410 JOURNAL OF SCIENCE. 
proceeds to show how plants growing in comparatively open ground 
tend to have large, flat undivided leaves, like rhubarb, in which the 
simple form is not much departed from, while those which grow in 
hedge-rows and tangled places, or which crowd together on the 
ground, like grasses, tend to have their leaves more or less cut up or 
reduced to filaments. I instance this case, because so few have 
tackled the subject at all, and because Allen in so many of his papers 
has thrown out numerous valuable suggestions, and has done much 
to popularize natural history. But there are just two objections to 
the above theory, either of which, to my mind, is fatal to it. One is 
that by the law of diffusion of gases, the per centage quantity of 
carbonic acid in the air of hedge-rows or tangled places is just the 
same as in open ground, because it tends to diffuse itself equally 
throughout the atmosphere, and I cannot see that there can exist any 
‘competition for it among leaves. In the second place leaves of simple 
undivided forms are by no means peculiar to open ground, or much 
divided leaves to thick tangled places. The opposite statement might 
be made with about as much accuracy. Here for example, is a kidney 
fern (Zrichomanes reniforme) with undivided kidney-shaped fronds, 
which will not grow except in damp shady woods, while this one 
(Hypolepis millefoliwm) with its myriad segments, is characteristic of 
open hil-sides. Again, look at these orchids (Corysanthes macrantha) 
—each with one entire heart-shaped leaf—which grow in thick damp 
woods, and compare them with this umbelliferous plant (Ligusticum 
haastiz) with its leaf-blades cut into feather-like fringes, and which 
lives 5000 feet up the mountain sides. Innumerable other examples 
might be adduced to disprove this hypothesis, but of course it is only 
fair to add that it is much more easy to criticize any theory of the 
kind than to advance a new one. And if Grant Allen or any one else 
advances a theory, which though not tenable itself, causes others to 
take up the question and examine it for themselves, he is always 
helping in the general search for the truth. But seeing that sunlight 
is an essential agent in enabling leaves to perform their functions, it 
is much more probable that their forms have been largely modified by 
their struggle to obtain the maximum quantity of light. But how 
this has acted I am unable to suggest. It is worthy of remark as 
showing the different ways in which botanists of fifty years ago worked 
as compared with those of to-day, that in all the older manuals great 
value is attached to the modifications of forms among leaves; they 
were divided into simple and compound, and every form of outline had 
a distinctive name. For the sake of systematic botany this immense 
nomenclature is still kept up to a certain extent, but as a matter of 
fact the form of the leaf is dependent in the first place on the forms 
assumed by the bundles of vascular tissue of which the ribs and veins 
are composed, and in the second on the extent to which the green 
cellular tissue is developed and spread over the frameworks thus 
formed (reference was made here to diagrams on blackboard.) In 
submerged water-plants the leaves are always reduced to a filamen- 
tous condition, and this we can understand at once. They have to 
obtain their carbonic acid in solution from the water, and as the rate 
of diffusion does not hold in water as it does in the atmosphere, and — 
further as the supply is ina more attenuated condition, as it were, 
the leaves have to spread out the largest possible surface to obtain an 
adequate supply. And to show that this explanation is correct, we 
