Nj
.A

CANNON

PLATE 1

Dorsal and ventral surfaces of typical leaves of Juglans californica, taken from a tree growing
in Santa Rosa, California. One-third natural size.

Studies in Heredity as Illustrated by the Trichomes

of Species and Hybrids of Juglans, Oenothera,

Papaver, and Solanum

BY

WILLIAM AUSTIN CANNON

WASHINGTON, D. C.

PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON

1909

STUDIES IN HEREDITY AS ILLUSTRATED BY THE TRICHOMES OF SPECIES
AND HYBRIDS OF JUGLANS, OENOTHERA, PAPAVER, ANDSOLANUM.

INTRODUCTION.
SCOPE AND PURPOSE OF THE STUDY.

In investigating the structural features of plants two or more methods
of approach, each especially adapted to the particular aim of the study,
whatever it may he, may be employed. The entire anatomy of a few forms
may be worked up, and appropriate comparisons instituted, or, on the other
hand, special features of the plant's structure, such as the trie-homes, may
be minutely studied, both in the mature state and in embryonic condition, in
a relatively large number of individuals as well as great variety of species.
Although the former method has hitherto been the one chiefly employed
in anatomical studies on plant hybrids, it seems appropriate, in view of
the present tendency to regard a plant as a complex of more or less inde-
pendent units, to institute the more special study. Whatever theories may
be held, this manner of studying structures in plants, as in animals has
lony been found, has several advantages. It permits, in the first place,
as suggested above, an examination into the behavior of the particular
organ, or tissue, in a much larger number of plants than might otherwise
be practicable, as, in the present instance, the trie-homes of some 20 differ-
ent kinds of plants, and many more individuals, have been passed under
observation. It also favors an investigation into the variation of the par-
ticular structure, as well as into the relation of the variation to the organic
and the physical environment of the tissue as possible causal factors of
variation. Also, not only the mature structures, but the embryonic stages
likewise, may be studied and compared, so that developmental stages may
easily be contrasted, and, in favorable material, the possible origin of the
structures may either be traced, or at least strongly surmised. And,
finally, what is particularly important in a study on heredity, the behavior
of the same structure may be advantageously observed throughout differ-
ent generations. For these and other reasons the particular rather than
the more general method of the study of the anatomy of the hybrids has
been adopted in the present investigation.

In selecting plant structures for comparative study, one or two points
were kept uppermost in mind. All of the plant tissues are not equally favor-

1

2 HEREDITY AS ILLUSTRATED BY TRICHOMES.

able for such an investigation, for the reason, which is apparent, that they
are either entirely inclosed by other tissues or so intimately related to such
other tissues that the extent as well as the manner of development is
largely conditioned by the development and position of such contiguous
Itissues. From the nature of things, structures so enveloped can, to a very
united degree only, whether representing "units" or not, develop as inde-
pendent org'ans. However, even in wholly immersed structures, as the
conductive system, the single-structure method of approach has been pro-
ductive of rich results. But for the purpose of the present study it was
essential that the tissues compared be as independent of other tissues as
possible, and, in addition, should possess well-marked characteristics, both
as mature structures and in embryonic stages; hence it was decided to
select for the comparative study, as being most likely to give satisfactory
results, the trichomal system.

The immediate objects of the investigation were, in brief, to study the
origin, development, inheritance, and variation, and, so far as practicable, to
observe the causes of variation of the trichomes in several pure species and
the hybrids of the species. In addition, it was desired to trace the behavior
of the structures in as many plants as possible, to the end that something
exact might be known as to the relation between such tissues and unit-
characters; that is, to be more specific, it was proposed to learn, for ex-
ample, whether the trichomal system as a whole constitutes a single unit,
or whether each separate type of trichome of a plant is to be considered a
distinct and independent character.

PURE SPECIES AND HYBRIDS EXAMINED.

The following hybrids, together with the pure lines, were studied:
Jug lans calif arnica ^.Juglans nigra, F,, P.,, and F:1; fiiff/a/is calif arnica ^Jug-
tans rcgia, F,, F2, and F3; Oenothera lamarckiana X Oenothera cniciata, F2
andF3; Papaver somniferum X Papaver orientals, F,; Papaver somniferum X
Paf>aver pi/osuw, F,; Sola mini ril/oston X Solanum guinense, F^ F2, and
later generations.

The material for the study of the trichomes of the Oenothera hybrid was
collected at the New York Botanical Garden in August, 1906, where it had
been formed three years previously. In this cross* the characters of the
pollen parent (Ocnot/iera critciata) are said largely to dominate, although
none appear to be transmitted unchanged. Particularly in the flower there
is a singular blending of the characters of both parents, as, for example, some
of the hybrids have short stamens, /ainarckiana-\\ke, and are incapable of

•Mutants and Hybrids of the Oenotheras, by D. T. Macdougal and others, Carnegie
Institution of Washington Publications 24 and 81, 1905.

INTRODUCTION. 3

self-pollination, and others have long stamens, critciata-lilm, and may be
self- pollinated. Thus the hybrid does not in any character revert consist-
ently to either parent. This is an example of the first cross breeding true.

The material for the study of the Juglans hybrids was collected at Mr.
Luther Burbank's experimental grounds in Santa Rosa and Sebastopol,
California, in 1907 and 1908, where, about 20 years before, Mr. Burbank
had made the original crosses.

Juglans californica X Juglans nigra is represented by one tree of the first
generation called the "Royal," and by numerous smaller trees of the sec-
ond and third generations, the latter called the "Beeson," in the nursery
at Sebastopol. Plates 4, 5, 6, and 9 illustrate, though very inadequately,
the variability of the leaves of the first, second, and third generations of
this hybrid and show that the greatest range is probably to be found in
the second generation, although those of the third are also extremely
variable. The young trees of the later generations exhibit also great
differences in size, as variation in this regard exceeding 500 per cent has
been observed between those of like age and growing under apparently
identical conditions.

Juglans californica X Juglans regia is represented by several trees of the
first generation, known as the "Paradox," in Santa Rosa, and by num-
erous second and third generation seedlings at Sebastopol. Four Paradox
trees are growing in the street in front of Mr. Burbank's residence. Al-
though only 17 years from the seed, these trees are about 27 meters in
height and their trunks are about 75 centimeters in diameter. The very
luxuriant character of the tree is extended to the leaves, which in some
instances are surprisingly long and are composed of leaflets which in num-
ber and size are said to exceed those of either parent. As will be pre-
sented somewhat in detail later in this paper, the leaves are also very vari-
able in surface characters, in shape, size, and contour. Certain of these
variations in the first and second generation plants are shown in plates 7,
8, and 10.

The hybrids Papaver somniferuin X Papaver orientate and Papaver somni-
ferum X Papaver pilositm were produced by Mr. Burbank and are now grow-
ing at his experimental grounds in Santa Rosa, California. The parents
of these hybrids, natives of southeastern Europe, the Caucasus region, and
Asia Minor, are either annual (somniferum) or perennial, but the hybrids
are perennial. Both of the Papaver hybrids are extremely variable, espe-
cially as regards the size, shape, and texture of the leaves and the size,
color, and form of the flowers, and they are either wholly sterile {P. som-
niferum X P. orientale) or to a certain degree fertile. On account of the
complete or partial sterility of these hybrids only the first generation has
been available for study.

4 HEREDITY AS ILLUSTRATED BY TRICHOMES.

Solatium villosum X Solatium guinense was produced by Mr. Burbank
and is now growing- in his experimental grounds at Santa Rosa, California.
Solatium guinense, a native of central west Africa, is a perennial of bushy
habit of growth, and bears black fruit of unpleasant flavor; Solatium villo-
sum, from Chile, is an annual, dwarfed and procumbent, and bears clusters
of small, hard, and green berries.* The hybrid is a low, bushy perennial,
and produces berries agreeable to the taste, the flavor of which recalls that
of the low-bush blueberry of the eastern United States. The hybrid has
not been known to revert, although its culture, begun in 1897, has been
carried to the fifth generation; it, therefore, is an instance of the breeding
true of a first cross.

The material for study of all hybrids procured from Mr. Burbank was
either collected by the writer and fixed in the usual manner, or was put up
by Dr. G. H. Shull in weak formaldehyde. To both of these gentlemen
thanks are due for the preparation of material, and to Mr. Burbank, who
so generously gives both his plants and his time for the forwarding of the
study of plant hybrids, acknowledgment is especially and gratefully made.
Acknowledgment should also be made to the directors of Hopkins Seaside
Laboratory, Pacific Grove, California, for the use of a research-room dur-
ing the summer of 1907, and to the Carmcl Development Company, Carmel-
by-the-Sea, California, who provided a laboratory which was used during
the summer, 1908.

•Year Book, Carnegie Institution of Washington, 1907.

TRICHOMES OF OENOTHERA HYBRIDS AND PARENTAL LINES.

TRICHOMES OF OENOTHERA LAMARCKIANA x OENOTHERA
CRUCIATA AND OF THE PARENTAL LINES.

Throe types of trichomes are found in both Oenothera lamarckiana and
Oenothera cniciata, and each species bears all three types (.fig. 1). These
are awn- or awl-shaped, club-shaped, and pear-shaped, and are referred
to as such in the following account of them. All of the trichomes are
unicellular.

Ki<;. 1.— a to (I, Oenothera lamarckiana; e to </, O. cruciata; htaj, O. la-
marckiana X cruciata. a, pear-shaped trichomes from the stem
( • 535); 6, club-shaped trichome from the veins, lower leaf-surface
( • 535); c, awn shaped trichome from ventral surface of leaf, be-
tween veins ( 81); <l, pear-shaped trichomes from ventral surface
of leaf, between veins ( * 3f>2); e, awn-shaped trichome from be-
tween the veins, ventral leaf-surface ( • 3U2); /, pear shaped tri-
chomes from- ventral surface of leaf, between veins; Q, awn-shaped
trichome from stem ( •; 3tf2); h, giant awn-shaped trichome from
capsule (X 84); i, pear-shaped trichomes from ventral leaf-surface
( • 535); ./, club-shaped trichome from capsule ( 362).

The awn-shaped trichomes of the older leaves are lifeless. They vary
in length to a marked degree, an(j m extreme instances range from 71.4 M
to 1.8 mm., although for the most part the differences in length He be-
tween 200 M and 1.00 mm.

6 HEREDITY AS ILLUSTRATED BY TRICHOMES.

The club-shaped trichomes, as well as the pear-shaped ones, are gland-
ular, and hence living. Both are less variable in size than the awn-shaped
trichomes and both have lengths characteristic of each species. The lead-
ing characters of the glandular trichomes are sufficiently shown in fig. 1
and do not require further description in this place.

OENOTHERA LAMARCKIANA.

In the pure parent Oenoihera lamarckiana, the three kinds of trichomes
above described are to be found on the lower surface of the leaves, on the
stem, and on the capsule. The variation of the awn-shaped trichome, which
was referred to above, may be shown by the following measurements,
expressed in /*: ventral surface of the leaf, 163.8, 168, 191.2, 202.6, 222.2,
223.1, 244.6, 248.8, 265.6, 306.6, 344.4, 399.0, 420.4. The trichomes above
measured were selected at random; the following measurements are of tri-
chomes which were in the region of the veins of the leaf, expressed in /*:
126.0, 231.0, 239.4, 252.0, 252.0, 336.0, 399.0, 420.0, 453.6. On the stem
the measurements were 126.0, 159.6, 176.0, 269.6, 387.2, 440.0, 721.6,
756.8, 844.8, 880.0, 915.0, 1144.0, 1196.8. On the capsule these trichomes
were less numerous but were all relatively long, as the following measure-
ments, in M, indicate, 844.8, 897.6, 915.2, 922.8, 968.0, 1003.6, 1073.6,
1073.6, 1144.0, 1196.8.

The club-shaped trichomes occur also on the ventral surface of the leaves,
on the capsule, and on the stem. This type of trichome is more uniform in
size in any given area, as between the veins or on the veins of the leaf,
than the awn-shaped trichome, and also the trichomes of different areas
are more nearly the same size than those of the other (awn-shaped) type.
This is also true, and perhaps more consistently so, of the pear-shaped
trichomes, the measurements of which will be given below, from which
circumstance a study of them as inheritable qualities is of value. The fol-
lowing measurements, in M, were made of the club-shaped trichomes taken
from the lower leaf-surface and between the veins: 168.0, 176.4, 180.6,
184.8, 189.0, 189.0, 197.4, 201.6, 210.0, 227.0, 231.2, 231.0, 247.8, 252.0,
253.0. Trichomes on the veins measured as follows, in M: 168.0, 168.0,
180.6, 189.0, 191.2, 201.6, 205.6, 211.0, 211.0, 231.0, 252.0. Club-shaped
trichomes on the stem had the following lengths, in /*: 131.2, 172.2, 176.4,
180.6, 180.6, 201.6, 210.0, 210.0, 222.6, 248.8. On the capsule these tri-
chomes measured as follows, in /*: 155.4, 163.8, 176.4, 189.0, 205.6, 211.0,
222.6, 231.0, 235.2, 252.0, 252.0. The average lengths of the club-shaped
trichomes for the given areas are as follows: lower surface of leaf, on vein,
200.08 p; between veins, 209.2 />*; on stem, 193.4 p; on capsule, 208.5 /*.

Pear-shaped trichomes were studied on the ventral surface of leaves, on
the stem, and on the capsule. The following measurements, in/*, give the

TRI.CHOMES OF OENOTHERA HYBRIDS AND PARENTAL LINES. 7

lengths of these trichomes for the different areas noted. Ventral surface
of leaf , between the veins: 37.8, 37.8, 37.8, 39.9, 42.0 42.0, 42.0, 42.0, 42.0,
46.2, 46.2, 46.2, 50.4, 50.4, 50.6; on the veins, 37.8, 42.0, 46.2, 50.4, 50.4,
54.6, 54.6, 54.6, 54.6, 54.6, 54.6, 63.0, 63.0, 67.2; on the stems: 37.8, 37.8,
39.9,39.9,42.0,42.0, 26.2,46.2,46.2, 50.4, 54.6, 54.6,163.0,67.2,67.2. The
pear-shaped trichomes were not so abundant on the capsules as elsewhere;
9 trichomes from the capsule measured as follows, in ^: 37.8, 46.2, 48.3,
50.4, 65.6, 58.8, 58.8, 58.8, 58.8. The averages in length for these hairs
are: (l) between the veins, on the lower surface of the leaf, 43.5/^; (2) on
the veins, 53.4 /*; (3) on the stem, 54 /*; and (4) on the capsule, 52.5 /*.

In comparing the relative development of the two kinds of glandular tri-
chomes on equivalent areas we find that those on the stem are rather small
and that those on the capsule and on the veins of the leaf are relatively
large, but the relation of the size of the two trichomes to the given areas
is not strictly consistent. For example, the smallest pear-shaped trichomes
are to be found between the veins on the lower surface of the leaf, while
it is in such areas that the longest club-shaped trichomes occur. This
relation is shown in table 1 .

TABLF. 1 . — Relative lengths of the club-shaped and the pear-shaped
trichomes of Oenothera lamarckiana.

Where found.

Club-
shaped.

Pear-
shaped.

Leaf, between veins....

M
209.2

i"
43-9

Stern..

CO •}

Capsule

^08 c

O2 s

OENOTHERA CRUCIATA.

The three forms of trichomes which are to be found in Oenothera lamarck-
iana are present in cntciata also (fig. 1). In form and in other charac-
ters, the trichomes of cm data are like those of the other species, only they
are consistently smaller.

The awn-shaped trichomes, upon the lower surface of the leaves and
between the veins, are relatively uniform in length and usually straight.
The extremes in length which were observed were 71 .4 /"• and 131. 4 p. On
the veins the trichomes are somewhat more variable and range in length
between 71.4 M and 399 /*. Mixed with the shorter awn-shaped trichomes
there are on the stem numerous giant trichomes of the same character,
save as to size. These were seen as long as 1.3 mm. The giant trichomes
only, of this general type, were found on the capsule, where they attained a
length as great as 1.8 mm.

8 HEREDITY AS ILLUSTRATED BY TRICHOMES.

The club-shaped trichomes were not so abundant as in the other pure
species. None were found on the lower surface of the leaf, either upon or
between the veins, and none on the stem. The following- measurements,
in p-. were made of club-shaped trichomes on the capsule: 96.6, 121.8,
134.4, 134.4, 147.D, 163.8, 168.0. The average length was 138.0 M.

The pear-shaped trichomes occur on the lower surface of the leaves, on
the stem, and on the capsule. Other than being' somewhat smaller in size
they arc like those of Ot'itot/ii'i~a lamankiaiia . Trichomes between the veins
on the lower surface of the leaves were found to have the following' lengths,
in/*: 23.1, 25.2, 25.2, 29.4, 29.4, 29,4, 33.6, 33.6, 33.6, 33.6, 35.7, 35.7, 39.9,
39.9, 42. 0. The trichomes between the veins averag'c 32.5 P- in length. On
tile veins the trichomes g'avc the following' measurements, in p-: 25.2, 31.6,
33.6. 33.6. 33.6, 33.6. 35.7, 37.8, 37.8. 37. 8, 37.8, 39.9, 42.0, 42.0, 46.2.
The averag'e length of the pear-shaped trichomes on the veins was found
to be 36.5 f-. The following measurements, in !>-, were derived from tri-
ehoines on the stem: 29.4. 29.4, 29.4, 31.5, 33.6, 33.6, 33.6, 33.6, 33.6,
33.6, 33.6, 35.7, 35.7. 35.7. The trichomes on the stem averag'e 33.4 p- in
length. On the capsule the measurements, in p.. were: 27.3. 29.4, 29.4,
31 .5, 31 .5. 31 .5. 53.6, 55.6, 55.6. 55.6, 55.6, 55.7, 57.8, 57.8, 5.7S. The pear-
shaped trichomes of the capsule averag'e 33. 2 p- in length. The relation of
the pear-shaped and club-shaped trichomes to the areas where they occur
is shown in table 2.

