The Substance Which Is Alive in Plants 161 



The sap-cavities are as simple in structure as they look. In 

 very young cells they are absent, as a later picture illustrates 

 (figure 129) ; but in those that are older little rifts appear in the 

 cytoplasm and gradually grow larger until finally, in the fully 

 mature cell, they become merged into a single cavity of very large 

 size, as the figure of the conventionalized cell clearly shows 

 (figure 48). This cavity is filled with water in which sugar and 

 other useful substances are dissolved. It thus represents a store- 

 house of useful materials, but serves secondary functions, like- 

 wise, in pressing the cytoplasm against the wall, and in aiding 

 growth, by methods which will later be described in the suitable 

 chapters. 



It is of interest to note that not only does new protoplasm in 

 general originate only from preexisting protoplasm, but new 

 cells originate only from cells, nuclei from nuclei, and plastids 

 from plastids; while the same thing has been claimed even 

 for cell-wall and sap-cavities, or rather for the part of the 

 cytoplasm which forms them, though here the evidence is not 

 conclusive. 



The question is now appropriate, why does protoplasm sep- 

 arate into cells at all, and what makes them of such minute size 

 as they are? It is sometimes assumed that the plant-structure 

 becomes cut up into cells in order to provide structural units of 

 convenient size and form, after the manner of the bricks of the 

 builder; but the analogy is wholly misleading, since the skeleton 

 of plants is not built at all from originally separate units, as brick 

 buildings are, but rather from a continuous mass of cell-wall 

 substance comparable with the cement construction now coming 

 into use. Another explanation maintains that each nucleus 

 can control only a limited quantity of cytoplasm; and thus are 

 established certain administrative units between which, natu- 

 rally enough, the walls are built, the resultant being cells. As 

 to the reasons why their sizes are so small as to require a mi- 

 croscope to show them at all, we have again a few guesses, but no 



