26 CELL-DIVISION 



rectangular or polygonal, and fit closely together, without any 

 interspaces. A little further back, representing an older phase, 

 they increase appreciably in size and often become more or less 

 rounded off, as a result of which small spaces, the intercellular 

 spaces (Fig. 27, i.p.), appear between them, particularly at the 

 angles. 



The increase in volume of the cells is almost entirely due to 

 the intake of water which results from the formation of substances 

 that bring about an increase in osmotic pressure. These sub- 

 stances are produced during the living processes carried on in 

 the cells, and, as a consequence of their solution, a number of 

 small vacuoles containing cell-sap appear in the cytoplasm. 

 These vacuoles gradually increase in size, and ultimately coalesce 

 to form one (e.g. Onion, Fig. i) or few large vacuoles. Apart 

 from its presence in the vacuoles, however, water also permeates 

 the protoplasm and cell-wall, forming the so-called imbibition- 

 water. 



With the gradual assumption of the mature condition the cells 

 generally lose their power of division, though this faculty may 

 be again stimulated into activity, as when a plant is wounded. 

 The growth of any organ of the plant is thus mainly the outcome 

 of multiplication of cells at its growing point, their subsequent 

 increase in volume leading both to a lengthening and gradual 

 widening of the tissues. As they reach maturity the new units 

 mostly become specialised in various directions to serve diverse 

 needs. Those, however, which have not undergone profound 

 alteration, but have retained their juvenile character, often 

 retain also their power of division and capacity to develop in 

 different ways as the demands of the organism may dictate. 

 It is the visible enlargement of plants that is popularly spoken 

 of as growth, but in reality this involves the three phases of 

 cell-division, enlargement, and the final differentiation which 

 will form the subject-matter of the next chapter. 



