2 THE PLANT CELL 



taken collectively, or to the box alone when the living parts have 

 disappeared, and the word protoplast will be used to designate 

 the living part alone. 



The box or cell-wall is manufactured by the protoplast for 

 its own stability and protection, and the protoplast must, there- 

 fore, exist before the wall which encloses it. Since the pro- 

 toplast is the living structural and physiological unit of the plant 

 body, and since everything that the plant performs is really the 

 work of its individual protoplasts, it necessarily follows that a 

 satisfactory comprehension of plant anatomy and physiology 

 is impossible without a knowledge of the nature of the proto- 

 plast itself. 



The Protoplast. If we study under high magnification 

 sections of an onion root tip that has been fixed, imbedded, sec- 

 tioned, and put up in permanent, stained mounts as directed in 

 the chapter on The Preparation of Sections, we shall find near 

 the root apex young cells that have not yet secreted their entire 

 cell-walls, and we shall often find some degree of plasmolysis or 

 shrinkage of the protoplasts away from their cell-walls so that 

 their contour can precisely be made out. Fig. i, A, shows us 

 such a protoplast. In the root all of the parts here shown were 

 living, excepting possibly the nucleolus b. The cytoplasm c 

 constitutes the bulk of the protoplast; the nucleus a is imbedded 

 in the cytoplasm; the plasma membrane d is a specialized outer 

 portion of the cytoplasm; the plastids e are relatively very min- 

 ute parts of the protoplast, but have a special work to do, as will 

 be learned later on. In older cells farther back from the root 

 tip we find that the protoplasts have secreted a wall, as shown in 

 Fig. i,B. 



.Comparing A and B of Fig. i, we see that in the younger 

 protoplast the cytoplasm looks something like a sponge w r ith 

 very fine meshes, while in the older protoplast the cytoplasm 

 does not so completely fill out the space inclosed by the plas- 

 matic membrane, some of the meshes having widened into rela- 

 tively large rifts. It seems that as the protoplast grows older 

 the cytoplasm does not keep pace with the general increase in 



