VARIOUS CAUSES OF GROWTH, 695 



and non-growing parts (as a bulb suspended and putting out leaves in the air), is not 

 nourished as such from without. The growth of certain parts is therefore no indication 

 of nutrition of the whole. Still less necessary is the connection between growth and 

 nutrition from without ; the special organs of nutrition, the green leaves, do not grow 

 after they are mature, although they carry on the process of nutrition. The two pro- 

 cesses may coincide both in place and time, /'. e. in the same cell; but may also be 

 separated in both space and time ; and this is indeed usually the case, as has been suffi- 

 ciently shown in Sect. 5. 



Sect. 12. Various causes of Growth. Growth, like vital activity, takes place 

 only when certain favourable external conditions coexist. These are the presence 

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

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

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

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

 ing 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 w^hile the secondary condi- 

 tions intervene only in certain cases, and exert their modifying influence very differ- 

 ently on the corresponding parts of diff'erent plants. 



The conditions spoken of as Necessary and Secondary Conditions depend upon 

 ihe environment of the plant, and act upon it from without. They may therefore 

 be described as External Conditions or causes of growth, in contradistinction to the 

 Internal 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 

 arc 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 

 Irom an oak-root. The diff"erences of these processes of growth is at once manifest 

 in the diff"erence 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 