TAP.I.H '_'. — Relation of tin triclioints of Ofnothfra crttciata to tlie
position on the plant where found.

Where found. ,Pear-

shaped. shaped.

M f-

Leaf, between veins.... Absent .32-5

Leaf, on veins Absent 36.;,

Stem Al)sent 35. .|

Capsule i ;S.o 33.2

In comparing the lengths of the pear -shaped trichomes of the two species
we find that those ot (tciiolln'ra </ iiiiiila are smaller wherever found than
those of Oenothcra laniarclciana, but that in both species the trichomes are
of variable length. The longest trichomes, with hardly an exception, occur
in the region of the veins of the leaf, and the shortest occur between the
veins of the leaf. In both species, also, the trichomes of the stem and of
the capsule are, as a rule, intermediate in length. The absence of club-
shaped trichomes from the leaf and the stem of Ooiotlicra cniciata precludes
a comparison of this trichonie in the two plants, but it is significant that the
trichomes of this type on the capsule of Oi-nothcra friiciata are smaller than

TRICHOMES OF OENOTHERA HYBRIDS AND PARENTAL LINES.

those on the capsule in the other species, as might be expected from a com-
parison of the pear-shaped trichomes. The awn-shaped trichomes arc so
irregular in their variation that a comparison of them would he meaningless.

OENOTHERA LAMARCKIANA X OENOTHERA CRUCIATA

The three sorts of trichomes which occur in both pure parents, the awn-
shapcd, the club-shaped, and the pear-shaped, arc to be found in the hybrid
also (fit;', l). The trichomes, except as to size, arc apparently like those
of the parents; they arc intermediate in size. The figures show the lead-
ing characters of the trichomes and a further description of them in this
place is unnecessary.

The awn-shaped trichomes on the ventral surface of the leaf, between the
veins, measure from 84.0 /* to 323. OM long; those on the veins ran. ye from
84. Of- to 1073.6 f- in length. Awn-shaped trichomes from the stem are from
96. 6 f- to 1214.4 /J-; those on the capsule are all of the lon.y kind and measure
from 915.2 f- to 12S4.8 n in length.

The club-shaped trichomes were not seen on the lower surface of the
leaves or on the stem, but were abundant on the capsule. The following
measurements, in f-, were made of such trichomes from the capsule: 165.8,
168.0, 180.6, 193.2, 201.6, 205.8, 210. o, 222.6, 239.4, 243.6. These tri-
chomes average 202 8 /* in length.

The pear-shaped trichomes occur on the lower surface of the leaf, on as
well as between the veins, on the stem but not on the capsule. Trichomes
from the lower surface of the leaf, between the veins, measured as fol-
lows, in f-: 29.4, 29.4. 29.4, 31.6, 33.6, 33.6, 35.7, 35.7, 37.8, 37. s. 37. S,
42.0, 43.2; on the veins, 33.6, 33.6, 35.7, 35.7. 37.8, 37.8, 42.0, 42.0, 42. o.
42.0, 44.1, 44.1, 46.2, 50.4. The average length of the former is 34.9 M,
of the latter is 41.3 f-. The following measurements, in f-, were obtained
from trichomes on the stem: 33.6, 33.6, 35.7, 35.7, 57.8, 37.8, 42.0, 42.0,
42.0, 42.0, 42.0, 44.1, 44.1, 46.2. 50.4. The average length of the pear-
shaped trichomes on the stem was found to be 4O.6 /*.

Table 3 gives the relative lengths of the trichomes of (.~>iiin>tlicni lantan'k-
iaiia X Ocnotlicra in/data.

TAKI.K :\.— Relative te/igt/ts of cliib-s/inpeit and pear-shape
of Oenothera laniarckiana X Oenothera iruciata.

Where found.

Club-
shaped.

Pear

shaped.

M

M

Leaf, between veins

Absent

34 -y

Leaf, on veins

Absent

1 1 ^

Stem

Abspnt

4 ' • J

40 6

Caosule...,

202 . S

Absent

10 HEREDITY AS ILLUSTRATED BY TRICHOMES.

COMPARISON OF TRICHOMES OF THE OENOTHERAS.

Upon comparing the extremes in length of the trichomes of the hybrid
with those of the parents we find (l) that the shortest pear-shaped trichomc
in Ocnothcra cruciata is 23.1 /"•; the shortest in Oenothera lamarckiana, on the
other hand, is 37.8 /*, while the shortest in the cross is 29.4 M, and (2) that
the longest pear-shaped trie-home in Ocnothcra lamarckiana is 67. 2 /*, the
longest in the hybrid is 50.4 /*, and in Ocnothcra cniciata is 46.2 M. In every
instance the shortest pear-shaped trichomes occur on the lower surface of
the leaf and between the veins, although in Oenothera lamarckiana short
ones occur also on the other areas; the longest are to be found on the veins,
although in the hybrid long ones may occur on the stem as well. As will
appear repeatedly in this study, this relation of the length of trichome to
area where found is so consistent in all plants that there may be some com-
mon underlying' cause, as, for example, nutritive conditions, which induces
the differentiation. At any rate, the variation is constant and must be
taken into account in the comparison of the trichomes of related forms.

Reference to tables 1,2, and 3 will show that in every instance where
analogous trichomes from similar areas are compared, those of the hy-
brid are intermediate in size, those of Ocnotlicra lamarckiana are larger,
and those of Ocnotlicra cruciata are smaller. In every instance also, the
trichomes of the hybrid are somewhat less than one-half the sum of the
other; that is, they are not average in measurement. In such hybrids as
the above, when both parents possess identical, or practically identical, tri-
chomes, whose sole apparent difference is that of size, we would perhaps
expect such a result. This would be based on one of at least two grounds:
First, either that the trichomes of the hybrid would represent the equal in-
fluence of both parents, as McFarlane* showed long ago for several hybrids;
or second, if there was inconsistent working' of the law of dominance so
that a portion of the trichomes would show reversion to one and a portion
to the other parent and the third portion be indeterminate in this regard,
the average of the whole would be intermediate. So far as the present
studies would indicate, there is no clear proof that reversion may not occur
as stipulated in the second alternative, since 60 per cent of the pear-shaped
trichomes of Ocnothcra lamarckiana come within the range of size exhibited
by these types in the hybrid, and 80 per cent of the pear-shaped trichomes
of Ocnothcra cniciata are within the range of size of those of the hybrid.
But it seems more probable, from the general intermediate condition of
the hybrid, that the trichomes may each also show the influence of each
parent in approximately equal measure.

*A comparison of the minute structure of plant hybrids with that of their parents,
and its bearing on biological problems. Trans. Roy. Soc. Edinb., 37 : 203, 1892.

TRICHOMES OF OENOTHERA HYBRIDS AND PARENTAL LINES. 11

A study of the range of variation of the pear-shaped trichomes shows
that those of the hybrid in nearly every instance are less variable than
those of either parent. The equations in table 4, which are numbers based
on the extreme lengths of the trichomes, present the relative variability in
a graphic manner.

TABLE 4. — Comparative variability of pear-shaped trichomes of tlic Oenothcras.

Where found.

O. lamarck-
iana.

O.lamarckiana
X O. cruciata.

O. cruciata.

Leaf, between veins....

149

177

146

I W

181

181

Stem

I ^O

121

148

l6d

1 4

The distribution of the trichomes shows the influence of both parents in
an unequal degree. The awn-shaped trichomes are found on all areas,
i. e., on leaf, stem, and capsule, but the club-shaped and pear-shaped tri-
chomes are variously distributed. The former occurs on all of the areas
in Oenothera lamarckiana, but only on the capsule in Oenothera cruciata and
the hybrid. The distribution of the pear-shaped trichomes is much more
general and uniform; they occur on the leaf and the stem in parents and
the hybrids, and on the capsule in Oenothera cruciata, but are not present on
the capsule in the cross, and only sparingly so on the capsule in Oenothera
lamarckiana. Where there are several kinds of trichomes present the
distribution of each kind in parents and hybrid offspring must be com-
pared in order to derive any exact knowledge of the influence of the parents
or the behavior of the pubescent covering throughout the generations of
hybrids studied. It is doubtful whether the quality of hairiness in plants
can rightly be considered a "unit-character," and the analysis must be
carried vet further.

12 HEREDITY AS ILLUSTRATED BY TRICHOMES.

TRICHOMES OF PAPAVER HYBRIDS AND THE PURE SPECIES.

TRICHOMES OF THE PURE SPECIES.

The thive speck's of l'a/>aw which were studied have only one type of
trie-home, which is niulticellular. awn-shaped, and usually slightly curved
(fit;'. 2). The trie-homes may attain a length so great as 7.0 mm. {Papaver

). although they usually are shorter than this.

FHJ. 2. — Trichomes <if I'ttparrr pure lines and hybrids. l\timt:fr somniferum: a, multieellu-
lar trichome from youny; stein ( lln b, detail of tip of trio home, showing appressed dis-
tal ends of the superficial cells ( :',ii'J). l'ai>ai-er oritnlalr: r, tip of triohome, to show the
projection »rdisi:il ends of snprrtlnlal (c'lls ( :ili2): </. detail from middle portion of tri-

(•home < W2} . />nf>ar<ir ]>i/»xinn: r, '2 triuhomes, s
projection of superficial cells { 84), Pajtarrr stun)
middle part of a typical trichome, losliow the Inooi
only a portion of which project ( .'W2); </, tip ol'tricl
/'. ftifoxitnt: It and /, Iricrhoinc^ from a leaf, illustrat
of superficial cells ( si).

•ini-diagrainniatic, to illustrate the
i/rritm I*, oricntafe: ./, detail from
sistent behavior of superficial cells,
oine ( ;t(i'2) . Palaver somniferutn
ng relatively prominent projections

The trichomes have a curious structure, unlike any other encountered in
the course of this investigation, Their growth appears to be indetermin-
ate. As a direct result of this character the length of the trichomes is
very unequal, so that comparative study of them from this standpoint would
he without value. The trichomes end in a sin<>-le much-eloncrated cell

TRICHOMES OF PAPAVEK HYBRIDS AND PURE SPECIES. 13

(fig. 2,6). Somewhat away from the tip there are 2 cells in cross-section,
still further there are 3 cells, and the number of cells of the cross-section
increases as one goes towards its base. At the base the triehome expands
suddenly, so that in longitudinal section it is conical. All of the cells of
the triehome are elongated in a direction parallel to its longer axis. The
increase in length takes place at the tip of the triehome, where new cells are
cut off by somewhat oblique walls. Growth in diameter appears to occur
by longitudinal divisions of the inner cells, but the exact sequence of these
divisions was not studied. The trichomes of the three species are similar
in form and size, but they are unlike in quality of roughness. In Pafnu'ci
orientate and 1'apa-ccr pilosuin the distal ends of the superficial cells project
beyond the general surface of the triehome and turn out at a rather acute
angle (fig. 2, c, d, and c) . In Papa-rcr soiiniifcntm, however, these cells did
not extend beyond the general surface, with the elTeet that the trieliomes
of this species are smooth (fig. 2, a).

The leaves, stem, and flower-stalk of Pafxircr orientals and of Pa/'aa f
pilosuin are well clothed with trichomes; but Papavcr sonuiifcrnin is prac-
tically glabrous, as only a few trichomes were observed on the flower-stalk.
and none on stem or leaf.

PAPAVEK SOMN1FEKUM PAI'AVEK OKIENTAI.K.

The leaves and the stem of the hybrid are well covered with trieliomes,
which in size and in form resemble those of Paparn' cricn/a/i'. but in certain
regards they are unlike the trichomes of that species. They, also, are dif-
ferent from the trieliomes of I^afiarcr soinnifrrniii, and in each instance the
variance lies in the character of the peripheral cells. The distal ends of
the peripheral cells, which in the seed parent are suppressed, but which
project with much regularity in the pollen parent, in the hybrid are ex-
tremely variable in their behavior. In some instances they do not project
at all, in others the projection is well marked, perhaps accentuated, and
frequently a middle condition was noted (fig. 2, /). In this particular,
therefore, the hybrid is fairly intermediate and exhibits the influence of
both parents.

PAPAVER SOMNIFERUM X PAPAVER P1LOSUM.

The hybrid Papaver somnifenim '' Papaver pi lositm is especially well
covered with trichomes which closely resemble those of the other Paparcr
hybrid studied. That is, in one character, namely, that of the surface of
the trichomes, they are intermediate between the pure parents. The pro-
jections of the superficial cells are, if anything, even more irregular than
in the other hybrid (fig. 2, // and /). In some instances the projections
are suppressed entirely, in others they are exaggerated and the triehome
has the appearance of bearing branches, but an intermediate condition also
occurs. This variation is suggested, but nowise adequately shown, in the
accompanying sketches .

14

HEREDITY AS ILLUSTRATED BY TRICHOMES.

TRICHOMES OF SOLANUM VILLOSUM X SOLANUM GUINENSE
AND OF THE PURE SPECIES.

SOLANUM VILLOSUM.

There are 2, or possibly 3, kinds of trichomes in Solanum villosum,
all of which are multicellular and glandular (fig. 3, a, 6, and c~). These
for convenience are in this paper called the awn-shaped, the giant awn-
shaped, and the big-headed trichomes. The trichomes were observed on
the stem, on the lobes of the calyx, and on the ventral surface of the leaves.
On the stem and on the calyx all 3 types occurred, with no apparent
choice of position, but on the leaf the distribution was as follows: only the
awn-shaped trichomes occurred between the veins; all were found on the
veins.

\

KIG. :!.— Trichomes of Nulnnnm pure species and hybrids. Solanum
nillOKum: a, awn-shaped secreting trichome from leaf, region of vein;
ft, giant awn shaped secreting trichoino from region of midrib, veu-
tral leaf-surface; r, big-headed glandular trichomes from young stem.
ftolanumgniiicnie: <l, big-headed glandular trichomes from ventral
leaf-surface; e, awn-shaped non-secreting trichome. Solanum villa-
sum < iV. nmnensc: /, big-headed glandular trichome from ventral
surface of leaf; </, big-headed glandular trlctiomr. (For size of tri-
chomes see text; reduced one-half.)

The awn-shaped trichomes are composed of a single row of cells, usually
5, which constitute the stalk, and a single terminal cell, which is some-
what swollen and contains coarsely granular matter. The stalk-cells are
provided with a delicate protoplasmic lining and are hyaline. They meas-
ure from 176fMo 352 ^ in length, with the greatest number about 193 p
long.

TRICHOMES OF SOLANUM HYBRIDS AND PURE SPECIES. 15

The giant awn-shaped trichomes are different from the awn-shaped ones
mainly in the quality of size; they measure from 792 p- to 1.4 mm. No tri-
chomes were observed of the awn-shaped type which measured longer
than the longest awn-shaped trichomes, 352 /*, or shorter than the shortest
giant awn-shaped trichomes, 792 p-, for which reason the giant awn-shaped
trichomes ought probably to be considered a distinct type.

The big-headed trichome is somewhat curved, so that it is frequently
closely appresscd to the surface. In structure it is composed of a stalk of
2 cells and a head of an undetermined number of cells, probably 4. The
stalk-cells are hyaline; the cells composing the head are densely filled with
a coarse granular substance. These trichomes measure from 63 h- to 75.6 M

in length.

SOLANUM GUINENSE.

Two types of trichomes were observed in Solatium guinense, of which
one is glandular and similar to the big-headed trichome of Solanum villosum,
and the other is not glandular and is unlike any trichome seen in the other
species (fig. 3, e).

The trichomes were found mainly on the veins of the ventral surface of
young leaves and on young stems; the older stems and older leaves were
usually glabrous.

The big-headed secreting trichomes were either straight or mostlv curved
and appressed to the surface. They measure from 67. 2 c- to 71 A ^ in length.
As in structure, size, and general appearance these trichomes are similar
to the analogous ones in Solanum villosum, a further description of them is
unnecessary.

The non-secreting trichome (the uncommon type) is broadly awn-shaped,
is usually curved, and more or less closely appressed to the surface. It is
composed of 3 or 4 cells, which rest on a multicellular base. The trichomes
measure from 239.4 p to 378.0 /«• in length.

SOLANUM VILLOSUM X SOLANUM GUINENSE.

Two types of trichomes were observed in the hybrid Solanum. These
were in all respects like those of the species Solanum gniiiense, and conse-
quently were the big'-headed secreting" trichome and the awn-shaped non-
secreting trichome (fig. 3, e and/). The trichomes are on the ventral leaf-
surface, where they are confined to the veins. The big-headed trichomes
measure from 67.2 f- to 80 /* in length, and the a\vn-shaped trichomes from
252.2 Mo 316.8 /* in length.

The account above given is from a study of first-generation plants.
About a dozen second-generation plants were examined also and a condi-
tion quite like that observed in the first generation was found. Measure-
ments of the big-headed secreting trichomes of second-generation hybrids
showed that they ranged from 63.0 M to 71.4 p on the leaves, and 71.4 /n to

16 HEREDITY AS ILLUSTRATED MY TRICHOMES.

75.6 /* on the stem. The non-secreting a\vn-shaped trie-homes measured
from 218.8 /* to 316.8 p in length.

The general relation of the triehomes of the hybrid and the pure lines of
Solanum are .^iven graphically in table 5.

TAIILE •"). — Comparison of the triehomes of Solatium villosuin X Solatium guinense

with those of the pure species.

Kind. S. villosuin. The hybrid. S. guinense.

Awn-shaped, glandular 176 n to 352 M Absent Absent.

Giant awn-shaped •.. 792 M to i . .) mm... Absent Absent.

Big-headed.. 'v, M to 75.6

67 . 2 MO 80 /* 67 . 2 MO 7 1 . 4 /u.

*(63 Mo 71 . } M)...
*(7i .4 m to 75.6 11)

Awn-shaped, not glandular... Absent 252. 2 Mo ',16.8,11 239.4/410378/1.

