INTRODUCTION 3 



study is not, however, mainly or directly economic: 

 it is biological. The structure of plants is chiefly 

 considered with reference to their functions as living 

 beings, i.e. physiologically, unless it is viewed merely 

 as an indication of greater or less affinity with other 

 plants, and thus as a clue to their evolutionary ancestry. 

 The life of the individual plant is recognised as a function 

 of two variables, viz. the characters which it inherits 

 from its parents, and the result upon its development 

 of its environment. This environment does not consist 

 wholly in climate and soil, although these are factors 

 of supreme importance. The plant will require a certain 

 amount of heat to sprout, to flower, or to ripen seed: 

 it will require a certain amount of moisture in the soil 

 and some small modicum of soluble saline substances 

 also available at its roots: its healthy development will 

 be influenced by its exposure to light and wind; but 

 its existence may be also conditioned by other living 

 beings, either plants or animals. As a seed about to 

 sprout, as a delicate slender seedling, or even at a much 

 later and more robust stage of its life, the occupation 

 of the surrounding soil by other plants, or the shade 

 of their overhanging foliage, may prove fatal to it. 

 Substances not otherwise available, or even harmful, 

 to its roots may be rendered useful if certain bacteria 

 exist in the soil to set up suitable processes of fermenta- 

 tion. The fertilisation of its seeds may depend upon 

 the conveyance of its pollen from one flower to another 

 by the agency of insects ; while the dispersal of its seeds 

 to spots where they may be freer from competition 

 may depend on their being carried by birds or entangled 

 in the hair of passing animals. 



The presence of any plant in any one place, moreover, 

 does not depend solely on the suitability of that place 

 for that species of plant. Climate and soil may be 

 suitable: there may be unoccupied ground; and all 

 necessary organic environment may also be present; 

 but in some manner or other, whether as seed or spore 

 or growing plant, the species must reach this suitable 

 spot. It must either be transported from some other 

 spot, near or far, where it has come into being; or it 

 must have originated de novo in the area where we now 

 find it. 



As vegetable physiology considers alike the external 



