GROWTH. 49 



divided horizontally, and the upper surface and the lower 

 surface are commonly different. Variations in form are 

 dependent not only on variations in the direction of growth, 

 but upon the place where growth is taking place, and 

 whether it be limited, as in the case of the growing points 

 and cambium tissue already referred to, or general 

 throughout the mass. 



The form of the plant or of any particular part of it 

 will also of necessity vary according as the growth is con- 

 tinuous or intermittent, equal or unequal. These are all 

 circumstances readily understood, and they are referred to 

 here because they furnish the reasons for the development 

 of bulb and root, as of turnip and mangel as contrasted 

 with that of foliage. In them also must be sought the 

 explanation of thin ears of wheat or defective hay crops. 



Phenomena associated with Growth and Activity. 

 Under this heading may be mentioned the various move- 

 ments in the liquid (cell-sap), contained within the cells, 

 and in the protoplasm, which are observed in living cells, 

 especially in those in which the vital processes are most 

 active. Here also may be mentioned the movements asso- 

 ciated more or less directly with growth, and the influence 

 of various agencies, such as of gravitation, light, tempera- 

 ture, &c., on plants and their several organs. These pheno- 

 mena and these influences are more manifest during active 

 growth ; and when they occur in living organs which have 

 ceased their actual growth, they do not essentially differ, 

 though they may do so in degree, and may also, to some 

 extent, be modified in character. 



Movements as Dependent on Growth. But a few 



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