152 THE LIFE OF THE PLANT 



tion reveals remarkable regularity in their arrangement. 

 The first -person who called attention to this fact was, 

 I believe, the famous Leonardo da Vinci ; but it was 

 not until the nineteenth century that the phenomenon 

 was studied in any detail by botanists. This regularity 

 of distribution is seen mainly in the way the leaves are 

 distributed on the stem, so that as far as possible they 

 neither screen nor shade one another, and at the same 

 time leave no free spaces through which the rays of the 

 sun may pass to no purpose. This statement can be 

 verified by a mere glance at the rosette of leaves on the 

 plantain. They alternate in such a way that only the 

 ninth leaf covers the first (i.e. the very lowest). Cer- 

 tainly the more leaves are separated the one from the 

 other the less they shade one another ; but a great 

 development of the leaf system is only possible when 

 the stem has reached certain dimensions. In most 

 cases this can be attained only at great expenditure of 

 building material, because in order to bear a great 

 number of leaves the stem must be very steady and firm. 



There are plants, however, which do produce a great 

 number of leaves and reach a great height, while at 

 the same time economising their building material. 

 These are the climbing plants, the thin delicate stems 

 of which select other plants or inanimate objects as 

 supports. Twisting and turning round these they climb 

 to a considerable height, and produce a large mass 

 of foliage which they could not support unaided. Such 

 are, for instance, the hop, the bindweed, the ivy, and 

 many other plants growing in tropical forests and known 

 under the general name of lianes. 



In general the stem has a twofold function : it must 

 bear leaves and conduct the nutrient sap from the root 

 to the leaf and from the leaf to the root. For this 

 purpose it must evidently be equipped with something 

 that will give it solidity, firmness, elasticity, and other 

 mechanical properties ; but at the same time it must 



