78 



COMPENDIUM OF GENERAL BOTANY. 



ing cells tliat causes the movement ; in the former the elastic force 

 of the walls themselves is brought into requisition (Schwendener). 

 Physiology leaves it an open question whether or not milk-sap also 

 contains products which are useless in the further processes of nu- 

 trition. Milk-sap may also serve collateral functions. The sap 

 escaping from injured tissues forms an air-tight protective coating. 



Fig. 46. — Smaller vascular bundle of the rhizome of Polypodium glaucophyllum. 



(After Potoni6.) 



The questions relating to the function and use of milk-sap require 

 further study, however. 



Having now obtained a knowledge of the cell-forms, we shall 

 next proceed with the anatomical (in part also the developmental) 

 and physiological discussion of the chief plant-organs of the entire 

 vegetable kingdom, beginning with the mosses, 



(c) The Stein- Structure of Mosses and Vascula/r Cryptogams. 



In the stem of mosses (example, Polytrichum) the central 

 bundle of thin-walled cells represents the vascular system ; a strictly 

 peripheral ring of thick-walled cells functions as the mechanical 

 system (Haberlandt.) ' 



> Beitrage zur Auatomie und Physiologie der Laubmoose, Pringsheim's Jahr- 

 biicher, 1886. 



