1890.] on the Shapes of Leaves and Cotyledons. 105 



climate most leaves are glad of as much sunshine as they can secure, 

 and are arranged with reference to it. The width of the beech leaves 

 — about 1£ inch — is regulated by the average distance between the 

 buds. If the leaves were wider they would overlap. If they were 

 narrower there would be a waste of sjDace. The area on the one hand, 

 and the width on the other, being thus determined, the length is 

 fixed, because, to secure an area of 18 inches, the width being about 

 If inch, the length must be about 2 inches. This, then, explains the 

 form of the beech leaf. 



Let us apply these considerations in other cases. I will take, for 

 instance, the Spanish chestnut and the black poplar. In the Sj)anish 

 chestnut the stem is much stronger than that of the beech. Conse- 

 quently it can carry a greater leaf-surface. But the distance between 

 the buds being about the same, the leaves cannot be much wider ; 

 hence they are much longer in proportion, and this gives them their 

 peculiar sword-blade-like shape. 



Again, if we look at a branch of black poplar, and compare 

 it wdth one of white poplar, we are struck with two things : in the 

 first place, the branch cannot be laid out on a sheet of paper so 

 that the leaves shall not overlap ; the leaves are too numerous and 

 large. Secondly, in the white poplar the upjDcr and under surfaces 

 of the leaf are very different, the lower one being covered with a 

 thick felt of hair, which gives it its white colour ; in the black poplar, 

 on the other hand, the two surfaces are nearly similar. 



These two characteristics are correlated, for while in the white 

 poplar the leaves are horizontal, in the black poplar, on the contrary, 

 they hang vertically. Hence the two surfaces are under very similar 

 conditions, and consequently jDresent a similar structure ; while for 

 the same reason they hang free from one another. 



Let us again look for a moment at the great group of Conifers. 

 Why, for instance, do some have long leaves and some short ones? 

 This, I believe, depends on the strength of the twigs and the 

 number of years which the leaves last ; long leaves dropping after 

 one, two, or three years, while species with shorter ones retain 

 them many years — the spruce fir, for instance, 8 or 10, Abies Pinsapo 

 even as many as 18. 



[Here Sir John dwelt on and q^xplained the forms of several 

 familiar leaves.] 



Seedlings 



I now come to the second part of my subject — the forms of 

 cotyledons. Any one who has ever looked at a seedling plant must 

 have been struck by the fact that the first leaves differ entirely 

 from those which follow — not merely from the final form, but even 

 from those which immediately follow. These first leaves are called 

 cotyledons. The forms of many cotyledons have been carefully 

 described, but no reason has been given for the forms assumed, nor 

 any explanation offered wdiy they should differ so much from the 



