THE CELL. 25 



tannin-spheres^ are of frequent occurrence within the cell. Fatty oil 

 often supplants starch during the processes of nutrition. Osmic 

 acid reacting upon these oil-droplets is reduced to black osmium. 

 Alkanua tincture stains the oil-globules red. Tannin-spheres and 

 probably other chemically related substances are stained brown with 

 potassium bichromate. According to Stahl tannic acid serves prima- 

 rily as a "chemical protector" against destructive animals. (For 

 example, snails devoured leached clover-leaves much more greedily 

 than fresh ones.) As the author states, this fact must not be 

 considered as all-inclusive, as tannin is very common in widely 

 different plant-tissues. Pfeffeb' expresses the opinion that 

 tannin is the result of the decomposition of albuminoid substances. 

 G. IvRAUs' (Halle), to whom we are indebted for a more thorough 

 study of this substance, looks upon the antiseptic powers of tannic 

 acid as that property which would suffice to play an important 

 part in the plant economy. This suggestion is highly inter- 

 esting and is worthy of further investigation. Concerning my 

 own investigations with regard to the physiological significance of 

 tannin ^ I came to the conclusion that tannin is erratic', for ex- 

 ample, in summer it wanders from above downward in the bark 

 and vascular system of the stem of Querciis peduncidata ; also that 

 it stands in a genetic relation to the formation of albuminoids. 



lY. THE CELL- WALL. 



As soon as the plant anatomist has progressed somewhat in his 

 work he will observe the significance of one of the points touched 

 upon in this chapter, namely, the growth in thickness and surface 

 of the cell-wall. The most important forms of cells are the result of 

 localized or uniform growth in thickness of the cell-wall. Upon 

 the growth in surface depends in general the size as well as form 

 of every plant-cell and therefore every plant-organ. We will first 

 treat of the internal structure and manner of growth of the cell- 

 wall, then of its chemistry, and finally in a special chapter, of the 

 growth-products of the cell-wall. 



' Physiologie, I. 306. 



' Giundlinieu zu einer Phj'siologie des Gerbstoffes. Leipzig 1888. 



^ Sitzuu^sbericbte der Berliner Akadcmie, 1885, 1887. 