•(218. 8/1 to 316.8/4!

"The numbers in parenthesis refer to hybrids of the second generation.

The data at hand are not sufficient either in amount or in kind to permit
the tracing of the influence of the pure lines on the hybrid offspring. This
much is apparent, however: the triehomes of the non-seerctin.u' awn-shaped
type which the hybrid inherits from So/a mini u'liiiit'iisi' are of full, not of
half six.e, as MacFarlanc found in certain hybrids investigated by him
where unilateral inheritance also obtained. v

*MacKarlane, I. c.

CANNON

Dorsal surface of leaves of Juglans nigra, which was growing near the Sebastopol, California,
ranch of Luther Burbank. The leaves were selected to show the extreme variation in
size and number of leaflets, and size of leaves. The basal pair of leaflets in the figure to
the right are only partly shown. One-third natural size.

PURE SPECIES AND HYBRIDS OF JUGLANS. 17

PURE SPECIES AND HYBRIDS OF JUGLANS.
LEAVES OF THE PURE SPECIES.

The English walnut and the eastern black walnut are common in culti-
vation in and around Santa Rosa, where they grow thriftily. The Cali-
fornia walnut is native at Santa Rosa and is also extensively cultivated as
a shade tree in the city, as elsewhere throughout the State. Material for
the study of the California walnut (///ff/a/is californica) and the English
walnut (.Juglans regia) was obtained from trees 16 and 13 meters in height,
respectively, which are growing on the grounds of two of Mr. Burbank's
neighbors in Santa Rosa. Material of the third type (Juglans nigra) was
collected from a second-growth tree, about 10 meters high, which was
growing by the roadside between the town of Sebastopol and the Burbank
ranch near that place. The collections were made in May, 1907, and May,
1908. It is not known whether the trees from which the collections of
material were made were the actual parents of the original crosses or not.
It is assumed in this study that the alternative is the case.

JUGLANS CALIFORNICA.

The leaves of Juglans californica bear from 9 to 10 pairs of leaflets and
1 terminal one (plate l), which is frequently the smallest one of the leaf.
The leaflets are ovate lanceolate and taper gradually to the tip; the base
is abrupt and even cordate; that of the terminal leaflet is attenuate. They
are somewhat roughened on the upper surface and are irregularly serrate,
with rather obtuse teeth. A close study of the leaflets would show con-
siderable variation, especially in size and in form, although the extent of
the variation has not been especially examined.

In May, 1907, when the first collection of leaves, which were exclusively
studied, was made, they were fresh-green and free from fungus; in the
following September, when the photographs were made, the upper surface
was badly infested with a black fungus which to an extent shows in the
plate .

JUGLANS NIGRA.

The leaves of Juglans nigra are extremely variable, particularly as to
size and number of leaflets, which number from 6 to 10 or more pairs and
which are irregularly disposed on the rachis (plate 2). In addition to the
variation of the leaves as a whole, the leaflets, also, are far from uniform
and in outline range from nearly cuneatc to ovate; the terminal one is
usually the smallest. They taper gradually to an attenuated apex. The
bases are abrupt, but usually they are not cordate. The leaflets are serrate,
with fairly regular and sharply pointed teeth; the ventral surface, in tex-
ture, is generally smooth.

18 HEREDITY AS ILLUSTRATED BY TRICHOMES.

JUGLANS REGIA.

The leaves otjtiglans rcgia are composed of from 2 to 3 pairs of leaflets
and a terminal one. The leaflets usually grade in size from the basal ones
to the terminal one, which is largest. They are broadly ovate and taper
rather abruptly to the apex, as well as to the base. The base of the
terminal leaflet is somewhat attenuated. The margin of the leaflets is
entire; the upper surface of the leaflets is minutely roughened or nearly
smooth, with the veins projecting somewhat above it (plate 3).

Table 6 presents in a condensed form the leading characters of the leaves
of the 3 species oijiiglans.

TAHLE (i.

Leaves with 5 to 7 leaflets J. regia.

Leaves with 19 to 21 leaflets... J. californica

Leaves with 13 to 21 leaflets J. nigra

Leaflets serrate J. californica J. nigra

Leaflets entire J. regia.

Leaflets narrow ovate J. californica

Leaflets ovate J. nigra

Leaflets broadly ovate J. regia.

Leaflets, apex abrupt J. regia.

Leaflets, apex gradual taper J. californica

Leaflets, apex attenuate f. nigra

Leaflets, base abrupt J. nigra

Leaflets, base cordate J. californica

Leaflets, base gradual taper J. regia.

TRICHOMES OF THE PURE SPECIES.

There are 4, or perhaps 5 kinds of trichomes in the pure species of
Juglans studied, all of which are found in each of them, which are trans-
mitted kind for kind to the hybrids, so that all types are to be found in the
cross in the first and, so far as observed, in the second and the third genera-
tions also.* The trichomes will be called (l) awn-shaped, which may form
groups which constitute stellate trichomes; (2) disk-shaped; (3) long
secreting trichomes, apparently of two kinds; and (4) short secreting-
trichomes. All of the trichomes, except the awn-shaped, are glandular
(figs. 4, 5, 6, 7, and 8).

The trichomes are distributed in time and in space in a manner which
is fairly consistent and characteristic. In the youngest leaves the most
abundant trichomes are the awn-shaped ones. In old leaves, although not
plentiful, the short secreting trichomes may be the only trichomes, or
nearly the only ones, to be found. In what may be called the leaves of
middle age, all of the trichomes occur, none perhaps predominating. All
the glandular trichomes are to be found on both surfaces of the leaves, but

*In the present study observations have been confined to the trichomes of young
and old leaves; only in one or two cases has the study been extended to the stem. No
attempt has been made to treat the general distribution of trichomes either of the pure
lines or of the hybrids.

PURE SPECIES AND HYBRIDS OF JUGLANS. 19

in the old leaves they may have disappeared from the upper surface while
yet some remain on the lower (ventral) side. The awn-shaped trichomes,
on the other hand, are apparently confined to the ventral surface of the
leaves, whatever may be their age. In this connection an apparent ad-
aptation, which is a nice one, is interesting to note. In old leaves the
ventral (or lower) surface is protected in various ways (more especially
presumably by the position) and the awn-shaped trichomes are absent;
in the youngest leaves, on the other hand, the ventral surface of the leaves
is the outer surface and is most exposed and at the time it is well provided
with awn-shaped or stellate trichomes.

In addition to the variation in kinds of trichomes which are present at
different stages of development of the leaf, or to the variation in their
distribution on the leaf, either of which have only been touched in the
foregoing, another factor also must be taken into account, namely, the
variation of the trichomes in size which accompanies their distribution.
For example, small secreting trichomes may be larger on the veins than
between them, and larger on the dorsal surface than on the ventral surface
of the leaf. Also, the trichomes may be larger in one stage of development
of the leaf than in a subsequent stage, or there may not be this difference.
Whatever may be the causes of the variation in size, the fact that the tri-
chomes are variable makes it necessary that trichomes of analogous stages
of development, as well as of analogous positions on the leaves, be com-
pared, which has been done in all instances. These circumstances, which
were noted in the other hybrids, and were observed notably in those of
Oenothera, greatly extended the work, but at the same time quite as much
enhanced the interest of the investigation.

Jl'GLANS CALIFORNICA.

Four or possibly five kinds of trichomes are to be found on both surfaces
of the leaves of Juglans californica (figs. 4 and 5). These are (l) the awn-
shaped trichomes, (2) the disk-shaped trichomes, and (3) the long and (4)
the short secreting trichomes. Of the third type there may be two kinds.
The awn-shaped trichomes measure about 360 ^ in length; the disk-shaped
trichomes are closely appressed to the surface; the long secreting trichomes
are 140 /* more or less long, and the short secreting trichomes measure 58 ^
more or less in length. A more detailed description of the trichomes, their
origin and development, and their variation, will be given in the following
paragraphs.

The awn-shaped trichomes are more variable than the other types, and
since there were no apparent causes of the variation this type of trichomes
was not especially studied, either in the pure species or in the hybrids.
The awn-shaped trichomes are unicellular and frequently are to be found
in groups (fig. 4, a). They occur on the ventral surface only and are
especially abundant on the young leaves.

20

HEREDITY AS ILLUSTRATED BY TRICHOMES.

The disk-shaped trichomes are glandular and are to be found on both
surfaces of the leaves (fig. 4, /to «).

The developmental history of the trichomes was not followed in all par-
ticulars. The following are some of the definite stages:

The trichome originates as broad and squat outgrowths of an epidermal
cell, from which it is early separated by a cross-wall that nearly coincides

Fia. 4.— Trichomes ofjur/lansualifornica: a, group of awn-shaped trichomes forming a stellate
trichome, leaf-surface « 81); 6 to n, various stages in the development of disk-shaped tri-
chomes (c, d, e, h,j, and ( X 535;/, g, i. and m X H60; n < 340); o, abnormal disk-shaped trichome
(X 535).

with the adjacent epidermal walls. From its first appearance as a papillate
projection the disk-shaped trichome is easily distinguishable from the be-
ginnings of the other forms. The young trichome becomes 2-celled by a
transverse division which separates the stalk from the portion which will

CANNON

PLATE 3

leaves of Juglans regia, from a tree growing in Santa Rosa, California, showing
extremes in size of leaves and number of leaflets. One-third natural size.

PURE SPECIES AND HYBRIDS OF JUGLANS. 21

become the head; all succeeding- divisions, therefore, differentiate the two
regions, namely, stalk and head. The second division is a longitudinal
one in the terminal, the head-cell, which is followed by a transverse divi-
sion of the stalk-cell. The stalk does not divide further in normal cells. A
single monstrosity was observed however (fig'. 4, <?), in which the stalk
had 4 cells. The subsequent cell-divisions are confined to the region of
the head of the trichome. The second and third divisions of the head are
by walls placed at right angles to the preceding division-wall, so that the
head becomes 4 similar cells (fig. 4,/).

Each quadrant is next bisected by a radial wall (fig. 4, g) . After this
the sequence of cell-division appears not to be regular, although the
octants usually undergo radial division; and this process is repeated until
a head of about 32 cells is the result. Whether the mature trichome may
have more than this number of cells was not determined, but in some in-
stances the presence of cell-walls placed at right angles to the radial walls,
and parallel to the long axis of the trichome, would indicate a larger num-
ber. The head of the trichome never exceeds 1 cell in thickness. At an
early stage in the development of the trichome the peripheral portions of
the head-cells become somewhat enlarged, so that in longitudinal section
the edge of the disk is bluntly angular. This is the beginning of the rim
which is a characteristic of the disk-shaped trichomes. By the more active
growth of the lower portion of the more peripheral cells the rim is pushed
upwards, /. e. , in a direction away from the surface of the leaf, until in
section the head of the trichome is markedly concave (fig. 4, //).

There is much variation in the depth of the concavity of the disk, as in
some trichomes the tip of the trichome was quite flat, while in others the
concavity was pronounced. The diameter of the heads in the mature tri-
chomes was more uniform, as the following measurements, in /«•, which
were made on representative trichomes selected at random, would indicate:
117, 100, 100, 117, 100, 109.

The long secreting trichomes are 126 /«•, more or less, in length (fig. 5).
The trichome takes its origin as a slender papillate projection of an epi-
dermal cell; it is early cut off from the epidermis by a transverse wall.
Subsequently it undergoes division, so that the young trichome consists of
3 cells, of which the terminal ones are the rudiment of the head of the tri-
chome. The third wall is, as in the short secreting trichome, almost surely
a transverse one in the stalk-cell. After this the sequence of cell-division
was not followed, but the final result is that the head is composed of 4
equal cells, radially disposed, and the stalk is composed of 4 lineally-
arranged cells. No further cell-divisions occur in this type of trichome.
The terminal cells which constitute the head become densely filled with
coarsely granular material; the cells of the stalk are poor in protoplasm.

22

HEREDITY AS ILLUSTRATED BY TRICHOMES.

Besides this type of long- secreting- trichome there may be another kind
with a larg-er number of cells in the head and also more than 4 cells in the
stalk. This form of trichome, doubtful in Juglans calif arnica, was often
met va. Juglans nigra, under which it will be described, and in the hybrid
Juglans calif arnica X Juglans nigra. A few trichomes with 6 stalk-cells
were found in Juglans calif ornica, one of which is shown in fig. 5, //, but
none were seen under conditions that made it possible to surely determine
the number of cells in the head.

FIG. .» — Trichomes of Jntjlanx catifnniirn: a to h,
secreting trichomes (a, c, 'I, /, and // .Vio; It
secreting trichomes ( ^ 53.r>).

various stages In the development of long
1200); i lo n, mature and young short

In studying this type of trichome it was found best to measure the head
only, since the entire trichome, which is relatively long-, varies consider-
ably. Wherever possible the position of the trichome on the leaf is given,
but whenever this is not done it is to be understood that in every instance

PURE SPECIES AND HYBRIDS OF JUGLANS. 23

trichomes from analogous situations only are compared, so that the results
in such cases may be considered just. Following are measurements, in /*,
on the length and diameter of the heads of the long secreting trichomes
which were taken from the veins of the leaves: length, 29.4, 31.5, 35.7,
33.6, 33.5, 31.5, 29.4, 25.2, 29.4, 29.4, 33.6, 25.2; diameter, 25.2, 23.1,
29.4, 25.2, 25.2, 27.3, 29.4, 31.5, 25.2, 25.2, 29.4, 27.3. The diameters of
the heads are the diameters seriatim of those whose lengths are given pre-
viously. The heads average 30.6/". in length and 26. 8 n* in diameter. In
length the variation from the average is 13 per cent, and the variation
from the average diameter is 15 per cent.

The material of fitglans calif arnica which was put up was not favorable
for the study of the youngest stages in the development of the short secreting
trichomes, so that a description of the mature organ only is possible in this
place. The trichome is made up of 6 cells, of which 2 constitute the stalk
and 4 the head (fig. 5, / to o), which are so placed as to form a disk 1 cell
in thickness."

A character which easily distinguishes the short secreting trichomes of
fiig/aiis calif arnica from those of Juglans regia or Juglans nigra is the length
of the head-cells. As the figures indicate, the cells of the head of the
short secreting trichome in fitglam califoniica are relatively long", which is
a consistent character of the long secreting trichome as well, and it is this
circumstance which makes advisable the comparison of the lengths of the
heads in both pure species and hybrids, although other measurements have
also been made. The measurements on the length and the diameter of
the heads, as well as the lengths of the entire short secreting trichomes,
together with notes on the positions occupied by the trichomes, are pre-
sented herewith.

The lengths of the heads of the short secreting trichomes are greater for
trichomes which are on the veins than for those which occur between them.
The measurements, in ^, are: length of heads, on veins, 35.7, 33.6, 27.3,
29.4, 27.3, 27.4, 27.3, 33.6, 33.6, 33.6, 33.6, 29.4; diameters of the same
heads, respectively, 25.2, 23.1, 29.4, 25.2, 27.3, 29.4, 23.1, 31.5, 25.2, 25.2,
29.4, 27.3; length of heads, between veins, 27.3, 21.0, 25.2, 29.4, 25.2,
29.4; diameters of the same heads, 25.2, 25.2, 25.2, 25.2, 25.2, 29.4. The
heads from trichomes on the veins average 30.9 /*, in length and 26.8 p- in
diameter. The average lengths of heads from between the veins is 26.2 M
and they average 25.9 /"• in diameter. The heads of trichomes on the veins
show a variation in length of 13 per cent from the average; those between
the veins, a departure of 16 per cent from the average. The variation
from the average in diameter for the two series is respectively 13 and 8
per cent.

Although it is recognized that the small number of measurements lessens
the value of the results in any instance, as a whole the number of measure-

24

HEREDITY AS ILLUSTRATED BY TRICHOMES.

ments made upon analogous organs is considerable, so that for a compara-
tive study they may not be without value . It is therefore of interest to note
that the departure in per cent from the average of trichomes on the veins
is the same in the long secreting trichome as in the one just considered.
Table 7 gives the length of the trichomes, the length and diameters of
the heads, and gives the area from which the trichomes were taken.

TABLE 7.— Measurements on short secreting trichomes ofjuglans californica.

VENTRAL SURFACE.

Length of
trichome.

Length
of head.

Diameter
of head.

M

M

M

37-7

18.9

(?)

37-8

21 .O

25.2

37-8

25.2

25-2

39-9

21 .O

27.3

37-8

18.9

23.1

46.2

25.2

25.2

42.0

23->

29.4

39-4

16.8

21 .O

33-6

18.9

21 .0

DORSAL SURFACE.

44.1
54.6
42.0
46.2

23.1

27.3
25.2
25.2

M

25.2
29.4

25.2
27.3

The average length of the entire trichome from the ventral surface is
39.2 /*, from the dorsal surface is 46.7 M. The average length of the heads of
trichomes from the ventral surface is 21 .0 /*, the average diameter is 24.6 p.
The averages for the length and the diameter of heads of trichomes taken
from the dorsal surface are 25. 2 ^ and 26.7 /*, respectively.

The measurements given in table 7 are of trichomes from old leaves;
those given just previously were from young leaves; we therefore have an
additional condition of the trichomes for comparison, /'. e., a comparison
of those from the young and from the old leaf. In the instance of the
young leaves the average length of the heads of trichomes which were on
the veins was found to be greater than the average of those which occur
between them. In the old leaves the heads of trichomes were larger on
the dorsal surface than on the ventral, although the number was so small
that their relation on and between the veins could not well be worked out.
A comparison of the average length of heads from the ventral surface of
the old leaf and of the young brings out the fact that the heads are longer

PURE SPECIES AND HYBRIDS OF JUG LANS.

25

in the young than in the old leaf, which condition must be taken into con-
sideration in such a comparative study as the present one.

JUGLANS NIGRA.

