20 Plant Physiology 



fungi, of slime molds, indeed of all plants, is much 

 alike. The cytoplasm is evidently a semiliquid trans- 

 lucent substance, and it contains granules, resulting usually 

 in a distinctly granular appearance. All protoplasm is 

 readily killed by a solution of iodine by which it is also 

 stained yellowish brown. This reagent is therefore con- 

 venient in demonstrating protoplasm in cells where it is 

 not readily visible. Moreover, the use of a strong salt 

 solution, causing contraction of the cytoplasm from the 

 cell-wall, is also important in demonstration. The outer 

 margin of the cytoplasm, or the margin bordering a vacu- 

 ole, possesses important physiological characters, and for 

 convenience it is called the plasma membrane. The 

 cytoplasm may inclose food-materials not easily distin- 

 guished from the usual protoplasmic granules. 



The structure of protoplasm has received much con- 

 sideration, and three noteworthy conceptions of its form 

 have been advanced as follows : (1) the netted or reticu- 

 lum theory; (2) the fibrillar theory, and (3) the alveolar 

 or foam theory. The present tendency is to regard it as 

 unnecessary to assign a definite structure persistent under 

 all conditions, and physiologically it is logical to believe 

 that the structure is simply a manifestation of a type of 

 activity. Stains of various sorts have been most im- 

 portant in the study of cytoplasm as well as of nucleus. 

 The chemistry, movement, and special responses of pro- 

 toplasm in general are considered later. 



13. The nucleus. The nucleus presents the appear- 

 ance of a dense or refractive protoplasmic mass, and it 

 does contain denser bodies. It is often nearly spherical, 

 but in certain old or specialized cells it may be irregular 



