PLANT PHYSIOLOGY. 



79 



carbonate, etc.). These are chiefly, if not entirely, ab- 

 sorbed by the roots, and in many 

 plants the tubercles formed by 

 parasitic organisms have been 

 thought to aid in the process (Fig. 

 47). In the higher plants it has 

 been shown that these compounds 

 undergo decomposition and re- 

 construction in the leaf, the re- 

 sult being the formation of proteid 

 substances; but it is also held 

 that probably every living cell is 

 capable of taking part in these 

 processes. 



138. Sulphur-assimilation. — Of 

 the assimilation of sulphur still less 

 is known than in the case of nitro- 

 gen. We know that sulphur is absorbed in the form of sul- 

 phates (of ammonia, potash, lime, and magnesia), and some 

 of these are to be found in the cells of plants, but where and 

 how they are broken up is not known. It has been suggested 

 that the crystals of calcium oxalate which occur in many 

 plants are residua of chemical changes by which sulphur 

 was set free from calcium sulphate. If true, this would 

 show that the assimilation of sulphur takes place in all 

 active tissues of the plant. 



139. Assimilation of other Substances. — Phosphorus is 

 absorbed in the phosphate of lime, which undergoes de- 

 composition in the tissues, but the details of the process are 

 not known. A number of other substances — e.g., potas- 

 sium, calciiim, iron, etc. — enter into the proper food of 



. plants as solutions of their salts, which afterwards undergo 



(Vicia; 



slightly reduced. 



Strasburger.) 