The following: kinds of trichomes are to be found on the leaves of Juglans
nigra: (l) awn-shaped, (2) disk-shaped, (3) short secreting1 trichomes,
and (4) two types of the long secreting trichomes (figs. 6 and 7).

The awn-shaped trichomes measure 252 p more or less in length and are
unicellular. They occur either singly or in groups of 3 or more. The awn-
shaped trichome is especially abundant on the young leaves, where it is
found on the ventral surface only, although it persists to a degree, so that
scattering trichomes are met on the older leaves as well.

The disk-shaped trichomes are relatively short and have broadly ex-
panded heads which are closely appressed to the surface. In longitudinal
section a disk-shaped trichome is seen to consist of a saucer-shaped head
and a stalk of 2 cells, although an occasional trichome is found which has
a stalk of 3 cells (fig. 6, X-). Certain stages in the development of the
disk-shaped trichomes were observed. It was seen to arise from the epi-
dermis as a squat projection which is soon separated from it by a transverse
wall. A second transverse division follows, by which the head is differ-
entiated from the stalk. The terminal cell next undergoes division by
longitudinal walls, so that a 4-cellcd condition results. The stalk-cell, by
the formation of a transverse wall, attains the adult number of cells, 2,
when cell-division of the stalk ceases.

In two cases the failure, to form a second transverse wall in the stalk
was noted. The cells of the head divide by the formation of longitudinally
placed walls only and are finally composed of 24 or more cells. The head,
therefore, is an expanded plate of cells 1 cell in thickness. The heads were
75 p- more or less in diameter and the depression of the head was 25 M more
or less beneath the rim.

Table 8 gives in detail the diameter of the heads and the depth of the
depression in several representative and mature disk-shaped trichomes.

TABLE 8. — Measurements of disk shaped trichomes of Jitglans nigra.

Diameter

Depth of Diameter

Depth of

Diameter

Depth of

of

depression of

depression

of

depression

trichome.

of head.

trichome.

of head.

trichome.

of head.

p

n

p

M

M

75.6

29.4

71.4

25.2

92.2

21.0

92.6

29.4

84.0

67.2

25.2

69.3

25.2

71.4

18.9

79.8

25.2

71-4

29.4

63.0

21 .O

84.0

2S-2

79.8

25.2

84.0

25.2

26

HEREDITY AS ILLUSTRATED BY TRICHOMES.

In table 8 the figures in the rig'ht-hand columns are the depths of the
depressions of the trichomes whose diameters are given in the correspond-
ing lines in the left-hand columns.

The averag'e diameter of the heads of the disk-shaped trichomes is 77.5 M;
the average depth of the depression of the heads is 25.0 /*. Reference to
the measurements on the disk-shaped trichomes in /u^/ans calif ornica, given

Fi(3. I). — Trichomes of Jttf/ltiitx nigru: a, awn -shaped trichome (ca., 242 H)\ l> to/;, development of
disk-shaped triehomes (/*, c, d, e, /, /, and/-, ,o3f): (/, ht and ./, .- N4j; Mo </, stages in devel-
opment of lone secreting tricboines ( ."A") : / 1200). (Reduced one-tin.li).

at page 21, will show that those of Jnglans nigra are noticeably smaller,
but the structure and the development, so far as known, of the two species
is the same.

The long secreting trichomes are of two types, of which one has a head
of 4 cells, and the other a head of about 8 cells. Of these the former will
be described first. The trichome is 100 M to 150 p in length (fig. 7, a to e).

PURE SPECIES AND HYBRIDS OF JUGLANS. 27

It takes its origin as a slender papillate projection of the epidermis, which
is early cut off by a transverse wall, and which may occasionally be laid
down somewhat above the general level of the epidermis. From a
comparison with this type of trichome in other species it is probable
that after the young trichome is separated from the epidermis it under-
goes one additional transverse division when the distal cell enlarges
and becomes the head and the proximal cell the stalk. Longitudinal walls
are laid down in the terminal cell in such fashion as to cause the organ-
ization of 4 similar and radially placed cells. With this the cell formation
in the region of the head is completed.

The stalk forms two other transverse walls (but it was not surely de-
termined whether the fourth stalk-cell was formed during or after the
divisions in the head as given above) when the cell-divisions in the region
of the stalk are completed. The trichome thus is composed of 8 cells, 4 of
which constitute the head and 4 the stalk. This relation was fairly con-
stant, and may have held in all cases, since when the number of stalk-cells
was more than 4 the head was relatively large and may always have had
more than 4 cells.

Table 9 gives the measurements on long secreting trichomes of the type
just described.

TABLE !l. — Measurements on long secreting trichomes of Juglans nigra.

Length

Diameter

Length

Diameter

Length

Diameter

of head.

of head.

of head.

of head.

of head.

of head.

M

M

M

M

M

M

21 .O

33-6

16.8

33-6

16.8

35-7

16.8

37-8

16.8

37-8

16.8

33-6

16.8

35-8

,6.8

39-S

i6.S

42.0

16.8

42.0

21.0

42.0

16.8

33-6

16.8

37-8

,8.9

39-9

16.8

3' -5

The heads of the trichomes average 17.6/"- in length and 37.4/w. in diam-
eter. The variation from the average length is 12.5 per cent, and the
variation from the average diameter is 14 per cent. From these measure-
ments it is seen that the heads of the long trichomes of the 4-celled type
are considerably shorter than the analogous ones vn. Juglans calif ornica and
that the relation of length to diameter of the head in the two species is
also unlike; the diameter is the greater in Juglans nigra and the length is
the greater \njuglans calif arnica. But the variation from the average length
is identical in the two species, and the variation from the average diameter
is nearly the same.

The long secreting trichome of the second type, that with more than 4
cells constituting the head, is fairly abundant in Juglans nigra. This type
of trichome has a stalk of from 5 to 9 cells and a head of about 8 cells
Only a few stages in the development of the trichome were seen, but these

28

HEREDITY AS ILLUSTRATED BY TRICHOMES.

were sufficient to indicate that this is essentially different from the long
secreting1 trichome of the other type. Fig'. 7, ,e, shows a cross-section of a
head which has reached the 5-celled stage. The fifth cell is formed by
the laying down of a radially placed wall by which the mother-cell is divided
unequally. In fig'. 7, ./', each of the 4 cells is divided by a radial wall, so
that the mature, or 8-celled, condition has resulted.

FIG. ".—Trichomes of Jiiyliinx niiirn: n to e, long secreting trichomes («, h, c, /I, and e X 585);

/ to ./, long secreting triohomes of the typo having a head of more than 4 cells (/, </, and /*,

•;535; /and.) • 84) ; k to /, short secreting trichomes, of which ?i to /-are from the region of the

veins of the leaves and » is from the leaf-margin (k, I, and «, 1200 : m, n, p, //, r, s, and (,

.%">). (Reduced one-flfth).

In several trichomes the head was observed to be composed of 7 cells
only, but whether these were immature or not was not determined. From
a careful study of this type of trichome it would seem that 8 is the usual
number of cells in the head. No measurements were made on the length
of the heads, but the following numbers, in /*, give the diameter for the
type of trichome in question. The measurements were made only on
what were thought to be mature forms. Diameter of heads: 54.6, 50.4,

PURE SPECIES AND HYBRIDS OF JUG LANS.

29

48.3, 50.4, 50.4, 56.6, 46.2, 52.6. That this is a type of trichome distinct
from the 4-celled one is apparent from the diameters of the head. The
smallest head of the 8-celled type is considerably larger than the largest of
the 4-celled form , while the largest is more than 25 per cent larger than these.

No study was made of the excretions or especially of the structure of
the secreting cells; the 8-celled type was the only one which gave indica-
tion of marked activity in this regard. In such trichomes as shown in fig.
7, c, it appears as if the secretion is held for a time between the layers of
the cell-walls which are on the distal end of the trichome. The appear-
ance figured was not observed in other trichomes.

The short secreting trichomes occur on both surfaces of the leaf; they
measure from 27.3 f- to 50.4 /* in length. Whatever may be the size of the
trichome the terminal group of cells is relatively short, so that the head
has a squat appearance.

The early, stages in the development of the short secreting trichome are
apparently similar to those of the same type of trichome \n Jug/aiis cali-
fornica. As usual for the trichomes, the origin is to be traced to rather
narrow outgrowths of epidermal cells. The young trichome undergoes
two successive transverse divisions, by which it is separated from the epi-
dermis and by which the initial of the head is differentiated from that of the
stalk. The following" division is probably longitudinal in the head-cell,
followed by a division of the stalk, by which the divisions in the stalk are
concluded, and this is followed by the second and third longitudinal divisions
of the head, which take place in such manner that 4 radially placed cells
result. With the organization of a head of 4 cells and a stalk of 2 cells
the cell-division in the trichome ceases.

TABLE 10.— Measurements on short secreting trichomes from young leaves,

Juglans nigra.

On the veins.

Between the veins.

Length of head.

Diameterof head. Length Diameter

..

t

i .

2.

head. head.

/J-

M

M

M

V-

21 .0

21. O

35-7

31.6

14.7

Not given

21.0

18.9

33-6

33-6

16.8

23-3

18.9

33-6

29.4

16.8

16.8

29.4

M-7

16.8

33-6

M-7

21. 0

35-7

16.8

16.8

3i-4

16.8

29.4

16.8

16.8

25.2

29.4

16.8

21. O

33-6

21.0

35-7

16.8

29.4

21 .0

3i-6

30

HEREDITY AS ILLUSTRATED BY TRICHOMES.

On the veins the average length of the heads is 20.8 M; the average
diameter is 31.5 n. Between the veins the average length of the heads is
16.2 /•<• ; the diameters are not given. The variation from the average
length of the heads of trichomes from the region of the veins is 31.5 per
cent; the variation in diameter is 60 per cent. The variation in length of
heads from trichomes which were from between the veins is 0.9 per cent.

TABLK 1 1. — Measurements of short secreting trichomes from old leaves,

Juglans nigra.

Dorsal surface.

Ventral surface.

Length of

Length of

Diameter

Length of

Length of

Diameter

trichome.

head.

of head.

trichome.

head.

ot head.

M

M

M

M .

M

M

27-3

'4-7

25.2

27-3

14.7

27-3

27-3

12.6

25.2

23-'

12.6

21 .0

*50-4

23.0

3' -5

25 . 2

12.6

25.2

37-8

21 .O

33-6

25.2

12.6

25.2

33.6

l6.8

27-3

25.2

12.6

21 .0

33-6

12.6

21 .O

*3I-5

*2I.O

*33-6

25.2

12.6

21 .0

27-3

12.6

12.6

25.2

11.5

18.9

*Vein.

The average length of heads from the dorsal surface is 16. 8 M; the aver-
age length of heads of trichomes from the ventral surface, including the
abnormally large one, is 13.4 M; omitting that one, the length is 12.5 P-.
The average length of the trichomes on the dorsal surface is 35.0 /*; the
average length on the ventral surface is 26.1 /*. The variation in length
of the heads from the dorsal surface is 20 per cent; from the ventral sur-
face 62 per cent.

From the study of the short secreting trichome it appears, therefore,
that trichomes from the regions of the veins in the young leaves have
longer heads than those from the areas between them, and that the length
of the heads of trichomes from the dorsal surface of old leaves is greater
than from the ventral surface. The entire trichomes also are longer on
the dorsal surface; and the heads of trichomes of young leaves are greater
than the length in old leaves.

JUGLANS REGIA.

The types of trichomes which were seen in the two species ot Juglans as
above described were observed in Juglans regia also. These are, (l) the
awn-shaped trichome; (2) the disk-shaped trichome; (3) one or perhaps
two types of long secreting trichomes, and (4) the short secreting tri-
chomes (fig. 8).

PURE SPECIES AND HYBRIDS OF JUGLANS. 31

The awn-shaped trichomes occur on the ventral surface of the leaves
only and are especially abundant on young leaves. They sometimes are
found in .groups, especially in young leaves, but in older ones they usually
occur singly; they measure 126 f-, more or less, in length.

As distinguished from the awn-shaped trichome, which is unicellular
and non-secreting, the other forms are glandular and are multicellular.
The mature disk-shaped trichome consists of a 2-celled stalk and a flat-
tened head, 1 cell in thickness, of about 32 cells. The youngest stages in
development were not seen. After the head has been differentiated and
divided into 4 similar cells, each quadrant becomes divided unequally by
radial walls, so that a head of 8 cells results (fig. 8, r). Later each octant
is divided by walls which extend outwards from the newly-formed octant
walls and a head of 16 cells is the result. The actual formation of walls
subsequently was not observed. Usually the cell-division in the head
proceeds in a regular manner, so that the product is symmetrical, but a
single monstrous trichome was seen in which this had not been the case.
The history of this trichome, naturally, could not be studied; it is shown
at i, fig. 8.

Measurements on the disk-shaped triehomes show that they vary both
in diameter and in depth in a regular manner, which is to be related to the
relative age of the leaf where found. Following' are given measurements,
in /*, on the diameter and the depth of disk-shaped trichomes on young
leaves: diameter, 105.0, 109.2, 105.0; depth, 8.4, 8.4, 12.6, 16.8, respec-
tively. The trichomes are not so abundant on old leaves and, as the fol-
lowing figures (/A) show, they are also of less diameter; the depth is not
given: diameter, 58.8, 50.4. 46.2, 46.2. 58.8, 46.6, 42.0, 42.0, 50.4, 58.8,
averaging 45.0.

The long secreting trichome measures 96 M, more or less, in length.
The mature trichome consists of a head of 4 cells and a stalk of 4 cells
(fig. 8, j to ;«)• It takes its origin as a papillate projection of an epider-
mal cell which is early cut off from the epidermis by a wall parallel to it.
The young trichome next undergoes transverse division, by which the por-
tion which is to become the head is differentiated from the portion which is
to be the stalk. The second division and the third division were not seen,
but are probably longitudinal in the head rudiment and transverse in the
stalk rudiments, respectively, as in the long secreting trichomes of the pure
species, as well as hybrids, where the sequence has been carefully followed.

Either following the divisions by which the head becomes 4-celled, or
part passu with these, the two final divisions of the stalk take place. Of
these divisions that of the outer cell occurs first. When the trichome is
mature the head is relatively long, but the material at hand was not favor-
able for a comparative study of the long secreting trichome, so that a more
precise statement can not at present be made. It was almost entirely
absent from old leaves and not very abundant on the young ones. The

32

HEREDITY AS ILLUSTRATED BY TRICHOMES.

FIG. 8— Tricbomes of Jnulunx rei/ia: a, awn-shaped trichomes (x 84): b to h, stages In the de-
velopment of disk-shaped tricbomesj ( 535): /, abnormal disk-shaped trlchome ( X 1UOO);
.;' to;m, long secreting trlchonies (.;' and A 1200: / and in 5:te): n to )•, development of
short secreting trichomes, ol which r Is a transverse view of </ (n, o, and p X 1200; q and
>• <5S5): .1 to r, short secreting trichomes from rather old leaf (;< 535); w, short secreting
trichomes from growing tip of stem ( < 535). (All figures reduced one-fourth.)

PURE SPECIES AND HYBRIDS OF JUG LANS.

33

second form of long secreting trichome, which was found in Juglans nigra
especially, was not surely determined for this species; if, indeed, it occurs
vn. fiiglans regia at all it is rare.

The short secreting trichome is to be found on both surfaces of the
leaves, both old and young;, although it is more abundant on the latter.
Only two of the young stages in development of the short trichome were
observed. After separation from the epidermis the next succeeding cell-
division cuts of the head-cell from the stalk-cell. When mature the tri-
chome consists of 2 stalk-cells and 4 head-cells (shown in fig. 8, n to w).

The short secreting trichome shows a considerable variation in size which
appears to be directly associated with the position occupied by it. The
variation is especially noticeable in the length of the head. Table 12
illustrates the range of variation in length of head in young and old leaves
and in stem.

TABLE Yl. — Measurements of short secreting trichomes,
Juglans regia. — Length of head.

Young Young
leaf. stem.

Old

leaf.

,

/*

M

29.4

21 .O

16.8

27.4

21.0 8.4

29.4

29.4 16.8

29.4

29.4

16.8

29.4

12.6

25.2

25.2

The average length of the trichomes from the dorsal surface is 29.2 /*,
from the ventral surface, 25.2 /*. The length of the heads of dorsally
placed trichomes is also slightly longer than those of the ventrally placed
ones; those of the former average 14.3 /«• and those of the latter is 13.6 /«•
(table 13).

TAIU.F-: l:;. -Measurements on short secreting trichomes, Juglans regia.

Dorsal surface.

V

Length of
trichome.

entral surface.

Length of
trichome.

ft

28.4

23-'
33-6
29.4

3" -5
29.4

Length of Diameter
head. of head.

Length of
head.

Diameter
of head.

f i"
16.8 27.3
14.7 25.2
'4-7 23.1
12.6 23.0
14-7 25.2
12.6 23.1

M
28 4
25.2
25.2
25.2
23.1
25.2

|U

14.7

12.6
12.6
12.6

'4-7
14.7

M

21 .0
21 .0
21 .O
25.2
25.2
21 .O

34 HEREDITY AS ILLUSTRATED BY TRICHOMES.

GENERAL COMPARISON OF THE LEAVES AND THE TRICHOMES
IN PURE SPECIES OF JUGLANS.

In leaf-characters the pure species of Juglans are fairly sharply separated
from one another. For example, in number of leaflets regia has less than
one-half those of either of the other species, and the range in number of
leaflets in calif arnica is considerably less than the range in nigra. In form
of the leaflets the species are not so sharply distinguishable. • The leaf-
character, however, is a good one to observe in the hybrids between these
species.

