the soil. However, when more com-
plete temperature observations are at hand, the winter rains may be found
to have a marked and important effect, particularly in the way of altering
the temperature of the uppermost soil levels.

From what is known regarding the temperature of the soil at a depth of
15 cm., it appears that the summer annuals are not subjected to very intense
heat, in fact not greatly exceeding 80° F., but those of winter may experi-
ence temperatures very close to the freezing point. At a depth of 30 cm.
the soil is not as cold as at less distance from the surface, but the minimum
is reached later in the season than at the higher level, so that the roots of the
winter annuals mav be subjected to the following conditions : During the
warmer days of winter and early spring the superficial soil ma}- attain a
temperature relatively high, while the deeper soil is yet cold. The condi-
tions for most favorable water absorption are not present in winter, there-
fore, and the effect is a limitation of the development both of root and of
shoot. Reversed conditions are to be found in summer, when the lower
soil lavcrs at the time of the rains and the appearance of the annuals are

PLATE 23

a, Brodiaea capitata grown in adobe clay, and b, in sand, in plant-house cultures.
Main root of clay-grown plant curved away from source of water supply.

PHYSIOLOGICAL KKATl'RES OF ROOT-SYSTEMS. 89

warmer than the upper soil layers. The growth conditions both of shoot
and of root in summer, therefore, are most favorable, with the result that
the summer annuals are very luxuriant, the shoots carrying a large leaf
surface and the roots being especially well developed.

The relations of the roots of perennials to soil temperatures are at once
similar and unlike the relations of the annuals. They are similar in that
the deciduous roots, which are developed on the more superficial laterals,
occupy the same soil horizon as the roots of the annuals, are developed at
a time when the annuals appear, and die with their passing. The persistent
portions of the roots of the perennials are subject to the greatest tempera-
ture extremes of the hojizon they occupy, which in the most superficial
roots, like certain of the cacti, must be very great, although the exact
measure is not known. The peripheral absorption roots of perennials,
both of the specialized and the generalized types of root-systems, have
still other temperature relations. It is probable that these roots in the
generalized root-systems, and in the Ephedra type of the specialized roots,
remain active throughout the year, but that in the cactus tvpe of the
specialized roots, especially in such forms as Opuntia arbusciila and Echino-
cacius wislizcni, they are formed at the time that the deciduous roots appear
and die when the latter kind of roots die. The temperature relations of
these roots are thus seen to be extremely complicated.

Most, perhaps all, of the perennials are active during the summer rainy
season, but many do not renew growth or come into flower in winter. This
is clearly a temperature relation and finds its reaction in the formation
of deciduous roots, and probably in other ways, in the root-systems. It
has been found impossible, for example, to induce the formation of decidu-
ous roots in Opuntia discata in winter, and deciduous roots have not been
seen in Fouquiei ia ipUndcns at this season, though when the examination
of the roots of Fouqmcria was made, temporary roots were seen in Encclia
jarinosa, Lvciuiii andeisonii, and other plants.

THE RELATION OF ROOTS TO WATER.

The water relations of the plants in the vicinity of the Desert Labora-
tory are verj' complex, owing partly to diversity in the plants themselves
and partly to essential differences as regards soil water in their habitats.
These differences lie chiefly in the origin of the water supply as well as in
the length of time which the water of the soil is available for use. On
Tumamoc Hill the entire supply is derived directly from the rains ; on the
bajada the water comes in the main directly from the rains, but some is
also received as superficial run-off or by seepage from Tumamoc Hill or
other higher ground ; on the flood plains, in addition to the rains and the
water which comes from Tumamoc Hill and other higher ground, there is a
water-table of unknown depth by West Wash, varying from 5 to i 2 meters
beneath the surface on the plain by the Santa Cruz. The water-table by the

y<) KiMPi IIAlilTs III' l>i;si:i<T n.ANTS.

Santii Cruz is sunkiciilh luai lln surlaci' l<i |Kriiiil its Ix-iii^' rt-aclicd lj\
llic roots of the larger plants, as Piosopis vclKtina. It lias been learned*
that the Santa Cruz is at present lower than the water table, so that tlie
river acts as a ditch. In limes not \ery remote there was no well-defineil
channel of the river, as at present, but the water spread over the plain,
which might indicate that formerly the water-table was nearer the surface
than it is now and thus more easily reached by the roots of plants. The
changes in the position of the water-table liaxe ]irobably operated to modify
profoundly the ilora of this habitat.

