Ch. hi, 5] PROTOPLAS^I 35 



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

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

 pass into and along the sieve-tubes. ^leantime 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 concUtions remain 

 favorable. Meantime something similar, as to the details 

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

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

 bright day through the summer. 



5. The Charactehistics of Protoplasm 



All study of physiological processes leads directly to pro- 

 toplasm, the h\'ing part of the organism. It is a perfectly 

 definite naaterial, with distinctive appearance and properties, 

 and it alone, of all the imrumerable materials or .substances 

 in nature, is aUve. 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 imderstand it, one must see the material for 

 himself in the laboratory. 



In most plant cells, as in those of the leaf lateh' stuched 

 (page 29), the U\'ing 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 Hke that of the accompam-ing 

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

 as a fining around tfie walls of the cyfindrical cell, but also 

 in irregular threads across the sap cavity. Protoplasm in 



