30 COMPENDIUM OF GENERAL BOTANY. 



Nageli in answering the how of starcli and cell- wall growth accord- 

 ing to the intussusception theory expressly omits i\\ewhy of cell-wall 

 growth. To find the causes for this why is impossible according to 

 his own statement because of the lack of knowledge in regard to 

 molecular cohesion, tension in various directions, distribution of 

 water and cell-wall substance, etc. In starch-grains a similar dif- 

 ficulty is met with, namely, the phenomenon of the smallest par- 

 ticles (micellae) surrounding themselves with a layer of water of 

 definite thickness. This also depends upon molecular forces "whose 

 nature is unknown." 



B. CHEMICAL COMPOSITION AND SECONDARY CHANGES IN THE 



CELL- WALL. 



The cell-wall is the product of living protoplasm ; that is, a car- 

 bohydrate is produced from an albuminoid. The details of this 

 chemical process are unknown. 



The wall of young cells consists principally of cellulose. Its form 

 ula, similar to that of starch, is given as n (C 6 H 10 O 5 ). Its molecular 

 weight is therefore unknown. Of the microchemical reactions the 

 following may be mentioned : swelling and final solution in con- 

 centrated sulphuric acid, in chromic acid, and arnmoniacal oxide of 

 copper; blue coloration w T ith a solution of iodine, dilute sulphuric 

 acid, and chlor-zinc-iodine. The cellulose of fungi is a modification 

 of cellulose which shows these reactions only after having been 

 treated for weeks with KHO. Before being so treated it gives a 

 yellowish or brown reaction with iodine and sulphuric acid ; it is 

 also less affected by acids. Another modification is the " reserve 

 celluloss," which will receive only brief mention. 



Very frequently cellulose undergoes a change in composition, 

 either throughout its entire thickness or only in certain layers or 

 areas. The following are the most important forms of changes oc- 

 curring in the cell-wall substance : (a) corky, (5) ligneous, (c) muci- 

 laginous, (d) deposition of coloring material and inorganic com- 

 pounds (mineral matter). Our knowledge concerning these is to a 

 certain extent very imperfect. 



(a) In a later discussion upon protection against loss of water 

 (epidermal system) the value of corky cell-walls will become more 

 apparent. The most useful property of corky or " cuticularized" 

 (suberized) membranes is the great, though not absolute, imper- 