Although somewhat difficult in all cases to speak with certainty, with a
single exception it appears that all of the types of trichomes occurring in
any one species may be found in all, and, further, that the leading points
in structure and development are the same wherever the trichomc is to be
found. Four types of trichomes occur in californica and regia and an ad-
ditional type in nigra. Of these, one form is unicellular and lifeless, the
others are multicellular and glandular. So far as could be determined
from the material at hand, each type of multicellular trichome appears to
have its peculiar manner of development, as suggested above, to which it
adheres with great consistency. After the trichome rudiment has been
formed the divisions of the short secreting trichome were, first, a cross-
wall separating the head from the stalk; second, a longitudinal wall in the
head; third, a cross-wall in the stalk; and fourth and fifth, two longitudinal
divisions of the head. The long secreting trichome experiences the same
sequence of cell-divisions and to them adds 2 in the stalk. The sequence
of division of the disk-shaped trichome is the same for the first two divisions,
after which it divides in another manner, which is thought to be, although
not actually proved to be, also consistent. As a result of the divisions the
heads of the various trichomes become 4, 8, and probably 32 celled.

Although abnormalities were observed they were not always intermediate
between any of the types, and in fact no intermediates were noticed.

The multicellular trichomes varied in size to a considerable degree, but
the variation was fairly consistent and was usually to be related to position
on the leaf or to age of the leaf. The trichomes and the heads of trichomes
also were each longer on the dorsal surface than on the ventral, on old
than on young leaves, and on the veins than between them. Nothing was
observed on the changes in size of the trichome-heads accompanying their
functional activity, if such occurs, and the variation is probably to be ex-
plained on other grounds.

The heads of the trichomes in the pure species Jnglans californica are
markedly longer than those of either of the other species; those of Juglans
nigra are shortest. The differences in length lie from 50 to 75 per cent,
so that this is a very good character to study in any cross in which fuglans
californica is one of the parents.

HYBRIDS OF JUGLANS. 35

HYBRIDS OF JUGLANS.

LEAVES OF THE HYBRIDS.
JUGLANS CALIFORNICA X JUGLANS NIGRA.

The walnut hybrid of this parentage was formed by Mr. Burbank in
1878 and is known as the "Royal." One specimen of the first generation,
which is the sole representative of this generation, is now growing at the
Sebastopol ranch of Mr. Burbank. It is a very handsome tree, about 20
meters high, with a wide-spreading top. Some second and some third
generation seedlings, the latter known as the "Beeson" walnut, are grow-
ing near the Royal at Sebastopol.

The Royal is distinguished by its rapid growth , by the fine grain of the
wood, and by the relatively wide annual rings of growth. The second
and third generation seedlings are extremely variable. This is noted in
size, in vigor of the plants, and in the color, size, texture, and other char-
acters of the leaves. The Royal fruits abundantly; the fruiting character
of the later generations is not known.

The leaves are composed of from 5 to 9 or more pairs of leaflets, which
may be strictly opposite on the rachis or may be more or less in alternation
(plate 4). The terminal leaflet is usually smaller than the other leaflets,
although it may also be larger than they are. It is generally single, but
occasionally a doubling is to be seen. In form the leaflets vary from lan-
ceolate, with a somewhat attentuate apex, to broadly ovate with an apex
which is acuminate. The bases may be either abrupt or even cordate. The
leaflets are serrate and exhibit all grades between fine and coarse serration.
The surface of the leaves is either smooth or roughened, the veins of the
dorsal surface may be either sunken or may be raised — generally the latter.
As a whole, any type of leaflet, or quality appertaining to a leaflet, holds
throughout all the other leaflets of the leaf.

So far as the general observations on the leaves of the three generations
of the walnut hybrid go, the leaves show some grade of intermediacy. No
leaf was seen which was not distinguishable from the leaves of either of
the pure race, and all of the leaves examined showed characters of both.
How far this remark will be good when each second or third generation
plant is studied separately is not known.

JUGLANS CALIFORNICA x JUGLANS REGIA.

This walnut hybrid, known as the "Paradox," originated from a cross
made by Mr. Burbank in 1887. Four trees of the first generation are at
present growing in the street in front of the experimental tract of Mr.
Burbank in Santa Rosa. Seedlings of the second (and third) generation
are at the Sebastopol ranch. The first-generation trees are about 28
meters high and about 25 meters spread. The bole of one of these trees,
at a point 1.5 meters above the surface of the ground, measured 30 cm. in

36

HEREDITY AS ILLUSTRATED BY TRICHOMES.

diameter. Like the other walnut hybrid (the Royal) this is a rapid grower;
the wood is fine-grained, and the annual rings are relatively wide. The
Paradox, unlike the Royal, however, bears little fruit.

The leaves of the first generation are extremely variable in number of
leaflets, in size, and in many other particulars. Leaves with 3, 4, 5, 6, and
7 pairs of leaflets were selected for photographing (plate 7).

The number of pairs of leaflets are said by Mr. Burbank -to reach as
great a number as 19, though I did not see leaves with so great a number.
Measurements on the extreme variation of the leaves were not made, but
observation would indicate that the range may easily exceed 500 per cent.

The leaflets vary in outline from ovate to broadly ovate. The apices
are acuminate or abrupt; sometimes they taper much as in the pure species
Juglans calif arnica. The bases are abrupt or even cordate; none are broadly
cuneate, as in pure species referred to. The leaf-margins are remotely
serrate. In all specimens examined the veins of the dorsal surface are
prominent. The leaf-surfaces are usually smooth, although they may also
be roughened. Thus each leaflet shows characters of both pure lines.
Whether a similar condition will be seen to obtain in the later generations
can not at present be stated.

The great tendency of the leaves of the Paradox hybrid to vary has been
remarked by Mr. Burbank, who tells me that in company of two eminent
scientists he selected 400 leaves or leaflets easily recognizable as different
from one another.

TABLE 14. — Leaves of Juglans californica X Juglans nigra, FH compared with
those of the pure species.

J. nigra.

J.c;

Leaves with 13 to 21 leaf-
lets
Leaflets ovate

Leave
let
Leafl

Leaflets, apex attenuate ....
Leaflets, base abrupt

br
Leafl

Leaflf

CO

J. californica X nigra.

J. californica.

Leaves with u to 19 leaf- Leaves with 19 to 21

lets. leaflets.

Leaflets lanceolate to ' Leaflets narrowly ovate.

broadly ovate.

Leaflets, apex acuminate Leaflets, apex tapers

gradually.

Leaflets, base abrupt or Leaflets, base cordate.

TABLE 15. — Leaves of Juglans californica X Juglans regia, Fv compared with
those of the pure species.

J. regia.

J. californica X regia.

J. californica.

Leaves with 5 to 7 leaf-
lets.
Leaflets broadly ovate

Leaflets, apex abrupt

Leaflets, base gradually
tapering.

Leaves with 7 to 15 leaf-
lets.
Leaflets ovate to broadly
ovate.
Leaflets, apex abrupt, or
acuminate.
Leaflets, base abrupt, or
cordate.

Leaves with 19 to 21
leaflets.
Leaflets narrowly ovate.

Leaflets, apex tapers
gradually.
Leaflets, base cordate.

CANNON

PLATE 4

Leaves of Juglans californica X Juglans nigra, FI, from Luther Burbank's Sebastopol ranch, to
illustrate the range in variation. One-third natural size.

CANNON

Leaves of Juglans californica X Juglans nigra, p2, to show extremes in variation of leaves and
leaflets. From Luther Burbank's Sebastopol, California, ranch. The figures are reduced
equally. One-third natural size.

HYBRIDS OF JUGLANS. 37

TRICHOMES OF JUGLANS CALIFORNICA x JUGLANS NIGRA, F,.

Four types of trie-homes occur on the leaves of this hybrid. These are
(l) the awn-shaped trichomc, (2) the disk-shaped trichome, (3) the long
secreting trichomc (of two sorts), and (4) the short secreting trichome.
The awn-shaped trichomes are unicellular and usually occur singly; they
are found upon the ventral surface only of the leaves and measure 211 /«•
more or less in length. While this type of trichome is especially abundant
in young leaves, it persists to a degree, so that it may be found in mature
leaves as well.

As in the pure species, the disk-shaped trichomes are composed of a
short stalk and a broadly expanded head, of which the center is depressed
(fig. 9). The supporting stalk is 2-cclled; the head is of many, perhaps
always of 32 cells, but is only 1 cell in thickness. The origin and develop-
ment of the trichome were not shown satisfactorily in the material at hand,
so that a description of these processes in hybrids of the first generation
can not be given. Measurements, in /*, on the diameter of the trichome
and on the depth of the depression of the head were made as follows:
diameter, 107.1, 109.2, 105.0, 100.8, 92.4, 92.2; depth, 8.4, 8.4, 18.9,
16.8, 21.0, 12.6, respectively.

The longer secreting trichomes measure 126 M more or less in length
and are of two types, which appear to be distinct. The more usual form
consists of a stalk of 4 cells and a head of 4 cells which are radiate. The
other form is composed of a stalk of a varying number of cells, usually
more than 4, and a flat expanded head of about 8 cells. Although both
forms occur on both surfaces of the same leaf, the latter is to be found
chiefly, perhaps, on the veins. The more common form will be described
first. After the young trichome has been cut off from the epidermis a
transverse wall appears which separates the head portion from the part
which will become the stalk. The next division of the trichome is also a
transverse one and occurs in the stalk-cell. The third division takes place
in the head-cell and is a longitudinal one. What the order of cell-divisions
was after this was not learned. The heads of the mature trichomes are
composed of rather long cells, so that in effect the head is neither like the
head of the long secreting trichome in Juglans nigra, nor like that in
Jiig/ans ca/ifornica, but holds an intermediate position. The following'
measurements, in/*, were made on the heads of mature trichomes: length,
21.0, 25.2, 25.2, 21.0, 25.2, 21.0, 33.6, 23.1, 21.0, 25.2, 23.1, 25.2. The
average of these is 24.1 /*.

The early stages in the development of the second type of long secreting
trichome, that with a head of 8 cells, were for the most part not seen, but
certain peculiarities in the cell -divisions of the head may be recorded. In
transverse sections of the heads of all of the other trichomes studied the

38

HEREDITY AS ILLUSTRATED BY TRICHOMES.

JfiO.g.—ftlehomefotJugtanteaHfornlea •: nir/m, Fj: a, awn-shaped trichome from a leaf; 6 to d,
dlsk-stiaped trichomes ; e to i, some stages in the development of long secreting trichomes :
.;', transverse section of head of mature long secreting trichome ; k to HI, long secreting tri-
chomes ; n to r, long secreting trichomes of the type having a head of more than 4 cells:
n, longitudinal sections of the head of a single trichome; o to </, views of long secreting tri-
chomes ; r, transverse sections of head of long secreting trichomes of different ages ; .», some
stages in;the development of short secreting trichomes. (All figures 535; reduced one-
fourth.)

HYBRIDS OF JUG LANS.

39

first three cell-walls are so arranged that a -(--shaped figure results. In
this type of the long secreting trichome, however, in which the 4, 5, and
6 cell stages of the head were seen, the arrangement of the walls is quite
different and indicates that there may have been a very different sequence
in the laying down of the walls from what ordinarily occurs. As nuclear
division was not seen in the heads, however, what the sequence of cell-
division was can not at present be told. In length of heads this type of
long secreting trichome is not different from the type having a head of 4
cells. The following measurements, in p-, were made: length, 21.0, 21.0,
25.2, 29.2. The diameter of such heads, however, is usually considerably
greater.

The short secreting trichomes were found on both surfaces of all leaves
but the oldest where they were apparently absent from the ventral side.
In length they range from 35.7 p to 54.6 /* and the size is evidently to be
associated either with position on the leaf or with its age. The heads also
of the trfchomes in both pure species vary greatly in length, and here,
also, the variation is probably in some manner connected with the position
occupied on the leaf, or with the condition of the leaf, but when taken into
consideration does not vitiate a comparison of similar organs which occur
under analogous conditions. The material studied did not permit an obser-
vation of the development of this type of trichome, so that for the present
a description of it must be omitted. The mature trichome consists of a
2 -celled stalk and a head of 4 cells placed radially. The first division of
the young trichome probably separates the stalk region from the head
region, as in both the pure lines, and the divisions of the head are longi-
tudinal. Measurements on the short secreting trichomes are given in
table 16.

TABLE Hi — Measurements on the short secreting trichomes of Juglans californica

X Juglans nigra, F^

Trichomes from veins
of young leaves.

Trichomes from dorsal sur-
face of old leaves. t

Length

Diameter Length of

Length Diameter

of heads.*

of heads, trichomes.

of heads, of heads.

21.0

21.0 ! 37.8

16.8 25.2

16.8

25-2 35-7

18.9 25.2

21 .0

25.2 39.9

18.9 23.1

21 .O

25.2 42.0

21.0 25.2

18.9

25.2 42.0

23.1 25.2

21 .O

27.3 J54.6

25.2 29.4

16.8

25.2

21 .O

25.2

21 .O

25.2

*The average length of these heads is 19.8 M; the average in diameter is 25.1
tThis type of trichome is absent from the ventral surface of old leaves.
IVein.

40

HEREDITY AS ILLUSTRATED BY TRICHOMES.

The average length of the heads of trichomes of old leaves is 20.6 p;
the average diameter is 25.5 M. Thus the lengths of heads in the old leaf
and in the young leaf are practically the same. In the pure species, where
the comparison is made, the young leaves were found to- have the largest
trichomes. It is possible that a comparison of trichomes from analogous
leaf-surfaces would in this instance also give similar results.

TRICHOMES OF JUGLANS CALIFORNICA X JUGLANS RF.GIA. Ft.

As in the pure species Jitglans californica and /uglans regia, so also in
the hybrid, 4 types of trichomes are to be distinguished, namely, the awn-
shaped, the disk-shaped, and the long and the short secreting trichomes.
The unusual type of long secreting trichome which was observed in the
other hybrid Juglans and in the pure species Jug/a us nigra appears not
to be present in this hybrid (fig. 10).

TAIILE 17. — Measurements on heads of long secreting trichomes o_f
Jitglans californica X Juglans regia, /•",.

Length.

Diameter.

Length.

Diameter.

M M

M

/»

21 .0

37-8

25.2

37.8

25.2

42.0

23-1

50.4

21 .0

46.2

23.1

39-9

21.0

33.6 | 21.0

37.8

The awn-shaped trichomes are most abundant in the young leaves, where
they occur on the ventral surface only. They are of the usual type, uni-
cellular, and range around 211.6 /* in length.

The disk-shaped trichomes, which in structure are quite like those in
both pure lines, consist of a flattened, multicellular head borne on a stalk
of 2 cells. In young leaves the trichomes are to be found on both sur-
faces; in older ones they occur on the ventral surface only. Mature tri-
chomes of this type vary greatly in diameter, but, so far as observed, the
differences in diameter are more or less closely associated with differences
in position on the leaves, and possibly also with the age of the leaf. The
following measurements, in /*, give the diameters of representative disk-
shaped trichomes: 71.4, 73.6, 67.2, 65.0, 75.6, 92.2, 79.8. Of these the
two last were from the region of the veins. The average diameter of the
heads of the trichomes is 74.9 M. A few measurements were made also to
trace an additional possible character— the depth of the depression — for
comparison with the pure lines. These measurements are as follows, in p:
diameter, 63.0, 84.0, 92.4, 79.8: depth, 12.6, 16.8, 12.6, 12.6, respectively.

The long secreting trichomes are most abundant on young leaves and
are more abundant on the veins than between them. They occur on both
leaf -surf aces. The trichome studied consists of a head with 4 radiate
cells and a stalk of 4 cells. None of the type with a head of 8 cells was

CANNON

PLATE 6

Leaves of Juglans californica X Juglans nigra, FS, to show the variation of the leaves and the
leaflets in size and in other qualities. From Luther Burbank's Sebastopol ranch. Two-
fifths natural size.

CANNON

PLATE 7

Leaves of Juglans californica X juglans regia, FI, from Santa Rosa, California. The "royal"
walnut of Luther Burbank. The figures of this plate, and of the following one, illustrate
the range of variation in size of leaves and in the number, size, and other qualities of the
leaflets. One-third natural size.

CANNON

PLATE 8

Leaves of Juglans califormca X Juglans regia, FI, from Luther Burbank's nursery, Santa Rosa,
California, showing the range of the variation of leaves antl leaflets. Compare with the
preceding plate. One-third natural size.

HYBRIDS OF JUGLANS.

41

seen. Although the development of this trichome was not studied par-
ticularly, the material examined indicated that the sequence of cell-divisions
may be the same as in the same type of trichome in both pure lines, and
thus it conforms with the sequence in cell-divisions in all of the other types
of multicellular trichomes in Juglans. Table 17 presents the measure-
ments made on the heads of this form of trichome. The average length
of the heads is 22.5 /*: diameter, 40.6

FIG. 10.— Trichomes of. fuglans californicn J. rei/ia, F,: <i, awn-shaped trichomes ( W); b tot-,
disk-shaped trichomes (6 and d, X SOOj; c and c, .">3o ; / and </, long secreting trichomes iff, X
585); h, transverse section of head of long secreting trichome of type with more than 4 cells in
head ( X 585); i to (/, short secreting trichomes; / ( •' 800) and.) ( < F3!>), young trichomes; k, tri-
chome from region of vein of leaf ( 535); (, trichome from dorsal leaf-surface ( < 535); m, longi-
tudinal section of short secreting trichome ( < 535): n to p, trichomes from old leaf ( ~&~>); 17,
transverse section of head of short secreting trichome ( < 8M).

The short secreting trichome is multicellular and consists of a 2 -celled
stalk and a head of 4 cells placed radially, as in both pure lines. Beyond
the first division of the young' trichome, which is a transverse one, no stages

42

HEREDITY AS ILLUSTRATED BY TRICHOMES.

in its development were seen. Measurements on the mature trichomes
show that the variation in length of the entire organ, but particularly of the
head, is in some manner associated with the position occupied by it, as has
been repeatedly noted in other forms. And, also, as in the other similar
measurements, this variation was found to be in a high degree consistent,
which is indicated by the tables of measurements (tables 18 and 19).

