RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 935 



with food-materials {Arum, Potato), or they become tendrils (the Vine), or spines 

 {Gleditschid) ', sometimes they assume the form of foliage-leaves {Ruscus, Xylo- 

 phyllum, &c.). The adaptations of roots are less numerous ; usually filiform, slender, 

 cylindrical, and provided with root-hairs for absorbing water and dissolved mineral 

 substances, they become tuberous reservoirs for reserve food-materials in the Dahlia ; 

 their tissue is loose and contains air and they resemble swimming bladders in 

 Jussieua ; in the Ivy, Ficus repens^ &c., they are simple organs of attachment for the 

 stem ; in Vanilla aromatica they play the part of tendrils ; but they never produce 

 sporangia or sexual organs. 



According to the definition already given of Purpose in the vegetable organ- 

 isation, its relationship to the morphological nature of the organ can also be illus- 

 trated by keeping in view the end to be served, i. e. the condition of the plant which 

 is most favourable in the struggle for existence, and then observing the means em- 

 ployed for attaining this end, i, e. what members of the plant become adapted, 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 considerable 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 fact that in 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 is 

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

 this the case in parasites and saprophytes {Orobanche, Neottia^ &c.), which vegetate 

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

 of upright growth, it is of interest to see in what various ways this one purpose is 

 attained in difi'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 



^ 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. 



