CH. Ill, 5] PROTOPLASM 35 



veinlet, of which it enters the sheath cells and there passes 

 along the veins to the stem, while the proteins in like manner 

 pass into and along the sieve-tubes. Meantime the mole- 

 cules of oxygen are moving out 'of the chloroplastids through 

 protoplasm and wall to the "nearest air passages, and along 

 them to the stomata and the external air, passing the entering 

 carbon dioxide en route. The movement of these materials 

 in their paths is of course impelled by definite and adequate 

 forces, and the mechanism is capable of continuous action, 

 which proceeds without break so long as the conditions remain 

 favorable. Meantime something similar, as to the details 

 of which we are ignorant, must be happening in the synthesis 

 of proteins. That is what every green leaf is doing every 

 bright day through the summer. 



5. THE 'CHARACTERISTICS OF PROTOPLASM 



All study of physiological processes leads directly to pro- 

 toplasm, the living part of the organism. It is a perfectly 

 definite material, with distinctive appearance and properties, 

 and it alone, of all the innumerable materials or substances 

 in nature, is alive. In Huxley's famous phrase, protoplasm 

 is the physical basis of life. 



Despite its importance, the protoplasm of plant cells has 

 an appearance so inconspicuous as to make it most difficult 

 either to describe or to represent in pictures. Therefore 

 in order to understand it, one must see the material for 

 himself in the laboratory. 



In most plant cells, as in those of the leaf lately studied 

 (page 29), the living protoplasm is rendered almost in- 

 visible by the thicker and denser walls which inclose it. 

 However, many epidermal hairs have walls so transparent 

 as to show the protoplasm clearly, in which case the mi- 

 croscope reveals an aspect like that of the accompanying 

 picture (Fig. 12). The protoplasm here extends not only 

 as a lining around the walls of the cylindrical cell, but also 

 in irregular threads across the sap cavity. Protoplasm in 