Not onlv do the sources of water vary in the dilTereiil habitats, but also
the periods during which it is available for the use of plants. Tumamoc
Hill and the bajada, partly for reasons of differences in water supply, are
the most arid, and the flood-plain of the Santa Cruz the least so. Tin-
character of the soil and its depth are also important factors in this con-
nection. The soil of the bajada and Tumamoc Hill is usually less than 50
cm. deep, while the soil by West Wash is over 2 meters and that of the flood -
plain of the Santa Cruz is 5 meters, more or less. Thus the arid condi-
tions of the bajada and Tumamoc Hill, with the least water supply, are
greatlv increased by the slight soil covering. The length of time elapsing
after a through wetting before the soil of the bajada, at a depth of 15 cm.,
becomes air-drv, is about three weeks, but moisture at this depth on the flood-
plain of the Santa Cruz remains for six weeks. In this instance, however,
in addition to the fact that more water is present in the habitat, the char-
acter of the soil as regards its fineness is an important factor.

Annuals are directly affected by the rains of the particular habitats
where thev are placed, while the perennials are in part directly and in part
indirectlv dependent on obtaining water by this means. On the bajada
and on Tumamoc Hill, water is mostly derived directly from the rains, and
only such perennials as are provided with water-storage organs, or are the
most resistant, can live or attain the best development in these habitats.

The depth to which the roots of annuals penetrate the ground is directly
controlled bv the depth of the penetration of the rains of the season and
the persistence of the annuals is mainly directly dependent on the length
of time the water remains in the soil where they are growing. As above
stated, within a period of three weeks following the rains, the superficial
soil on the bajada, to a depth of 15 cm., may become air-dry, while that
of Tumamoc Hill and of the flood-plain may retain moisture for a period
exceeding six weeks. Since the largest part of the roots of most annuals do
not reach deeper than 20 cm. it is seen that the period of their activity is
definitely fixed and comparatively brief.

Annuals having the most deeply penetrating root-systems, and which
are pro\ided with checks against excessive water loss, survive longest after
the wet season has passed. Kallstraemia grandiflora, for example, a sum-

*Communicated to the writer by Prof. G. E. P. Smith, of the Arizona E.xperitncnt
Station.

PHVSIOI.OGICAl, KKATl'KKS <)!•" RooT-SYSTUMS. ijl

mer annual, uutlivts many other forms, and should additicjual rains occur
somewhat out of season, but before the plant has succumbed, it will revive
and accompUsh an astonishing extent of growth. '■Among the winter
annuals, such deeply rooted forms as Amsinckia speclabilis and Phacclia
tanacctifoUa will survive long after Boidcsia lohaia, or other plants with
shallow roots, have perished.

How closely the character of the water supply, the amount of water,
and the progressive drying out of the soil, are related to the character of
the root-svstem of annuals, does not appear from the observations here
reported. In certain instances, however, the root-system is greatly modi-
fied by the water relations. For example, it has been repeatedly seen that
the longest and frequently the greatest number of laterals arise within 4 to 5
cm. of the surface of the soil. Without definite knowledge of the water
movements at that depth, it may be assumed that during the period
of greatest growth the soil at this depth has an adequate water supplv
and provides practically all of the water necessary for the plant, and that it
is onlv later, when by progressive drying out the upper soil becomes too
dry for benefiting the plant, that it obtains its water at the greatest depth
and mainly bv means of the elongated tap root. In certain cases, as in
Amsinckia, where precociously formed root rudiments are present, the
coming of unseasonal rains induces growth, and the superficial roots are
again in condition to absorb water.

The reaction of the root-systems of the perennials is much more complex
than that of the annuals, inasmuch as a portion of the roots live through-
out the year and a portion endure during favorable seasons only. The situ-
ation is further complicated by the fact that each habitat differs fnjm everv
other habitat as regards water relations. The great difference in the char-
acter of the root-systems of the perennials and their ]iossible variabilitv
under dift'erent conditions have also to be considered.

The peripheral rootlets of the generalized root-systems are probably
living throughout the year, since they are in soil which, even in dry seasons,
contains sufficient moisture for absorption, but the deciduous rootlets, both
of the generalized and the specialized root-systems, are present and func-
tional only during the more favorable seasons, chiefly in summer. The
peripheral roots of the specialized forms of the cactus tvpe, however, like
the deciduous rootlets, are present only during the more favorable seasons.
From these circumstances it is seen that the active period of absorption of
plants with the latter type of roots is a very restricted one, while the absorp-
tion period of the generalized root-system and of the Ephedra type of the
specialized system, although most active during the existence of the decid-
uous rootlets, is continuous from season to season.

