102 
natural history of the leaf will soon be fully 
known. 
The second lesson which Mr. Tyler’s paper 
enforces is the one which paleontology, as we 
have seen, had already taught. What the leaf 
struggles to secure is the maximum amount of 
light and air. In this effort it constantly tends 
to stretch out as far a possible from the stem. 
The proximal portion, which is most shaded, 
is then sacrificed to the distal portion, and the 
leaf is differentiated into petiole and blade. 
But in this process various stagesoccur. Those 
parts which are of any advantage to the plant 
are in part retained, and the parts sacrificed are 
selected in the exact measure of their failure 
to benefit the plant. 
Some may regard the principle of adaptation 
for securing the maximum light and air as in- 
adequate to explain these modifications of the 
leaf, but it is this principle that determines not 
only the form but also the arrangement of 
leaves, and when we remember that opposite 
leaves are also decussate, that in whorled 
leaves those of one whorl stand over the inter- 
vals between those of the next whorl below, 
and that even in plants with alternate leaves 
the phyllotaxy is so adjusted as to secure the 
longest interval between one leaf and the next 
one that must stand directly over it, we not only 
see with what rigorous exactness this principle 
works, but also what apparently trifling differ- 
ences in advantageousness are seized upon and 
made to count in producing manifest effects. 
The least useful portion of a leaf is not that 
at the very base, but that which is some dis- 
tance from the base, and even this may be par- 
tially retained as a wing to strengthen the leaf- 
stalk. The portion at the base is often preserved 
in one form or another, and we have seen, in 
the fossil and living species considered, how 
this may vary in the process of evolution, but 
in the most highly developed of our living flora, 
where it is retained at all, it is usually in the 
form of stipules, which have all conceivable 
shapes and differ in all degrees in their perma- 
nence, some being appropriated to other uses 
according to the law of vicarious function. 
Others are persistent as small organs of different 
forms. Still others are deciduous at varying 
stages in the growth of the leaf, some, as Mr. 
SCIENCE. 
[N. 8S. Vou. VI. No. 133. 
Tyler shows, never being seen except on dis- 
secting the bud. The last stage in this process 
is their complete atrophy and the resultant 
wholly exstipulate leaf. 
The third lesson that we learn from Mr. 
Tyler’s studies is that monocotyledonous plants 
constitute an early stage in the process of leaf 
development. This is what would have been 
naturally supposed, but there has been a ten- 
dency of late to cast doubts upon the position 
of the monocotyledons and to maintain that 
they are as highly developed and that they 
have been as late in their appearance in 
geologic time as the dicotyledonous angiosperms. 
It must be admitted that the paleobotanists have 
been the ones who have chiefly taken this view. 
This has been due to the exceedingly meager 
representation which the monocotyledons have 
in the fossil floras of the globe, and especially to 
the natural doubts which have arisen as to the 
botanical character of most of the fossil forms 
that have been regarded as monocotyledons by 
certain authors. The Yuccites of Schimper, 
from the Lower Trias, as also his genus Atho- 
phyllum, which he did not himself refer to that 
class, but which others have naturally regarded 
as a monocotyledonous plant, cannot certainly 
be claimed as ancestral monocotyledons, al- 
though the proof to the contrary is equally want- 
ing. Scarcely anything that has been discovered 
in the great Jurassic floras of the world has even 
been called monocotyledonous, and very little 
that is certainly such occurs even in the Creta- 
ceous. It is, therefore, been held by some that 
this class of plants first made its appearance 
with the palms of the Eocene, but so rich and 
varied is this Eocene palm flora that it presents 
acase analogous to that which until recently 
was offered by the dicotyledonous floras of the 
Middle Cretaceous, and requires the violent 
assumption that a great group of plants sud- 
denly burst in upon the world and attained all 
at once a high state of development in widely 
separated regions. This assumption is now 
thoroughly disproved in regard to the dicotyle- 
dons by the discovery of early embryonic types 
at a much earlier age, naturally leading up to 
the higher types referred to. 
The monocotyledons, from their very nature, 
are the least adapted of all forms of vegetable 
