'^'V^^ GENERAL CONDITIONS OF PLANT-LIFE. 



Sect, i o. — Action of Gravitation on the Processes of Vegetation ^ 



Since the attraction of the earth acts uninterruptedly on all parts of the plant, the 

 entire vegetable organisation must be so contrived that the weight of the separate 

 parts of the plant is serviceable, or at least not injurious to the various purposes of 

 the life of the plant. 



In observing these relaiionships the first thing is to distinguish between those 

 contrivances which l^ave for their object to bring the weight of the parts of the plant 

 into harmony with the purposes of its life — gravitation itself not taking any direct 

 recognisable part in the attainment of these objects' — and those phenomena of vege- 

 tation on the other hand which are brought into existence by the direct influence 

 of gravitation on the mechanism of growth. 



To the first of these groups belongs the fact that the branches and foliage of 

 upright stems are distributed nearly equally on all sides, and. that in larger plants the 

 firmness and elasticity of the masses of tissue in the stem is promoted by the form- 

 ation of wood, or is brought about by other means, as for instance in the trunk of 

 Musa. But since it is very common in the organic world for the same purpose 

 to be attained by very different means, slender delicate stems with but little wood 

 can protect themselves from sinking down and can expose their foliage to the light 

 by twining round firm supports, or by climbing with the help of tendrils, hooks, 

 spines, &c. The same purpose is evidently served by the various floating contriv- 

 ances of water-plants and those of fruits and seeds ; in all these cases the structure 

 is obviously adapted to make the weight of the part of the plant serviceable or at 

 least not injurious to its life ; although it cannot be maintained that gravitation takes 

 any part in the formation of wood, in the sensitiveness of tendrils, or in the produc- 

 tion of a floating apparatus. The only explanation of these arrangements lies in 

 Darwin's Theory of Descent ; viz. that, under the influence of long-continued natural 

 selection, only those structures are finally able to maintain their existence which, 

 while sufficient for the other requirements of life, are so arranged that the weight of 

 the part is not injurious or is even useful. It must not be inferred from this, nor 

 does observation render it probable, that gravitation takes any direct part in these 

 phenomena. 



Gravitation however exerts a direct influence on the growth of young parts of 

 plants as soon as the longitudinal axis of the growing organ is inclined obliquely 

 to the perpendicular and therefore to the action of gravitation. In this case the 

 growth in length of the oblique organ is different on the upper and under sides, 

 and the more so the more nearly horizontal the axis of growth. According to 

 the nature of the organ and its purpose in the economy of the plant, either the 

 upper side grows more strongly than the under side, or the reverse. A curvature 

 concave either downwards or upwards is thus caused by the influence of gravita- 

 tion and growth, and this curvature increases until the free-growing end is directed 

 vertically either downwards or upwards ; the former, for example, in primary roots, 

 the latter in many primary stems. In lateral branches, leaves, and secondary roots. 



' These statements are intended in the first place to draw the attention of students to the pro- 

 cesses of vegetation which are especially influenced by gravitalion. Its action on the mechanism 

 of growth will be fully described in Chap. IV, where also the literature is (quoted. 