In plants having generalized root-systems, it is probable that the pene-
tration of the roots, the character of the soil permitting, is equal to the
penetration of the rains. On the bajada this is limited by the presence of

92 KO(jT MAinrs OK DliSRKT IM.AN'TS.

tlic hard i)an which occurs within .scj cm., or less, of the surface, and on
Tumanioc Hill by the untkriyinK rock. On the Hood ])lain, where the soil is
sand and adobe, as by West Wash, the penetration is probably about 2
meters, but on the vSanta Cruz llood-plain water from the rains is thought
not to penetrate the <;roun(l over 1 meter, although it probably goes below
that depth in jjlaces.

The penetration of the roots of the cactus type apparently presents
another problem, since certain of the cactus TOot-systems, Opuntiaarbuscula
particularly, lie within 2 to 5 cm. of the surface or well within the depth
attained by water either during summer, when the most active growth
occurs, or in winter. The relation in this instance may be with a proper
air supply, rather than with the penetration of the rains alone.

Where there is considerable depth of soil and the water table is close to
the surface, as in places near the Santa Cruz river, the conditions are most
favorable for the deep penetration of roots. Under such conditions the
water chain during the rainy season may be continuous from the water
table to the surface and so favor deep penetration. It is here that Prosopis
vclutina and Populiis sp. may be in position to obtain perennial water and
attain a large growth. The roots of Prosopis have here been seen to reach
5 to 8 meters below the surface, which is apparently the greatest depth
reached by any plant in this vicinity.

RELATIONS OF THE ROOTS OF NEIGHBORING PLANTS.

The desert plants vary greatly in their abundance. It is well known
that shrubs inhabiting the bajada are comparatively remote from one
another. On the flood-plain the same species may form a fairly dense
thicket. In favorable seasons the annuals, both of the bajada and the
flood-plains, are occasionally so numerous as to completely hide the ground.
It is evident, therefore, that the roots of the different classes of plants,
annuals or perennials, hold varying relations to each other, which are dif-
ferent in accordance with the kind of habitat they occupy.

The mutual relations of the roots of annuals and of perennials vary also
with the character of the perennial roots, but only as the roots of annuals
find sufficient room for full development in all of the habitats.

On the bajada and Tumamoc Hill the roots of such annuals as Anvda
thuiberi, Kallstioemia graiuh'flora, Solanitin clccagnijolium , Erodium cicu-
tarimn, and others which have a deeply penetrating root-system, may
reach as deep as the roots of the perennials having a generalized type of
root-system. But on the flood-plain the roots of the two classes of plants
do not occupy the same horizon due in part to the fact that the roots of the
annuals differ little if any in habit from those on the bajada, but mainly
because of the deep penetration of the perennials. This can be illustrated
by a single example. The roots of Covillea on the bajada may extend
away from the main axis as far as 4 meters, while on the flood-plain the

PHVSIOI.OGICAU FEATURES OF ROOT-SYSTEMS. 93

lateral extent may be less than 2 meters. In the latter case there is the
additional fact that the root-system of Covillea is, as a whole, deeper than
on the bajada. Competition between annuals and perennials on the flood-
plain, therefore, may be considered a negligible quantity, but competition
between the two classes of plants on the bajada during the vegetative
seasons is undoubtedly very keen, and the presence of annuals on the
bajada is probably an important factor in bringing about the sparse char-
acter of the perennial flora of the habitat.

Since the roots of the specialized type of the Ephedia form penetrate
deep and reach laterally to but a small extent, the roots of the annuals do not
come into close relation with them, but the relation of the roots of annuals
and of the specialized roots of the cactus type is peculiar in that usually
the roots of such perennials are more shallowly placed than those of the
annuals. Owing, however, to the fact that in the root-system of annuals,
laterals are ordinarily prominent 4 to 5 centimeters beneath the surface,
during the rainy season, the roots of the two classes occupy the same horizon
and undoubtedlyenterintokeencompetitionforwater. Soon after the close
of the rainy period the superficial roots of the annuals and the deciduous
rootlets of the perennials, owing to the progressive desiccation of the soil,
can no longer remain functional. With the drying of the surface layers the
more superficial roots of the plants having the generalized root type also
cease absorbing, and the entire amount taken in by the root-system of
such plants comes from the deeper levels. At this period the most deeply
placed roots of such annuals as mentioned above are placed in close rela-
tion with the deeper absorbing roots of the perennials with generalized
roots, and the competition is transferred from the more superficial to the
more deeply placed roots both of annuals and of perennials. Finally the
annuals mature, their roots absorb in decreasing amounts, and competi-
tion with the roots of other plants ceases.

