22 BOTANY OF THE LIVING PLANT 



are seen, till the mature parts are reached. Various changes appear 

 in the cells. They alter their form and the character of their walls 

 and contents. As a rule the cells enlarge greatly. An important 

 change in the cytoplasm, which is usual in plant-cells, accompanies 

 this growth. It is known as vacuolization, and it may be well illus- 

 trated in the cells developing into the pith or cortex of an ordinary 

 stem. Starting from the embryonic state, where the wall is very 

 thin, and the cytoplasm and nucleus fill the whole space enclosed by 

 it (Fig. 12, A), the volume begins to increase with age and the wall 

 thickens. But the volume of the cytoplasm does not keep pace with 

 that of the whole cell, and vesicles or drops of clear liquid appear 

 within it. 1 These are called vacuoles, and they are filled with vacuole 

 fluid, or cell-sap, which is water with certain substances dissolved in 

 it (Fig. 12, B). The vacuoles are always completely enclosed in the 

 cytoplasm, which controls them and the substances dissolved in them. 

 The vacuoles may vary in number, size and position, and the position 

 of the nucleus is also inconstant ; sometimes it lies laterally in the 

 peripheral cytoplasm ; usually it is central. As the vacuoles enlarge 

 they may run together, and finally form a continuous cavity, in the 

 middle of which the nucleus is frequently suspended by radiating 

 threads of cytoplasm (Fig. 12, C). A condition is thus arrived at 

 which is characteristic of many cells in the mature state. 



Other cells may undergo changes of a much more marked character 

 than this, as they pass from the young to the mature condition. 

 Such changes fit them for performing their several functions in that 

 commonwealth of units of which the mature plant consists. Division 

 of labour is characteristic of the mature tissues of all the Higher 

 Plants, and it is the structural differentiation of the constituent cells 

 that makes this possible. It will be well here to explain briefly the 

 chief changes which may be traced in the different tissues, as they 

 are developed from the uniform embryonic cells that compose the 

 apical cone. 2 



(i) Changes in Size and Shape of the Cells. 



Practically all cells grow as they mature, and a simple case in 

 which the change of form is only slight has been seen in Fig. 12 (A-C). 



1 It is now known that minute vacuoles are present in embryonic cells. 



2 This analysis of the changes during differentiation finds its proper place 

 here, and forms the natural foundation for any rational study of mature 

 tissues. But it is open to the student to read it either before or after those 

 tissues have been described. 



