RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION, 935 . 
with food-materials {Arum, Potato), or they become tendrils (the Vine), or spines 
{Gledifschia) ; 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 different 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. 