Where the growth of annuals is most dense, the roots intermingle and
occupy the soil fairly completely. This, however, occurs only in the case
of perennials when, as on the bajada, plants of one species, or plants having
similar types of root-systems, are growing in proximity. The most strik-
ing instance observed of the encroaching of the roots of one species on the
territory occupied by the roots of another plant of the same species was
that of Covillea, where 60 roots of neighboring Covilleas were seen either in
physical contact or lying very near the roots of the plant being specially
studied.

The roots of plants with specialized root-systems of the cactus type do
not occupy the same horizon as the roots of perennials having the general-
ized forms of roots, and therefore do not compete with them for water.
This, however, is true only of such cacti as have the most superficial roots ;
those like Opuntia fulgida, with a root-system which approaches the gen-
eralized type in character, probably approach this type in their mode of
distribution in the soil, particularly where the soil is shallow.

<H KOOT IIAIUTS OI' DKSKKT ri.ANTS.

ROOT HABITS AND PI AN 1 IMSTRIBUTION.

I'lii.' cliaracler of till.' rool-syslfin of (IcSLTt plants as an irological factor
is often of gruat iniportancf. Tlu' direction taken by the roots as a factor
in the causes underlying the distribution of plants may be illustrated in a
simple manner, thus: I,et (.'. h. and c represent the three leading root tvpes,
namely, those having both laterals and tap root well developed, generalized
root-system (a), those with the tap root as the most jirominent feature (6),
and those with the laterals of special prominence (c ). Of plants having a,
h. or (■ forms of roots, other conditions including temperature relations
being equal, a will have the most general distribution, h will have the most
limited distribution, while ])lants with c roots are in a measure intermediate
in this regard. In plants with the last type of roots, however, it is probable
that factors other than the root character exercise the controlling influence.

Of plants with generalized root systems Acacia, Ccllis, Eticclia, Lycium,
Parkinsoniu. and Prosopis may be found to some extent in each habitat,
both where the soil is deep and where it is shallow. With these forms
should also be included the annuals except those with bulbous roots.
Plants with this root type are either evergreen or deciduous, but all have a
fairly large transpiring surface, and in certain of them, especially Encclia.
the leaves are of good size.

Plants having a prominent tap root, the /; type, include Condalia, Penio-
ccreiis greqqii. Ephedra. Kccrbcrlinia. and Zizyphns. These forms are re-
stricted to localities, preferably the flood-plain, where the soil has sufficient
depth for the development of the main root.

The cacti are nearly all provided with the c form of roots, but to them
should be added Jatroplia and Krainci ia, the latter a parasite. The cacti
occur both on the bajada and on Tumamoc Hill, and not on the flood-plain
of the Santa Cruz or by West Wash to anv extent. They are the plants
par excellence of the bajada.

Summing up the relation between the type of root-system and the dis-
tribution of plants, according to the notation given above, we have the
following: Plants with a type of roots may be found on the bajada, Tuma-
moc Hill, and on the flood-plains of West Wash and of the Santa Cruz.
Plants having the 6 tvpe of roots occur only on the flood-plains of the wash
and the river. Plants with the c type are chiefly on the bajada, although
they are common also on Tumamoc Hill, to which certain of them, notably
Carncgiea. are, for the most part, confined.

The diftercnt root types are diff'erently related to the distribution of the
plants. . The generalized roots (a type) facilitate distribution because of
their plasticity, while the speciaUzed roots of the b type restrict the distri-
bution because of their rigidity and the need of considerable depth of soil.

SUMMARY.

i . On Tumamoc Hill there is sufficient moisture in the soil at a depth of
30 to 40 cm. to be available to plants all of the year. The soil of Tumamoc
Hill and the flood-plain of the Santa Cruz at a depth of 15 cm. may be air-
dry within six weeks following rains and that of the bajada within throe
weeks.

