Vol. II., No. 9, MAY. 1885.1 
BOTANICAL EVOLUTION.* 
IT. 
a 
(That portion of the lectures dealing with the evolution of 
Flowering from Flowerless plants is omitted.) 
Leaving the question of the genealogy of Flowering Plants, I 
now wish to direct your attention to special applications of the 
theories of descent and development in their relation to the 
vegetable kingdom. And this evening I propose only to deal 
with modifications of stems, leaves, &c. If we take a sprouted 
pea or bean, we notice that it has a small straight root, which 
invariably descends into the soil, a small stem, which as inva- 
riably ascends in search of light, and two oval leaves, whose 
edges are quite uncut. This stage probably represents the primi- 
tive condition of all plants having two seed-leaves (Dicotyledons), 
for. it may be accepted as a general principle that these early 
stages in the life of the individual shadow forth the early stages in 
the development of the race; just as the caterpillar suggests to us 
the worm-like form from which the butterfly is descended. Nowa 
remarkable plant called Welwitschia (after its discoverer) which occurs 
in the western parts of tropical Africa, retains its primitive characters 
to this day, and has apparently never got beyond them. It may be 
looked on as a plant which never gets beyond its babyhood. It 
produces a stem which sometimes attains a diameter of four feet, but 
is seldom more than a foot high, and it bears two immense seed-leaves 
which are more than six feet long and two or three feet broad, and 
which last throughout the plant’s life, zc, more than a century. 
These are the only leaves it ever acquires. There is no other plant 
known in which this rudimentary condition is permanent. 
In all other cases differentiation takes place sooner or later, and 
after passing through the stage marked by the one or two seed-leaves, 
the structure becomes more or less complex. The leaves themselves 
change in the first place, because their primary function being: to 
breathe in and decompose the carbonic acid of the air under the 
influence of sunlight, they become modified in a variety of ways to 
effect this object. This subject of the forms of leaves has not received 
much attention from the evolutionary point of view, and I must 
confess my inability to explain it satisfactorily. The popular evolu 
tionist Grant Allen, in a series of articles on the forms of leaves, 
recently contributed to ‘ Nature,” explains the varied structures as 
due to the keen competition among the leaves for the carbonic acid of 
the atmosphere. And using this as the solution of the problem, he 
Continued from p. 373. 
