RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 837 



adapted for the purpose, and what metamorphosis they undergo. A few examples 

 will explain this'. 



It is obviously useful for the greater number of flowering plants — in other 

 words advantageous in the struggle for existence — that their stem should grow 

 rapidly to a certain height, because the conditions of assimilation (light and warmth) 

 are thus most perfectly fulfilled, and because — which is perhaps of greater importance 

 — the flowers are more easily detected by insects on the wing, and the pollen trans- 

 ferred by them from one flower to another. Even where (as in many Coniferae, &c.) 

 the light pollen is carried by the wind to the female flowers, this is accomplished 

 better when the flowers are at a greater height from the ground ; and finally by this 

 means the dissemination of the seeds by the wind or by frugivorous birds is pro- 

 moted, or their scattering by the bursting of the fruits. That these arrangements for 

 propagation are especially promoted by the upright growth of the stem is evident 

 from the large number of plants which develope their leaves in a rosette close to the 

 ground or on a stem that creeps along it, a rapidly ascending flower-stem being 

 formed only just before the unfolding of the flower-buds. Still more strikingly is 

 this the case in parasites and saprophytes (Orobanche, Neotda, &c.), which vegetate 

 below and blossom above ground. If we concede these and other special purposes 

 of upright growth, it is of interest to see in what various v;ays this one purpose is 

 attained in diff"erent species of plants. In many shrubs the growing stem is endowed 

 with sufficient firmness and elasticity to support in an upright position the weight of 

 the leaves, flowers, and fruits ; if it happen to be broken down, or if it must raise 

 itself from a previously creeping position, advantage is taken of the property of 

 geotropism. But the slender haulms of Grasses are not themselves endowed with 

 this power ; and in their case the basal portion of each leaf-sheath forms a thick ring 

 the tissue of which retains for a long time its power of growth ; and when the haulm 

 is bent by the wind, or is in its early stage prostrate on the ground, the elevation into 

 an erect position is brought about by the surface of the node which faces the ground 

 growing rapidly and strongly; a knee-shaped bend is thus formed by which the 

 upper part of the haulm is raised up. If, on the contrary, the stem is perennial, and 

 has to bear a great weight of branches, leaves, and fruits, contrivances of this kind 

 are not sufficient, and then the tissue becomes woody ; if the weight of the crown 

 increases year by year, the stem also becomes thicker each year, as in dicotyledonous 

 trees and Conifers ; if the weight of the foliage does not increase, as in Palms, the 

 stem only retains the same thickness. In such cases a considerable quantity of as- 

 similated food-material is necessary in order to produce the massive solid stem, while 

 in many other cases the elevation is attained at the expense of a very small amount 

 of organic substance, as in climbing and twining plants, such as are found in the 

 most widely separated families of Angiosperms. Plants with a twining stem like the 

 hop presuppose in general the existence and proximity of other plants which are able 



1 In these examples I am compelled to confine myself to the most important points. Most of 

 the adaptations are so complicated that a detailed description of them in even a single plant would 

 require a great deal of space. What was said in the . fourth chapter of this book on climbing plants 

 and in the sixth on the adaptation of the foliar organs of a flower to the purpose of cross- 

 fertilisation may be . consulted. .: .■"'■:.-' 