TABLE IS. — Length of short secreting trichomes ofjuglans californica
X Juglans regia, f,. ( Trichomes of old leaves.)

Dorsal

Ventral

Dorsal

Ventral

surface.

surface.

surface.

surface.

M

M

M

M

46.2

25.2

42.O

29.4

50.4

33-6

37-8

25.2

37.8

25.2

33 •(>

58.8

25.2

The averages in length of trichomes are: dorsal surface, 45.5^-; ventral,
28.2 p-. Similar results were obtained in another series of measurements,
which need not be given, in which the dorsally placed trichomes averaged
35. 9 M and the ventrally placed ones 28.1 p- in length.

TABLE 19. — Measurements on heads of short secreting trichomes of Juglans
californica X Juglans regia, Fr ( Trichomes of old leaves}.

Dorsal

surface.

ft

18.9

21 .0

16.8

Ventral
surface.

Dorsal
surface.

M

21 .O

16.8

16.8

Ventral
surface.

M
16.8
16.8
16.8

M
12.6

21 .O

14.7

The averages in length of the heads are: dorsal surface, 18.9 ^; ventral
surface 15.5 /*.

JUGLANS HYBRIDS-SECOND GENEKATION.

The material for the study of ihc Jitglans hybrids and their pure parents,
which has been reported on above, was collected in the spring of 1907.
This material, unless otherwise specifically noted, was of the first genera-
tion. The study was resumed in 1908 for the following purposes: It
seemed best to obtain trichomes which were undergoing nuclear division
in order to satisfactorily determine the sequence of the cell-divisions, and,
also, it seemed best to carry the study of the variation of the trichomes
somewhat further, and to learn if possible the relation between the vari-
ation of the trichomes and the variation of the plant bearing them.

In selecting the plants for study it was decided to take 2 each of the
largest and the smallest of both kinds of hybrids. The largest Juglans

HYBRIDS OF JUGLANS. 43

calif ornica X Juglans nigra, Nos. 1 and 2, were both about 1.5 meters
high; the smallest of this cross, Nos. 3 and 4, were 40 and 50 cm. high,
respectively. The largest specimens of Juglans calif ornica X Juglans regia,
Nos. A and B, were from 3 to 4 meters high; and the smallest, Nos. C and
D, were 70 and 150 cm. high, respectively. All of the second-generation
plants of either race were of the same age and were apparently growing
under similar conditions.

LEAVES OF THE HYBRIDS.

Fully developed leaves, taken from analogous portions of the plants, were
examined and the basal pair of leaflets were photographed (plates 9 and
10). Plate 9 shows the basal leaflets of 4 leaves of plant No. 4, Juglans
californica X Juglans nigra. No. 1 of the figure had 10 leaflets and was 18
cm. long; No. 2 had 11 leaflets and was 29 cm. long; No. 3 had 11 leaflets
and was 21 cm. long; and No. 4 had 11 leaflets and was 26.5 cm. in length.
Plate 9 shows, also, the basal leaflets of leaves from plant No. 1. Of the fig-
ure, No. 1 is of a leaf which was 40 cm. long and had 18 leaflets; No. 2 had
17 leaflets and was 34 cm. in length; No. 3 is of a leaf which had 17 leaflets
also and was 36 cm. long; and No. 4 is of a leaf which bore 13 leaflets
and was 25 cm. in length. Plate 10, lower figure, is of the basal leaflets of
plant No. A, Juglans californica >' Juglans regia. Of the figure, No. 1 is of
a leaf which bore 11 leaflets and was 49.5 cm. long; No. 2 is of a leaf which
bore 9 leaflets and was 51 cm. long; No. 3 is of a leaf which had 7 leaflets and
was 25cm. long; and No. 4 is of a leaf which had 11 leaflets and which was
55.5 cm. in length. Plate 10, upper figure, is of the basal leaflets of leaves
from plant No. C. No. 1 is from a leaf which bore 9 leaflets and which was
34.5 cm. long; No. 2 is of a leaf which had 7 leaflets and was 25 cm. in
length; No. 3 is of a leaf which bore 8 leaflets and measured 32.5 cm. in
length; and No. 4 is of a leaf which had 8 leaflets and which was 26 cm. in
length. The leaves and leaflets of D, which arc not shown, were very-
large, so that only two leaf-bases could be accommodated on the photo-
graphic screen at one time. One leaf of D had 12 leaflets and was 37.5 cm.
long; and another had 15 leaflets and was 66.5 em. in length.

There is thus a considerable variation in both series in size of leaves
and in number of leaflets, both as regards those of a single individual
and those from different plants of the same blood. As for other char-
acters, such as form of leaflet, shape of apex and of base, emargination,
and texture or character of surface, there is less variation between the
leaves of any individual than between the leaves from different plants.
From the examination the following generalizations seem to hold: (l)
The largest leaves have also the greatest number of leaflets; (2) the largest
leaves are most variable as regards the number of component leaflets. The
converse of these is true for the smaller leaves. Except for plant D, also,
the largest leaves are borne upon the largest plants; that is, the leaves ap-
pear to be an index of the vigor of the plants. The number of leaflets of

44 HEREDITY AS ILLUSTRATED BY TRICHOMES.

the leaves of both series does not indicate distinct reversion to pure lines
in any instance. In Juglans calif ornica X Juglans nigra, in plant No. 1,
the leaflets numbered from 13 to 18; in No. 4, from 10 to 11. In the pure
line calif ornica the leaflets numbered from 19 to 21, and in nigra from 13
tc 21. It therefore appears that the hybrid No. 4, which is small in size,
has a smaller number of leaflets than were seen in any leaf of either pure
line. Injiiglans calif ornica X Juglans regia, on the other hand, in num-
ber the leaflets are intermediate, since in A they range from 7 to 11, and
in C from 7 to 9, while in regia the range is from 5 to 7. It is, therefore,
concluded, as regards the plants which were especially studied of both
series, that they do not show reversion in any gross leaf-character to pure
lines in any instance.

TRICHOMES OF THE HYBRIDS.

The material for study was found to be especially favorable for the study
of the embryogeny of the trichomes, and, accordingly, an account will be
given here of the origin and the development of the leading types. It
should be stated at the outset that the cell-divisions were seen in numbers
sufficiently large to point to the soundness of the conclusions based on
this phase of the investigation.

Five forms of trichomes were found in the hybrid Juglans calif ornica >
Juglans nigra and 4 in the other hybrid. The type not common to both
is the long secreting trichome already noted as occurring in Juglans nigra
and in the first-generation hybrid with nigra blood. In addition to these
trichomes, which do not require any additional description here, 3 or 4
abnormal types were seen, all but one of which had already been noted.
These will be described below. vSince the development of any type of tri-
chome adheres to its peculiar pattern in whichever strain it is found, unless
especially stated to the contrary, the subjoined descriptions apply to both
lines.

The disk-shaped trichome takes its origin as a squat projection of an
epidermal cell and early undergoes transverse division. The first division
of the unicellular trichome thus formed is a transverse one by which the
portion which is to become the head is separated from the portion to
become the supporting stalk. The second division is a longitudinal one in
the end-cell. This sequence was observed without exception in the disk-
shaped trichome, and is the sequence of the first two cell-divisions in all
of the other multicellular trichomes, save only a single aberrant type which
will be mentioned below. In all cases examined the next divisions occur
in the head, which appears to become 4-celled at least prior to the trans-
verse cell-division which completes the divisions of the stalk ; and tri-
chomes were observed with the fifth and sixth head-cells forming, and one
with a head of 8 cells without the final division of the stalk-cell. On the
other hand, trichomes were seen which had the stalk fully developed,

CANNON

PLATE 9

L^iftll

*

Basal leaflets of leaves of Juglans califomica X Juglans nigra, p2, in plants " 4 " and " 1 ".
One-half natural size. Further explanation in the text

CANNON

PLATE 10

Basal leaflets of leaves of Juglans califomica X Juglans regia, FZ, of plants " C " and "A".
One-half natural size. Further explanation is given in the text.

HYBRIDS OF JUGLANS.

45

although the head consisted of 6 cells only. It is thought that there may
be a relation between the large number of cells of which the mature tri-
chome is composed and the lack of consistency in the sequence of its cell-
divisions. The head of the mature disk-shaped trichome is composed of
at least 32 cells.

PiQ. 11. — Trichomes of Juglans californica X -7. niyra, F-2, plant No. 1: a, figures
showing the cutting off of the young trichome from the epidermis of the
leaf; 6, first cell-division of trichome: c, second or longitudinal division of tri-
chome. (All figures X 1200.)

As opposed to the want of uniformity in the cell -division sequence shown
in the disk-shaped trichome, the divisions in the short and the long
secreting- trichomes follow a perfectly consistent sequence throughout.
After the short secreting trichome is cut off from the epidermis it under-
goes division by a transverse wall into a terminal cell, which will give rise
to the head, and a basal portion, the stalk. The second division occurs in
the end-cell and is a longitudinal one. The third division of the trichome
is a transverse one in the stalk, by which the divisions in the stalk region
are completed. The fourth and the fifth cell-divisions are longitudinal

46

HEREDITY AS ILLUSTRATED BY TRICHOMES.

ones in the head-cells. The trichome then consists of a head of 4 cells,
radially disposed, and a stalk of 2 cells, and cell-division ceases. Although
numerous examples of each stage in the development of the trichome were
seen, no exception to the sequence as given was noted.

The long secreting trichomu in Jitglans calif arnica X Juglans nigra is of
two sorts. One type has a stalk of 4 cells and a head of 4 cells, and the

FIG. 12. — Long and short secreting trichomes of Juytans cali/tn-nica X Juglans njV/ra, Fj, plant 1:
a, third cell-division Of trichome; b, 4-cellcd trichome, of which only 3 cells are shown; c, fourth
cell-division of trichome; cl, fifth cell-division shown in transverse and longitudinal sec-
tions. (All figures < 1200.)

other type has a stalk of about 8 cells and a head of about 8 cells. The
development of the first kind is as follows: The first cell-division of the
young trichome is transverse, by which the portion which is to become
the head is differentiated from the portion which is to become the support-
ing stalk. The second division is longitudinal in the head, the third is

HYBRIDS OF JUGLANS.

47

transverse in the stalk, the fourth and fifth are longitudinal in the head.
To this point the sequence of divisions of the short secreting and long
secreting trichomes is the same. The sixth division occurs in the upper
stalk-cell and is transverse, and the seventh and last cell-division is a cross-
wall in the lower stalk-cell. The successive divisions are illustrated by the
accompanying figures, which also indicate the sequence of cell-divisions
in the short secreting trichome.

The results of the study on the second type of long secreting trichome,
while not entirely satisfactory, indicate that it is a fundamentally different
form than the type with a smaller number of cells. The youngest stages
of this trichome either were not seen or could not be identified as belong-

FIQ. 13. — Trichomes of Juglanx califoniica X JuglHtm tiigra, K2, plant 1: a tor, long
secreting trichomes, showing sequence of cell-division of stalk; tl to /, some
stages in development of short secreting trichomes. «<toe, y 1200 ; c, .840; (/to
/, X 1200. Reduced one-fourth.)

ing surely to the trichome. A trichome with 5 cells, of which 2 were of
the head, had undergone division of the lower of the 2 stalk-cells. An-
other trichome having a head of 4 cells had just experienced the division
of the lower stalk-cell. That there is some irregularity in the sequence of
division is shown by another trichome in which the stalk consists of 4 cells,
although the third cell-division of the head is just taking place. The cell-

48

HEREDITY AS ILLUSTRATED BY TRICHOMES.

divisions of the head and of the stalk appear to take place independently,
which is especially and perhaps solely true of rather old trichomes. For
example, a trichome was seen which had a head with 7 nuclei, but a stalk
with the fifth cell just being- cut off, and another trichome was seen which
was forming the fifth or sixth cell in the head, although the stalk con-
sisted of 9 cells.

FIG. 14. — Disk-shaped triehomesof Jurftan * c<itifnrnica *. Jttglans itigra, F2, plant 1: a, trichome
with head consisting of 4 cells, of which 2 are in the process of dividing, and a 1-celled stalk; b,
trichomes with head of <i or 7 cells, and a. stalk of 1 cell which is undergoing division; c, tri-
chome with 1-cellert stalk and 4-ceiled hend; il, trichome with '2-celled stalk; e, transverse sec-
tion of head to show the formation of octants; /, trichome with a head of 7 or 8 cells and a
1-celled stalk. (All figures 1200.)

Additional evidence as to irregularity in the number of cells making up
the larger type of long secreting trichome was observed in the unusually
large number of head-cells. A trichome was seen, and is shown in fig. 16,
which had a head of 14 cells, although there were only 6 cells in the stalk.

HYBRIDS OF JUGLANS.

49

The reasons for this curious behavior are not apparent, but an explanation
may possibly be found in the following facts: The trichome in question was
seen only on the veins and is the last of the types to appear, that is, the
older leaves only showed its presence, although exactly when in the de-
velopment of the leaf this type was formed was not learned. Again, the

Fro. 1.3.— Tricliomes of Jug/tins culiftirnica < Juglans nlgra, F 3, plant 4. (Unless otherwise
stated the trichomes are of the type which has a head of more than 4 cells.) n, third cell-
division of trirhome; b, long secreting trichome undergoing the sixth cell-division; c,
second division of the stalk, which thus occurs in different sequence than in the regular
type; d, same stage as before; e, 2 trichomes, showing stalk of 4 cells and the third division
of the head ; /, trichome with a head of 8 cells, of which 3 were seen In the adjoining sec-
tion, and the tifth stalk-cell in the process of being cut off; fft abnormal trichome of long
secreting type. (All figures x 1200.)

great irregularity may be owing to the hybrid nature of the plant, as no
such inconsistencies were observed in the pure strain nlgra. Or, the fact
that the number of cells composing the trichome is relatively large may be
a factor which should be considered in this connection. This irregularity
is all the more striking because of the perfect consistence in the sequence
of cell-divisions which was observed in the smaller type of long secreting
trichome and in the short secreting trichome as well.

50

HEREDITY AS ILLUSTRATED BY TRICHOMES.

Aberrant and abnormal types of trichomes were seen in both lines of
hybrids. These either were modifications of existing trichome forms or
quite new types. Disk-shaped trichomes with more than 2 cells in the
stalk, and short secreting trichomes with stalks with 3 in place of 2 cells
were observed. The first abnormality noted was seen also in the pure
parents, nigra and regia. The abnormal short secreting' trichome has been
noticed only in the hybrid Juglans calif arnica X Jnglans nigra plants num-
bered 1 and 2. Abnormal long secreting trichomes were seen, as above
indicated, but none which were certainly of the smaller form of this trichome.

FlG. 1H. — Triohome of Jnglanx rtttifornica V Jiti/latif uitiru, I'\, plant 1.
Abnormal form of type of trichome which normally has a head of .s
and astsilk of S cells, (x 1200).

An additional type of the abnormal trichome, the
irregularity of which is not dependent on the modifica-
tion of a form now prevalent, was observed several
times in [uhlans calif arnica X Juglanx nigra and once
in the other hybrid. This trichome consists of a cell-
complex in which the customary supporting stalk is
' wanting; the trichome is made up of 3 tiers of cells, of
which each tier has 2 (fig. 18). Thus there are 6 cells
in the trichome, of which 2 are terminal, 2 basal, and 2 median. This
trichome arises as follows: After the young organ is separated by a cross-
wall from the epidermis it undergoes longitudinal division (fig. 18, «), in
place of a transverse wall, as commonly is the case. The circumstance that
the first wall is longitudinal necessarily affects all subsequent cell-divisions
in such a manner as to bring about the origin of a new form of trichome.
It is of interest to note that the trichome is not a modification of an exist-
ing type, but that it appears suddenly and owes its existence to the unique
placing of the first wall. It is of interest to note further that should experi-
mentation show that each type of trichome is a unit character, we have
here the origin of a character through mutation, in place of through gradual
change, as may be the case with certain of the other trichomes whose
origin and development have been reported on in this paper.

HYBRIDS OF JUG LANS.

51

Flu. 17. — Short secreting trichomes of Jtiylatixrali/vrnica -; Jut/lanx rr.yin, F^, plant C : a to <•, origin
and growth of trichome previous to separation from epidermis « 840);; d and c, separation of
tricliome ruiliment from epidermis ( • 1200); /, 1-celled stage (X 840);. </, first divison of triohorne
( : 12*1); li, 2-celled trlcliome( X40); /.second division of trichome; .y, ;i-eelled tricbome (> 1200);
/., third cell-division ( 1200); I, transverse section of leaf, showing long and short secreting tri-
chomes in place.

52

HEREDITY AS ILLUSTRATED BY TRICHOMES.

MEASUREMENTS ON THE TRICHOMES.

The measurements which were made on the multicellular trichomes of
the pure lines and first generation of fuglans hybrids indicate, as above
shown, considerable variation in size, but comparative study showed also
that the causal relations attending- the variations might be traced, or at
least surmised. Thus the variations were seen to be associated with the
age of leaf or with the position occupied by the trichomes on the leaf.
These conditions were shown to hold good for all the plants brought under

observation . It followed,
therefore, when these facts
were taken into account, that
comparison between closely
related forms was possible.
As the studies reported on
up to page 42 were made in
two successive seasons, 1906
and 1907, there remained the
possibility that variations in-
duced by seasonal differences
might in some degree alter
the measurements as given
in the earlier studies by modi-
fying the development of the
hybrids whose ancestors va-
ried in climatic experience as
well as in blood. There also
remained an instance of the
behavior of trichomes in re-
version to the pure lines to
be observed, since no hybrid
examined during the earlier

FIG. 18.— Abnormal and aberrant forms of trirhomes of
Juglans califvrnica .•; Jtigttini niyra, F2, plant 1: a,
young, and b and c mature stages of a new type of
trichome (the first cell-division of the trichome is lon-
gitudinal in place of being transverse, as is the usual

position); (/ and e, abnormal trichomes. (a,£>,and c,
<120»; d, XS40; i; '/.. 535.)

course of this study exhibited
this phenomenon. The meas-
urements to be given presently, therefore, are calculated especially to
show two things, namely, (l) whether the trichomes exhibit seasonal
variation in size, and (2) whether ancestral characteristics relating to
size of trichomes reappear in the second hybrid generation.

