SIMPLE RIGID SUPPORTING TISSUES 



59 



axiomatic and can be easily demonstrated. The development of the cell 

 and its rigid connective-tissue organ is shown to great advantage in a longi- 

 tudinal section of a growing root-tip. We have selected a root-tip from 

 the chestnut seedling. Young cells selected from near the tip are solid 

 masses of cytoplasm containing a nucleus (Fig. 60, A). They are sur- 

 rounded by a well-defined cell-wall of some strength and thickness. This 

 wall is the connective- tissue material and is visible from the very first. 

 When a little further developed, a vacuole appears in the cytoplasm, and 

 later one or more others appear near the first. As the cell grows larger, 

 these increase in size and push the nucleus over to one side. In an older 

 cell the two or more vacuoles have broken through the intervening 

 walls of cytoplasm and united to form one large vacuole (Fig. 60, B). 



FIG. 60. A-C. Three stages in the development of a plant cell as an organ of rigidity. 



In some cases more than one vacuole is left. At this time it will be seen 

 that the size of the cell is very much greater, especially in length. 



The vacuoles increase in length until we find a mere shell of proto- 

 plasm lining the thick cell-wall and a larger mass of protoplasm some- 

 where nearer the center (Fig. 60, C). This mass contains the nucleus 

 and is connected with the peripheral shell by strands and bridges of cyto- 

 plasm. During life, a circulation of the whole mass keeps the nucleus 

 in touch with, and in command of, its most distant portions of cytoplasm. 

 Thus the peripheral shell is able to make and preserve intact the impor- 

 tant cell-wall, which is the cell-organ in this case and provides rigidity. 

 The cell has enlarged during this development. 



The same result, a relatively greater surface on a given mass of 

 cytoplasm, is thus attained as it was attained in the Leidig's cell of the 

 lobster. On the other hand, one should notice that there is no renewal, 

 that the cell once formed goes through its direct form of development and 



