50 PLANT LIFE OK THE FAKM. 



concerned, at certain definite places called growing points. 

 The new tissues thus formed are at first wholly cellular, 

 some of the constituent cells retaining the faculty of sub- 

 division, though sometimes not manifesting it till a later 

 period ; while others become modified in various ways as 

 growth goes on, forming wood-cells, fibres, epidermis, and 

 so on. 



Form as Dependent on Growth, If we could sup- 

 pose the degree or intensity of growth to be equal on all 

 sides, and without impediment or obstacle, the result 

 would be a spherical plant ; and such plants do exist, 

 but, in the great majority of cases, the conditions are 

 such that growth is greater in amount in one direction 

 than in another ; or it may be that while pait remains 

 stationary another part grows, the result being a change 

 of form. In the case of the main root and stem, the 

 principal direction of growth is vertically upwards and 

 downwards ; in the case of leaves, the main direction of 

 growth is horizontal, so that while a stem or a root may 

 be divided from above downwards into two nearly equal 

 halves, one half the reflex of the other, a leaf must be 

 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 develop- 

 ment of bulb and root, as of turnip and mangel as con- 

 trasted with that of foliage. In them also must be 