JUGLANS CALIFORNICA X JUGLANS NIGRA, SECOND GENERATION.

Of the several forms of multicellular trichomes, measurements were made
on the disk-shaped and on short secreting types only, and of these the
latter offered most favorable material and was mostly used.

The following measurements, in /«•, were derived from disk-shaped tri-
chomes taken from the largest and from the smallest of the hybrids. They

HYBRIDS Ol: JUOLANS. 53

are of diameters only; it was found not practicable to make a special
study of the depth of the head. Disk-shaped trichomes from No. 4,
diameters of heads: 79.8, 84, 84, 79.8, 92.4, 100.8, 71.4; the average
diameter is 84.6. The following are the diameters of disk-shaped tri-
chomes from No. 1: 84, 79.8, 79.8, 84, 84, 79.8, 79.8, 79.8, 71.4, 84, 79.8,
71.4; the average is 79.8. The disk-shaped trichomes from the smaller
plant, No. 4, average larger than those from the larger one, No. 1. In
both instances the trichomes run smaller than in the first generation
(101.1 /J-), and also smaller than in the pure species Juglans calif arnica
( 107 /*) , and appear to be comparable to the trichomes of the pure species
Jiig/ans nigra; that is, there appears in this case to be a nigra reversion.
The short secreting trichomes were most numerous, so that accordingly
the measurements on them were most complete. Both of the hybrids
from which the disk-shaped trichomes just reported on were taken were
examined as regards the short secreting trichomes. On the trichomes the
following series of measurements were taken: (l) length of entire organ,
(2) length, and (3) diameter of the head. The relative age of the leaf
and the position occupied on it are both recorded in table 20.

TABLK 20. — Measurements on short secreting trichomes, hybrid No. /.*

Dorsal surfac

e.

Ven

tral surface

Entire Length

Diameter ; Entire

Length

Diameter

length. of head.

of head.

length.

of head.

of head.

n

M

M

/*

M

M

36.6 18.8

29.28

20.13

12. 8l

20. 1 3

4l.24(v) 21.96

27-45

31. n(v)

14.64

20.13

28.62 '6.47

20.13

27.45

16.47

21 .96

30. 10 16.47

18.30

27-45

16.47

23.79

34.77 18.30

25.62

2 I . 96

12. 8l

lS.30

32.94 18.30

23-79

32-94(v)

21 .96

2 I . 96

31.11

20.13

21 .96

29.28(V)

18.30

23-79

32-94

2O. 13

21 .96

23-79

12. 8l

18.30

29.28

16.47

2[ .04

29.28(V)

14.64

21 .96

36.6 (v)

25.62

29.28

31. ii (v)

16.47

25.62

29.28(V)

14.64

21 .96

23-79(v)

10.98

21 .96

23-79(v>

14.64

21 .96

*In this and subsequent tables (v) refers to the location on a vein; where the
letter is not given the trichome is understood to be placed between the veins.

The averages of these measurements are as follows: Dorsal surface, be-
tween veins, entire length, 32 /*; length of head, 18 /*; diameter of head,
24.7 /*. Ventral surface, between veins, entire length, 24 /*•; length of
head, 14.2; diameter of head, 20.5 ^; on veins, entire length, 28.7 /"•; length
of head, 15.9 /*; diameter of head, 22.5 /*.

The leaves of both series of hybrids were not entirely mature; they were
in each instance 84 /* in thickness.

54

HKKKIHTY AS II.LUSTKATHD liY TKICHOMICS.

Another study on the dimensions of the triehomes of No. 4, as will ap-
pear directly, gave somewhat different results. The leaves were of the
same thickness as those reported on above. The measurements, of which
there were 60, may be disregarded; the results are as follows: Dorsal sur-
face, entire length, 31.47 /•<•, length of head 16.5j«, diameter of head 21.6 /«•;
ventral surface, entire length, 29.2 /"•; length of head 16.2 /*, diameter of
head 22.6 M. Thus the length of the triehomes from the dorsal surface is
about the same in both cases, but the size of the heads is in the latter
instance somewhat greater. The dimensions of triehomes from the ventral
surface of the former are nearly the same as those from between the. veins
of those last given. In each instance the same relation of length to
diameter of head is almost exactly held.

TABLE L'l. — Afcasiirei/icnls OH short secreting trichoincs, hybrid A'o. 4.

Dorsal surface.

Entire Length Diameter

length. of head, of head.

36.6 (v)
36.6 (v)
*4 I . 26
36.6
36.6

38.4 (v)
38.43

> s • 3
,8.3
14.64

'3-3

1 4 . 64
18.30

16.47

20. I '

23.79
= 3-79
23-79

= 7-45

25.62
2O.I3
21 .96
25.62
= 3-79

Ventral surface.

Entire Length Diameter
length. of head. of head.

M

M

M

27-4S(v)

10.98

1 4 . 64

29.28

12. 8l

I O . 98

36.6

,8.3

2 I . 96

31.11 (v)

12. Si

21 .96

*32.94

16.47

2O. 13

34-77(v)

23-79

20.13

42.09 (v)

1 4 . 64

18.30

131 . i i

12. Si

18.30

{49.41

14.64

21 .96

27-45

14.64

23.79

31.11

14.64

21 .96

31.11

14.64

20. 13

*Attached to leaf-surface in close relation to a vein.
iThese triehomes occurred on the same vein.
IThese triehomes were on opposite sides of the leaf and very near a vein.

The averages arc as follows: Dorsal surface, entire length, between veins,
33.8 p, length of head 16.44 /*, diameter of head 25 A*; on veins, entire length
41.84 /•<•, length of head 1S.3/*, diameter of head 23.5 M; ventral surface
between veins, entire length 31.1 /»-, length of head 19 /«•, diameter of head
19.70 /A; on veins, entire length 35.5 i", length of head 15.16 P-, diameter
of head 19.26 /*.

The leaves studied were not fully developed; they were 84/*in thickness.
If we compare the average measurements of hybrids 1 and 4 we shall get
the following results: (l) triehomes taken on the dorsal surface, between
veins, are longer in plant 4 than in plant 1, and the heads also are larger,
but they retain similar proportions; (2) those between the veins on the
ventral surface of plant 4 are also longer than in 1, and the heads are
longer but of less diameter (califoniica character ?); (3) on the veins the
triehomes of 4 are larger than those on the veins of 1, but the heads are
shorter and of less diameter, that is to say, the triehomcs of hybrid No. 4

HYBRIDS OF JUGLANS.

55

are uniformly larger than those of hybrid No. 1, which is a larger plant.
The form of the heads is, with one exception, the same in both series of
plants.

It has been noted, in the tables and the averages, that the heads of the
trichomes are wider than long", and this also is a character that distinguishes
nigra from calif arnica. The relative form of the heads of the short secret-
ing trichomes in the pure species and the form of the head in this, the
second-generation plant No. 4, are shown in the figures. Therefore it
appears that we here meet a well-defined instance of reversion to the pure
species nigra.

JUGLANS CALIFORNICA X JUGLANS REGIA, SECOND GENERATION.

Measurements were made on trichomes taken from the leaves of plants
A and C, which were among the largest and smallest, respectively, of those
of this generation. The study was carried out mainly on the short secret-
ing trichomes, for the reason that this type was most numerous, although
some measurements also were made on the disk-shaped trichomes. The
long secreting trichomes in the material examined were not present in
numbers large enough to make a satisfactory study of them.

TABLE 22. — Measurements on short secreting trichomes, hybrid No. A.

Entire
length.

36.6 (v)

62. 22 (V)
32.94 (V)
32-94
36.6 (v)
29.28

34-77 (v)

32-94

32-94

34-77

40.26 (v)

36.6

34-77

34-77

27-45

38-45 (v)

•sal surface.

Ven

Length

Diameter

Entire

of head.

of head.

length.

M

M

M

I8.3

36.6

25.62 (v)

34-77

36.6

25.62 (v)

18.3

29.28

25.62

18.3

32-94

3'-" (v)

18.3

27-45

25.62

20. 13

29.28

25.62

16.47

21 .96

3'-'i (v)

16.47

32.94

29.28 (v)

'8-3

3I.II

27.45

2O. 13

32-94

34-77

25.62

40.26

25.62

•8.3

32-94

29 . 28

,8.3

3I.II

21 .96

20. 13

3«-4

27-45

16.47

21 .96

29.28 (v)

2O. 13

32-94

25.62

Ventral surface.

Length
of head.

12. 81
16.47
12.81
14.64
12. 81

12. Si

16.47
14.64

12. 8l

18.30

IZ.8l

16.41
12. 8l

14.64
16.47
16.47

Diameter
of head.

M

29.28

25.62
20. 13

27-45
27.45
29.28

27.45
29.28

27-45
32.84
27.45

25.62

27.45
27-45
27-45
25.62

The averages are as follows: Dorsal surface, entire length, between
veins 32.94 p, length of head 18.5 M, diameter of head 30.4 /"•; on veins,
entire length 40.2 /«•, length of head 21.7 /«•, diameter of head 32.1 p;
ventral surface, between veins, entire length 27.04 /*, length of head
14.4 M, diameter of head 27.8 /*; on veins, entire length 28.67 /•<•, length
of head 15.25 /*, diameter of head 27.75 /*.

56

HEREDITY AS ILLUSTRATED BY TRICHOMES.

Following are the diameters, in /"-, of disk-shaped trichomes from leaves
of plant A: 54.6, 67.2, 67.2, 63, 58.8, 67.2, 58.8, 67.2, 71.3, 63, 71.3, 67.2,
71.3, 71.3, 71.3, which average 66 i"-. The following dimensions, in/*,
were obtained of disk-shaped trichomes from C: 67.2, 71.3, 67.2, 67.2,
67.2, 63, 63, 67.2, 71.3, 71.3, 67.2, which average 67.5 M. The diameters
of the trichomes of the larger plant A and those of the smaller plant C
are nearly the same. The trichome appears not to revert to either pure
line, but holds an intermediate position, as in the first generation.

Dimensions of the short secreting trichome of both of the plants A and
C were especially noted. These included the entire length of the trichome,
the length, and the diameter of the head. Attempt was made to get
leaves which were of approximately the same age, but it happened that
those of A were 10 M thicker than those of C. The former was 94 p and
the latter 84 ^ in thickness. It will be recalled that the leaves of hybrids
numbered 1 and 4 were also 84 /«• in thickness.

The measurements show that the length of the trichomes of the dorsal
surface, as well as both dimensions of the heads of trichomes from this sur-
face, are larger than the corresponding dimensions of trichomes from the
opposite leaf-surface, and also that all of the dimensions of trichomes
from the veins are greater than the dimensions of trichomes from between
them. The fact of reversion is not clear. The diameter of the heads of
the hybrid trichomes is considerably greater than that of either cat if arnica
or regia, and the length of the head is intermediate between the lengths
of the heads of this trichome in the two pure species. The proportions of
the head of the hybrid, on the other hand, are practically the same as the
proportion in the pure species n-gia. It is to be noted that the form of the
head is nearly as in the first generation of this hybrid (fig. 19).

TABLE '2'.',. — Measurements on short secreting trichomes, hybrid No. C.

Dorsnl surface

Ventral surface.

Entire

Length

Diameter Entire

Length

Diameter

length.

of head.

of head. length.

of head.

of head.

p-

^

M M

M

M

36.6 (v)

18.3

27.45 25-62

16.47

25.62

31.11

16.47

27.45 2g.28(v)

14.64

29.28

34-77

18.3

23-79 27.45

14.64

27-45

45-"5(v)

2O. IT.

29.28 36.6 (v)

21.96

27-45

4o.26(v^

•8-3

23-79 43-92(v)

21 .96

34-77

42.og(v)

20.13

27-45 32-94

20.13

23-79

36.6 (v)
4<D.26(v)

30.13
21 .96

29.28
23-79

27-45
27.45 (v)

16.47
04.64

31.11

21.96

»7.45 (v)

I8.3

21 .96

34-77(v)

18.3

27-45

36.6

20. 13

29.45

34-77(v)

18.3

29.28

36.6 (v)

16.47

25.62

23-79

10.96

23-79

38-43 (v)

2O. 13

32-94

21 .96

14.64

21 .96

34-77

I8.3

29.28

31.11

14.64

25.62

34-77 (v)

I8.3

31.11

31.11

16.47

29.28

29.28

14.64

25.62

42.09(v)

23-79

27-45

32-94(v)

16.47

20. 14

32-94(v)

18.3

i

29.29

HYBRIDS OF JUG LANS.

57

The averages of the measurements are as follows: Dorsal surface be-
tween veins, entire length 33.5 /*, length of head 17.6 p, diameter of head
27.4 p; on veins, entire length 37.69 M, length of head 19 p, diameter of
head 26 P; ventral surface, between veins, entire length 27. 6^, length of
head 15.5 M, diameter of head 26^; on veins, entire length 35.2 /*, length
of head 18.9 /*, diameter of head 28.3 M.

From the averages it appears that the trichomes which occur on the
veins are larger than those between them, and that those on the dorsal

surface are larger than those
on the ventral. There is very
little difference between the
measurements of trichomes of
plant A, which is among the
largest of those of the second
generation, and of plant C,
which is of the smallest of this
generation. The difference is
so slight that the sum of the
averages of all dimensions of
the trichomcs of A are only 5
more than the sum for C . The
form of the head is the same
as in A.

In hybrid numbered A, so in C, the fact of reversion to the pure lines
is not clear. The heads of the trichomes of the hybrid are broader as well
as shorter than those of regia, but they are smaller in these dimensions
than the heads of californica. This condition is shown diagrammatically
by fig'. 19, which is based on the average measurements of the two pure
lines and the hybrid. It will be seen that, in form at least, the heads of
the trichomes of the hybrid agree very well with the pure species regia, to
which they appear to revert.

GENERAL COMPARISON OF TRICHOMES IN THE PURE SPECIES
AND HYBRIDS OF JUGLANS.

The study of the trichomes of the Jugtans hybrids and pure species has
been carried on with the purpose of comparing not only the mature organs
of the hybrids with those of the parents, but also their origin and devel-
opment step by step so far as possible. As the study progressed it was
found that material for comparison of mature structures was abundant
enough, but material for the observation of the developmental stages,
which depends on the presence of mitotic figures, was not at all easy to
get, so that, as the second-generation hybrids were fairly rich in this
particular, the account of the development of the trichomes is practically
confined to plants of this, the second, generation.

Kl«. 19.— Diagrams, based on numerous measurements,
showing reversion of the heads of short secreting tri-
ehomes in the second-generation Jut/Ian* hybrids.
tr, ,T. califfH'itira J. rfffin, plant A. Legend: solid

line ( — — ), J. citlifnrnicit: dotted line ( ), J.

retfia: broken line ( ), hybrid, b, J. c-ali/or-

nit-a J. nifirn, plant 1, Legend: solid line ( )

J. ralifiirniea; dotted line ( ), ./. nii/ra: broken

line ( ), hybrid.

58

HEREDITY AS ILLUSTRATED BY TRICHOMES.

Four types of trichomes were seen in the pure species californica, nigra,
and regia, and in the hybrids Jnglans californica X Juglans nigra and
Juglans californica X Juglans regia, but in the species nigra and the hybrid
into which it enters an additional form of trichome, not observed in the
other plants, was also seen. In addition to these, the regular types, there
were also abnormal forms of trichomes which were clearly modifications
of those more commonly present, and one type which was essentially
different from those most abundant. With one exception all types of
trichomes were multicellular and glandular.

In the second generation of the hybrid Juglans californica X Juglans nigra,
two of the multicellular trichomes, namely, the short and the long secret-
ing trichomes, are composed of 6 and 8 cells,
respectively. The sequence of cell-divisions
up to the formation of 6 cells is identical,
hence the larger trichome may be regarded
as a further developed short secreting one,
or vice versa, the smaller type may be held to
represent an arrested stage in the develop-
ment of the long secreting trichome (fig. 20).
It does not seem probable that both have
descended from a common ancestor (so to
speak) which had intermediate characters,
since the simplicity of the structure seems
to preclude this. Of the two other types of
multicellular trichomes which occur in the hybrid, and which have from
16 to 32 or more cells, the disk-shaped trichome agrees in the sequence of
its divisions with the two trichomes mentioned above in the first two
divisions only, and the fourth type of multicellular trichome agrees in
sequence with the two trichomes mentioned in the first 3 and possibly the
first 5 cell-divisions.

The sequence of all of the cell-divisions in the 6-celled and the 8-celled
trichomes was found, by repeated observations, to be perfectly consistent,
and the first two or first three divisions also in the larger trichomes were
seen to be consistent for the particular type of trichome, but the subsequent
cell-divisions of the larger trichomes appeared not to occur in consistent
sequence. The last statement is made with reservation, however, as
further and careful study of later stages, which would be a difficult task,
might reveal an unexpected regularity of cell -division in such trichomes.

The abnormal trichomes were of the following types: they were short
secreting trichomes with one additional stalk-cell, or a disk-shaped tri-
chome with 1 or 2 additional cells in the stalk. There seemed also to be
abnormal forms of the long secreting trichome in which the stalk had an
unusually large number of cells and the head also had a larger number of

Fi«. 20. Sequence in cell-division

in the short and the long secreting
trichomes of Juglunx. The num-
bers give the succession of cell-
will formation in proper sequence.

