776 MECHANICS OF GROWTH. 



of assimilated food-material, water, oxygen, and a sufficiently high temperature. 

 Under these conditions individual cells or masses of tissue may grow, provided that 

 their organisation permits it. But independently of these conditions there are others, 

 as we have seen in the last chapter, which, without absolutely causing or arresting 

 growth, nevertheless influence it; as light, gravitation, and pressure. The first- 

 named may be called the necessary, the last the secondary conditions of growth. 

 In all growth all the necessary conditions must concur while the secondary conditions 

 intervene only in certain cases, and exert their modifying influence very differently 

 on the corresponding parts of different plants. 



The conditions spoken of as Necessary and Secondary depend upon the en- 

 vironment of the plant, and act upon it from without. They may therefore be 

 described as External Condi/tons or causes of growth, in contradistinction to the 

 Jnierfial Conditions dependent on the organisation of the plant. The existence of 

 the latter conditions is most strikingly manifested in the fact that all parts of plants 

 are able to grow only during a certain time ; when this time — the period of youth 

 and development — is past, they no longer grow, even when all the favourable 

 conditions concur. This shows that the internal organisation undergoes changes, 

 which at length render the continuance of growth impossible. But even in organs 

 which are still growing a certain independence of external circumstances may be 

 perceived ; an Oak-leaf invariably grows differently from an Elm-leaf, an Oak-fruit 

 from an Oak-root. The differences of these processes of growth is at once manifest 

 in the difference of form and of the other properties of the organ ; and no com- 

 bination of external circumstances has the power of giving to a root, by change in 

 its growth, the form of a leaf or to an Oak-leaf the structure of an Elm-leaf. There 

 are also certain internal conditions of growth which do not decide, like the age of an 

 organ and the necessary external conditions, whether growth shall take place, or at 

 what rate; but determine how it shall proceed, and what specific and determinate 

 organisation shall be attained by it. This latter circumstance depends only on the 

 parent plants, or in other words on the species or variety to which it belongs. 

 Descent determines the specific character of the growth; all the other conditions 

 determine only whether growth shall take place at all, and with what rapidity and 

 energy. The innate internal conditions that regulate the nature of the growth of 

 the plant, when once present cannot again be destroyed or reversed ; while the ex- 

 ternal conditions may be at one time brought into action, at another time set aside. 

 The internal and external conditions of growth may therefore be distinguished as 

 the historical and the physical ; but those properties of a plant which have been 

 obtained historically are generally termed hereditary. This expression is not open 

 to objection unless heredity be considered, as has recently been done by many, 

 as a kind of natural force requiring no further analysis. For in distinguishing 

 hereditary conditions of growth — i.e. those that have been acquired historically — 

 from physical ones, it is not meant that the former do not also owe their existence 

 to physical causes, but only that besides the accidental concurrence of physical 

 conditions, it is also necessary to take into account certain characters which the 

 plant has acquired when in the embryonic condition (in the broadest sense of 

 the term) in the form of definite specialities of organisation through the influence 

 of its parents. 



