1 62 The Living Plant 



exact knowledge. Possibly the chromosomes need a certain 

 size in order to perform their functions; this would establish the 

 size of the nucleus, and hence (on the explanation above noted) 

 of the cell. Another explanation rests on a mathematical basis. 

 We may assume that the typical cell is a sphere filled solidly 

 with protoplasm. When a sphere enlarges in size, its bulk in- 

 creases much faster than its surface, the bulk increasing as the 

 cube of its diameter and the surface as the square thereof. Ob- 

 viously it is through this surface that the spherical cell must 

 absorb the oxygen for the respiration of the entire bulk of its 

 protoplasm; it is therefore quite evident that there must be a 

 certain size of the cell in which the surface is just sufficient to 

 aerate the bulk of protoplasm within, — and that size would de- 

 termine the average cell size. If the cell were to grow larger its 

 surface would not suffice to aerate the bulk, while if smaller the 

 surface would be needlessly great. In a general way this con- 

 clusion is sustained by the fact that where conditions for respira- 

 tion are harder the cells are smaller, and vice versa. Moreover, 

 the very largest cells occur in places well situated for aeration, 

 and besides, possess accessory arrangements, — viz., the flattening 

 cf the protoplasm in a thin layer against the wall (figure 45), and 

 protoplasmic streaming, — which aid to that end. These features 

 prevail in the hair cells already observed by the reader, and in 

 consequence those cells become large enough to be visible to the 

 eye without the aid of a lens. In general, therefore, it does seem 

 true that the relation of bulk to surface in a solid as affecting res- 

 piration is one of the principal factors, if not indeed the principal 

 one, in making the size of cells what it is. 



In comparing the functions of the cells of plants with those in 

 animals, it soon becomes obvious that plant cells exhibit a far 

 lower degree of division of labor; and this involves a remarkable 

 consequence. It seems to be a fact that when protoplasm con- 

 tinues to perform a single function for long periods of time, as it 

 does in the highly-specialized organs of the anim.al body, it grows 