TRICHOMES IN PURE SPECIES AND HYIiRIIXS. 59

cells than normal. All of these abnormal trichomcs were found on the
veins of leaves, but were not very abundant. An additional form of ab-
normal trichome, which was plainly different from any trichome to be
found on the Juglans hybrid, and pure lines as well, was found in F, of both
hybrids. This type had a stalk 2-ranked in place of 1 -ranked, as usually
is the case. The head also was 2-celled and not 4 or more celled, as in
the other multiccllular trichomcs. This type had its origin in a trichome
rudiment which is divided at first longitudinally, in place of transversely,
as is the case in the other trie-homes. From this circumstance alone we
here observe the beginning of a new type of trichome, not by the modifi-
cation of a preceding1 form, as is the case of the 6-celled and the 8-celled
trichomes above mentioned, and probably the other multicellular trichomcs
also, but by a sudden change in the sequence of cell -division.

Numerous measurements, which were made on the length of the tri-
chomcs and the length and the diameter of the heads, showed that there-
is considerable variation in size, and further study revealed a definite
relation between this variation and the position occupied by the trichome,
or the age of the member bearing it. By keeping these facts in mind,
just comparisons could be made between trichomcs placed in analogous
locations and under similar conditions, which otherwise would not be pos-
sible. The general facts of the variation as induced by such environ-
mental conditions may be expressed briefly thus: the trichomes and the
heads of trichomes which are situated on the veins of the leaves or in
their immediate vicinity arc usually larger than trichomes of the same
sort which occupy a position between or relatively remote from the veins
of the same leaf; trichomcs on young members are usually larger than
those on member's that are old; trichomes from the dorsal surface are
usually larger than the corresponding trichomes from the ventral surface
of the same leaf.

As regards reversion to ancestral characteristics, the trichomes of such
of the first-generation hybrids as were stiulied were intermediate in size and
hence did not exhibit reversion, and those of the second generation were
not uniform in this particular. It is probable that study of a large number
of plants of the second generation will show alternative inheritance. In the
second-generation hybrid, in which iiigra enters, the disk-shaped and the
long" secreting' trichomcs both show nigra characters and may be regarded
as reverting to that pure species, while as to these trichomes in the hybrid
in which regia enters the inheritance is not so clear. In this case, the disk-
shaped trichomes of the hybrid are intermediate in size between those
of californica and regia, while the heads of the short secreting" trichomcs
have the form of those in regia, but are much larger. Should the short
secreting trichome of the Juglans californica X Juglans regia be considered
a reversion, as probably is the case, we here have another indication that

60 HICREDITY AS ILLUSTRATED BY TRICHOMKS.

a trichomal system should not be taken together as a single unit, but should
be separated into as many units as there are trichome types.*

The hybrid plants of the second generation which were taken for study
were among the largest and among the smallest of all of this generation,
and all were of the same age and apparently had had an equal chance in life.
The opportunity was thus given to observe the relation between vigor of the
plants as a whole and the size of the trichomes which they bore. The
results are contradictory, and because of this fact the value of the trichome
in a study of this character is shown to be great, as it probably is less
influenced by conditions attending general plant development than other
organic systems of the plant. The disk-shaped trichomes, and the short
secreting trichomes also, of plant 4 were somewhat larger than those in
the larger plant numbered 1 of the hybrid Juglans calif arnica X Juglans
nigra, while these trichomes in plant A and in plant C of the other hybrid,
like and like, were practically of the same size.

*See page 1 1.

COMPARISON OF TRICHOMES IN PURE SPECIES AND HYBRIDS. 61

CONCLUSIONS AND RESULTS.

The 11 different species and the hybrids derived from them, which were
employed in the course of the investigation , represent an interesting; range
of habits and life conditions as well as types of variation and reversion.

The pure species are both arborescent (.Jitglans) and herbaceous. The
herbaceous plants are annuals (.Papaver somniferum, Solatium villosum) ,
biennials (.Oenothera lamarckiana, Oenothera crnciata} , or perennials (So/a-
num guinense, Papaver orientate, Papaver pilosum) .

The feature of fertility or sterility of the hybrids is as follows: Those
wholly fertile are Solanum villosum ' Solatium giii noise (5 generations
have been observed); Oenothera lamarckiana < Oenothera crnciata (3 and
more generations have been studied with regard to this characteristic);
Jiiglans calif ornica X Jiiglans nigra. The wholly sterile hybrids arc Papaver
somniferum X orientate. Jitglans californica X Jiiglans regia and Papaver
somniferum arc partly fertile.

Several types of reversion are known among the hybrids, so far as this
feature has received particular study, and further observation would prob-
ably reveal still other reversion characteristics.

Two hybrids, Oenothera lamarckiana >< Oenothera cruciata* and Sola nit in
giiinctise X Solatium villosum, are fixed forms in the first as well as the
succeeding generations, so far as observed. In the Oenothera hybrid
there is a mingling of the parental characters, no new character appearing,
but the Solanum hybrid exhibits, in the fruit, a condition not found in
either pure parental species. The Papaver and the Jiiglans hybrids, first
generation, are extremely variable as regards the leaf-characters. The
type of reversion vn. Jiiglans hybrids, of the second and third generations,
has not been closely studied, but may be Mendelian. The Mendelian
characteristic of dominance was not observed in any hybrid.

Two general forms of trichomes were seen in nearly all of the hybrids
and the pure species — living forms (which arc glandular) and non-living
forms. After some study it was learned that the living forms of trichomes
had more of interest for the subject in hand, so that for the most part this
paper reports the behavior of such trichomes. In the Oenotheras the
glandular trichomes are unicellular, but in the other species they are mul-
ticellular, and in certain of them, particularly in Jiiglans, the develop-
ment of the trichomes was followed and was found of value for comparative
purposes.

As repeatedly observed during the course of the study, the trichomes
were found to present considerable range in size, which is meant to apply
to any type, and this was found to be the case both in pure lines and in
hybrids. The extremes in variability were no more marked in the latter

*Mutations, Variations, and Relationships of the Oenotheras, by MacDougal, Vail,
and Shull. Carnegie Institution of Washington Publication No. 81, 1907.

62 HEREDITY AS ILLUSTRATED BY TRICHOMES.

than in the former case, which was somewhat unexpected, inasmuch as in
certain instances (Juglans callfornica X Juglans nigra, for example) the
leaves of the hybrid far exceed those of either parent in the range of their
variability. The extreme variability of the leaves of the hybrid referred to
was estimated at 500 per cent, which has reference to size merely; if other
characters were measured the range would be found quite as great, while
the extremes in size of the trichomes of these leaves fall under 100 per
cent and usually much below that figure.

A comparison of the trichomes shows in general that there is a direct
connection between the size and the condition under which they are placed.
For example, trichomes which are located on or close to the veins of a
leaf are larger than those of the same kind between the veins of the same
leaf, and also trichomes of the dorsal surface are usually larger than those
on the ventral surface. The difference in the size of the entire trichome
extends also to the portion active in secretion — the terminal cells of the
multicellular types. Owing to such relation between size and position oc-
cupied by the trichomes, it is probable that the variation is to be associated
with differences in physiological conditions, as nutritive relations, quite
as in the fluctuating variability of larger plant organs.

In general it was determined that each species has trichomes which in
form and in size are characteristic of the species, and should it chance, as is
usually the case, that both parental lines of a hybrid have trichomes type
for type the same, but differing only in size and form, the corresponding
trichomes of the hybrid, at least of the first generation and frequently in
later generations, hold some degree of intermediacy. But if one pure
line bears trichomes unlike those of the other pure line, the odd trichomes
are transmitted unchanged either in form or in size. The Solan 11 in hybrid
was the only clear ease of unilateral inheritance, although Juglans cali-
I'ornica ', Juglans nigra almost surely has this type of inheritance also.

Not only do the trichomes vary in size, but certain observations indi-
cate that they have an unequal distribution, so that if in size and form of
trichomes the inheritance may be said to be bilateral it may no longer
be considered such when the facts of distribution of the trichomes on the
members bearing them is taken into consideration. This condition was
especially noted in Ocnothrra hybrid. In the hybrid and in the pure parents
the trichomal distribution was observed to be as follows: The pear-shaped
trichomes in Ocnothera lainarckiana was found on leaf, on stem, and spar-
ingly, on capsule; in Ocnothera cruciata the same trichome type was on
the leaf, stem, and abundantly on capsule: in the hybrid it occurs on leaf
and stem, but not on the capsule. The club-shaped trichomes in Oenothera
laniarckiana occur on leaf, stem, and capsule, but in Oenothera cruciata and
the hybrid this type is to be found only on the capsule. This observa-
tion, and others also, make it probable that the trichomal system is not

COMPARISON OF TRICHOMES IN PURE SPECIES AND HYBRIDS. 63

to be taken as a single unit, but rather that it is to be eoneeived as being-
comprised of as many units as there are distinct types.

In most of the plants studied it was not practicable to study the origin
and development of the trichomes, either from a lack of suitable material
or because the trichomes were unicellular. But in Jug fans the material
for study was the most favorable, so that the ontogeny of the trichomes
was followed to a certain extent in all plants, both hybrid and pure species,
but particularly was this done in the second-generation material. In
/iig/ans both unicellular and multicellular trichomes are found, of which
4 types occur in calif or nica and rcgia, and an additional type in nigra.
The hybrid strain with nigra blood has 5 trie-home types, while that with
regia blood has but 4. The trichomes common to all Juglans studied
are (1) awn-shaped — unicellular, (2) short secreting, (3) long secreting,
and (4) disk-shaped types. The extra form, found in nigra and its deriv-
atives, is a long type with a number of cells in stalk and head. The short
secreting trie-home has 6 cells, the long secreting type has 8 cells, while
the disk-shaped form has 32 or more cells. The 6-celled trie-home has a
certain sequence in development which it follows with perfect consistency,
which also is true of the 8-celled trie-home. The latter trie-home up to
the 6-celled stage has the same sequence in its cell-divisions as that of the
smaller form, and to these adds 2 others, which also have a proper
sequence. Therefore it seems probable that the two trichomes are espe-
cially closely related, and either that one developed out of the other or
that one represents an arrested stage of development of the other.

The first 3 cell-divisions of the odd type of trie-home, that peculiar
to nigra and its descendants, agree in sequence with the corresponding-
stages in the short and the long secreting trichomes, but in the later
development it is different from either. Only the first 2 cell-divisions
of the disk-shaped trie-home agree with the- course of development of the
three trichomes just mentioned. This type of trie-home, consequently,
probably is not so closely related to either of the preceding types as these,
particularly the first two mentioned, are to each other.

In addition to the more common form of trichomes \n/iigfans, abnormal
forms and one aberrant type were observed both in the pure lines and in
the hybrids. The abnormal trie-homes were very evident modifications of
the prevailing types; that is, they were short secreting trichomes with an
extra cell in the stalk, long secreting trichomes with a stalk of more than
4 cells, or (in trichomes with nigra blood) the odd trie-home with more
than 8 cells in the head, or, finally, disk-shaped trichomes with a stalk of
3 cells. The aberrant trie-home was observed in the second-generation
material of both hybrid strains. It had a structure quite different from
that of the other multicellular trichomes and it originated in a manner
peculiar to itself. It therefore is taken to represent a new type of tri-
chome, and is with little doubt a mutation.

64 HEREDITY AS ILLUSTRATED BY TRICHOMES.

From a study of the development of the trichomes \njuglans, as just
summarized, it would appear that there may be at least two ways they
may take their origin. They either may be modifications of types already
existing, as illustrated by the abnormal forms and indicated by a com-
parison of the development of the trichomes, or they may arise suddenly
through the circumstance that the initial cell-division is a unique one. In
addition to the origination of the trichomes, in the manners described,
observations suggested that at least compound unicellular trichomes might
arise in quite another manner.

In most of the Jitg/ans studied the awn-shaped trichomes occur singly,
but in a few instances they were aggregated into small groups, the elements
of which, however, were quite the same as the single trichomes. In such
cases it is evident that the more complex trichomes, referred to in fore-
going descriptions as stellate trichomes, owe their origin to the fact that
several adjacent epidermal cells all give rise to awn-shaped types, and
that therefore they can not become separated in the subsequent develop-
ment of the leaf. The primitive types of trichomes oijuglans may, there-
fore, be conceived of as being the awn-shaped trichome, in addition to the
short secreting trichome and the aberrant type. The possible relationships
of these types, and their relation to the other trichome in Jug latis, as con-
ceived from their study, is graphically shown in fig. 21.

Abnormal Long secreting

type trichome in

niyra only

Long secreting Abnormal
trichome (ype

^ Abnormal

type

Unicellular Aberrant Short secreting-

triehome trichome trichome

FIG. 21.— ProlMilde origin and relationship of Hie trichomes of Jiiplans.

SUMMARY. 65

SUMMARY.

(1) The trichomes of the following- hybrids, and of the pure parental
species, were passed under observation: Jnglans californica X Juglans
nigra, F, and F2; Juglans californica X Juglans regia, F, and F2; Oenothera
lamarckiana X Oen othera crucia ta, F2 and F:1; Papavcr sotnniferum X Papaver
orientate, F,; Papaver soiiiiiifemm X Pa paver pilosum, F,; Solatium villosiini <
Solatium guinense, F,, F2, and later generations.

(2) The Oenothera hybrid inherits characters from both pure parents,
but does not revert to either line.

Three types of trichomes, all of which are unicellular, are found in both
parents and in the hybrid Oenothera. Those of lamarckiana are larger than
those of cruciata, while the corresponding trichomes are, in the hybrid, of
an intermediate size. Both in the hybrid and in the pure parental lines
the trichomes which occiir on or in the close vicinity of the veins of the
leaf are larger than those between the veins of the same leaf.

The distribution of the trichomes is unlike in the two pure lines and in
the hybrid. In lamarckiana both types of glandular trichomes are found
on stem, leaf, and capsule; in cruciata the pear-shaped trichome occurs on
the same regions, but the club-shaped trichome is found on the capsule
only. In the hybrid the club-shaped trichome only occurs on the capsule,
in this particular exhibiting a cruciata character, but no other type of tri-
chome is found on the capsule in which the hybrid is different from cither
pure parental line. The peculiar quality of distribution is held to indicate
the independence of each type of trichome as a character.

(3) Two strains of Papaver hybrids were examined, both of which were
sterile, so that the first-generation plants only were available.

Only one type of trichome is found in Paparer, both in the hybrids and
pure species, which has, in the hybrids, an intermediate structural character.

(4) In the Solatium hybrid and pure species 2 types of trichomes are
found, but of these one type only is common to both parental lines and
the hybrid. The second type occurring in the hybrid is inherited from
the guinense line.

Although as regards the trichomal character the Solatium hybrid has
unilateral inheritance, in another regard it possesses a new characteristic,
the flrvor of the fruit, which, in all the material examined, was seen to be
a constant character.

(5) In Juglans, pure species, 4 types of trichomes, 3 of which are mul-
ticellular and glandular and 1 unicellular and lifeless, are found in each
species. Nigra and nigra derivatives have, in addition, a form peculiar to
themselves. Each form of trichome has a manner of development to which
it adheres with great constancy. Numerous measurements show that the

66 HEREDITY AS ILLUSTRATED BY TRICHOMES.

trichomes of each pure species have a characteristic size and form, but that
a considerable range of variation, which is always associated in a very defi-
nite manner with the position occupied on the leaf, or with the age of the
leaf, is to be found both in form and in size.

The first and the second generations of thejuglans hybrids were examined.
The first-generation hybrids of both strains, as regards the leaf -char-
acters, are intermediate although not strictly so. Dominance was not
observed. Each hybrid bears all of the trichomes found in both parents,
which in the nigra derivative is one more than in the other hybrid, owing
to the fact that pure nigra has one more type of trichome than occurs in
regia or calif arnica.

So far as followed each form of trichome, type and type, had the man-
ner of development which was seen for it in the pure lines. In size and
form the trichomes are intermediate between the same characters of the
trichomes in the pure lines, and also in the hybrid they exhibit a varia-
tion directly associated with the position which they occupy on the leaf
or with its age.

The second-generation hybrids of Juglans do not exhibit reversions in
gross leaf-characters to either pure line. The course of development of
trichomes in both strains was followed closely and it was learned that each
trichome type has the same sequence of cell-division in development as
was seen for the particular trichome in F, and the pure lines. The 6-celled
trichome has a certain form of development, which the 8-celled type fol-
lows exactly up to the 6-celled stage and then it adds 2 divisions peculiar
to itself. The other multiccllular trichomes agree with the sequence of
these divisions in the early stages only.

In the hybrids of the second generation, both strains, abnormal trichomes
were seen. These were very evident modifications of types already exist-
ing. One aberrant type also was found, which was quite different from
any occurring commonly on the leaves. The aberrant trichome originated
by divisions which were essentially different from those of the usual tri-
chomes, and there are no intermediate forms between the aberrant trichome
and the other types, from which facts this type is held to be a mutation.

(6) The trichomes of Juglans are thought to arise from three types,
namely, the awn-shaped, the 6-celled (short secreting), and the aberrant
trichomes. By the conversion of all of the epidermal cells of a group into
awn-shaped trichomes, stellate types arise; by the processes of arrested
development, or the fixation of minor variations (such as the abnormal
trichomes), or by mutation, the multicellular trichomes have originated.
In Juglans the largest number of types of trichomes seem to have arisen
by the second and third means. The aberrant form represents a type which,
by such variation, would be the ancestor of still other kinds of multicellular

SUMMARY. 67

trichomes. Should each kind of trichome represent a distinct character,
as seems possible, we have here several methods by which unit characters
may take their origin.

(7) \T\Juglans there is no direct relation between the size or vigor of
the leaves or plant and the size of the trichomes, although position on
the leaves directly affects size and form of the trichomes.

(8) In the second generation of Juglans calif arnica X Jug Ian s nigra the
short secreting trichome, in size and form, reverted to nigra. No other
case of reversion to one pure line, of a character of trichome common to
both pure lines, was observed \njng/ans.

UNIVERSITY OF TORONTO
LIBRARY

Acme Library Card Pocket

Under Pat. "Ref. Index File."

Made by LIBRARY BUREAU, Boiton