2. The highest soil temperatures are reached in Jul)' jusl ]iieceding the
summer rains: the lowest temperatures of the soil, at a depth of 15 cm.,
are reached in midwinter and a depth of 30 cm. in late winter.

3. The roots of most annuals do not penetrate the sfiil deeper than 20
cm. The largest development of laterals of annuals takes place 4 to 5 cm.
from the surface of the ground. Rudimentary roots of the first order
were seen in Aiiisiiickia. Eyodiuin, Kntrichinm. Ilaipagniiella, Malva, and
Pcctocarya among the winter annuals, and Aiuaiinilhus, Bociluiavia, Clado-
thrix, and Tnanllictna of the summer forms. These rudiments are further
developed onlv under favorable moisture conditions, such as out-of-season
rains, when the}- become of great imjjortance to the plaiU.

4. The root-systems of the summer annuals, which are distinct from the
winter annuals, are especially well developed. This characteristic is due
to the great luxuriance of shoots of the summer forms, owing to the favor-
able vegetative conditions of this season, not least of which is the warm soil.
In winter the soil at the depth attained b\' the roots of the annuals is
colder than the superficial layers and does not present the most favorable
conditions for water absorption. In winter, also, the shoot development
of annuals is less than in summer.

5. Perennials have three types of roots, namelv, the generalized type,
with the tap root and the laterals both well developed, and two specialized
forms, of which one type has a prominent tap root and the other promi-
nent laterals. Covillca friJentata and Prosi}pis vcliiiina are representatives
of plants with generalized roots, while Kavbrrlinia sf^hiosa and a few other
plants have specialized roots of the first ty]3e, and most of the cacti have
s])ecialized roots of the second kind.

6. The roots of perennials growing on Tvunamoc Hill and on the bajada
do not penetrate as a rule deeper than 30 cm., the depth of the available
soil ; while those on the flood-plain of West Wash attain a depth of 2 meters,
and those by the Santa Cruz may reach as deep as 3 meters or more.

7. The most shallow rooted perennials observed were ('piinlia iiihiisciila
(■ whose roots frequently do not lie more than 2 cm. below the surface)
and hlchinocadiis wislizcni. The arborescent opuntias have roots which
approach the generalized type.

9<» SUMMARY.

8. Most cacti have two divisions of the root-system: an anchoring and
an absorbing system. In Carncgica gigantca the anchoring roots, in old
])Iants, are assisted in their mechanical function by the enlarged bases
of the laterals. This to a certain extent is true of the arborescent cacti
as well, but the lack of secondary development of this character in Echino-
cactns u'lslizmi is the chief reason why plants in age lean sharply, or fall.

9. Fleshiness in the roots of the opunlias is a condition resulting directly
from an abundant supply of water. Whether some species exhibit greater
tendency toward lleshincss than others was not determined. A similar
reaction was seen in Brodicca capilata.

10. The most superficial roots of the generalized tj-pe of root-systems
and a few of the specialized tvpe of the cactus-form bear, on the larger lat-
erals, filamentous roots in groups of 6 or more. These are formed during
the rainy season, especially in the summer, and die with the drying out
of the soil. The deciduous rootlets arc of great advantage to the plants
in that they greatly and quickly increase the absorption surface, without
at the same time increasing the distance of water transport — a factor of
undoubted importance where the transpiration rate is high.

1 1 . Competition as evidenced by the relationship of the roots of neigh-
boring plants may be summarized as follows: The roots of annuals inter-
mingle and often occupy the same horizon. Wiere perennials of a single
species occur together on the bajada, the roots of one plant may reach to
and intnide upon the root-area of its neighbor. Thus in studying Covillea
on the bajada, 60 roots of neighboring Covilleas were encountered which
either were in contact with the roots of the plant studied or were in the
same horizon. On the flood-plain competition among the roots may not
be so keen as on the bajada. The roots of annuals growing on the bajada
reach as deep as most perennials in the same habitat, and since they
occur in large numbers competition with them inust be an important
causal factor contributing to the sparscness of the perennial vegetation of
the bajada. The annuals also come into competition with the shallow
rooted perennials through the laterals which are developed on the tap root
of the annuals 4 to 5 cm. beneath the surface of the ground.

12. Perennials with the generalized type of root-system have the widest
local distribution, and those with a pronounced development of the tap root
have the most limited distribution. Plants with laterals well developed,
the cacti especially, are most abundant on the bajada and on Tumamoc
Hill, where the soil is shallow, and seldom occur on the flood-plains.

8099