THE CELL WALL. 



123 



sonous, but there are a uumber that are 

 inert. 



Resins, Oleo-Resins, Gum-Resius, Ethe- 

 real Oils and Balsams, all exist singly or 

 in combination, either in the cell sap or 

 in special passages, excretory passages 

 or glands distributed throughout the 

 plant body. Medicinally they form an 

 important class of products. 



The Gum-Resins are often found in 

 the so-called milky juices of the plants. 

 Balsams generally contain either Ben- 

 zoic or Cinnamic acid. They are very 

 complex mixtures. Caoutchouc is found 

 quite extensively in milky saps, 

 . The Volatile Oils are widely distrib- 

 uted and are to be considered as secre- 



tions. They may be in the cell sap or 

 may exist in special glands or intercellu- 

 lar spaces. 



Ferments form an interesting class of 

 nitrogenous plant products. In the ger- 

 minating seed they are of vital import- 

 ance, converting many products for the 

 use of the young plant. Diastase, Pa- 

 pain, Emulsin and Myrosin are among 

 the most important. 



riant Acids. The aromatic plant 

 acids are widely distributed. Some are 

 important to the constructive metabol- 

 ism of the plant body, but their offices 

 are as yet not fully known, while others 

 are employed in destructive metabolism, 

 or katabolism. 



CHAPTER IV. 



THE CELL WALL. 



The cell wall is the limiting membrane 

 of the cell. When present, which is the 

 rule, it generally consists of a mixed 

 substance called Cellulose, which has 

 been produced by the activity of the pro- 

 toplasm of the cell. At first it is quite 

 delicate, but as growth continues cer- 

 tain modifications take place which may 

 be studied '(6 extern dJ under the mor- 

 phology of the cell wall, for they deal 

 with the markings, thickenings, etc., 

 and have nothing to do with the inter- 

 nal changes or chemical modifications. 



Morphology. 



In the young developing cells the cell 

 wall appears like a thin veil, which 

 grows both in thickness and in surface. 

 How the growth in thickness takes place 

 is not yet perfectly understood, (a) 



Whether the cytoplasm which is found 

 in the cell wall is transformed into cel- 



lulose, or whether its surface layers are 

 added externallyj is a subject of contro- 

 versy. The truth probably is that both 

 processes are at work at the same time. 

 This growth in the cell wall may take 

 place centrifugally, from the inside out- 

 ward, as in the w^alls of spores and 

 pollen grains and epidermal cells; or cen- 

 tripetrally, from the outside inward, as 

 in stone cells, and in spiral and annular 



ducts. 



The growth in thickness gives rise to 

 many irregularities which will be stud- 

 ied later under the subject of pores and 



(a) Zimmermann, Pflanzenzelle, p. 154. 

 Strassburger (ZeU und Zelle-bildung). 



markings. The growth on the surface 

 also gives rise tO other irregularities 

 which are determined by the amount of 

 pressure exerted upon the growing cell. 

 Cells, when uninfluenced by outside 

 forces, tend to grow in a spherical shape, 

 but this condition is rarely found in the 

 higher forms of vegetable life, and un- 

 equal pressures produce cells of almost 

 all degrees of irregularity, hexagonal, 

 cubical, pyramidal, star-shaped, brick- 

 shaped, flattened, etc. 



The degree of variety is enormous, and 

 in many cases cell shapes are so peculiar 

 that they afford an excellent means for 

 the identification of certain drugs. 



Chemistry. 



The chemistry of the cell wall is an 

 intricate problem. The student is re- 

 ferred to Zimniermann, 1. c, for a clear 

 exposition of what can here be hardly 

 more than touched upon. 



Cellulose, which is generally taken as 

 the substance of the typical cell wall, is 

 a carbohydrate, with the empirical for- 

 mula, c/HioOs. Its micro-chemical re- 

 actions will be considered in a special 



chapter. 



Botanists have considered five modifi- 

 cations of the typical cell wall, which are 

 the results of the incrustation or satura- 

 tion of certain chemical substances upon 

 or in the walls of cellulose. These are: 



1. The mucilaginous modification. 



2. Lignification, 



3. Suberization. 



4. Cutinization. 



5. Mineralization. 



