14 METABOLISM 



to the colouring matter as to the cane sugar. In certain cells, however (nec- 

 taries, Lecture V ; roothairs, Lecture VIII ; endosperm cells, Lectures XIII, 

 XIV), the protoplasm is found to be permeable to various substances, such as 

 sugar, asparagin, peptone and proteid. In other cases we must admit the fact 

 of exosmosis without being able to say definitely what the substances are which 

 undergo exosmosis. Take, for example, Bacteria, which require organic com- 

 pounds for their nourishment ; if these organisms are able to thrive at the 

 expense of green Algae it is manifest that exosmosis of organic materials must 

 take place from the cells of the Algae. Again, the seeds of Lathraea and 

 Orobanche germinate only in the presence of a host plant, and this fact cannot 

 be explained otherwise than by assuming that an exosmosis of organic com- 

 pounds takes place from the host. But in general it may be said, as shown by 

 DE VRIES in the case of beet, that no exosmosis of the contents of the vacuole 

 takes place. 



A comprehensive knowledge of the phenomena connected with the per- 

 meability of protoplasm cannot, however, be obtained by a study of exosmosis 

 only, because we are limited to the accidental occurrence of substances in the 

 vacuole, our acquaintance with which, moreover, is only rarely exact. In 

 studying endosmosis, on the other hand, we are unlimited in our choice of the 

 materials which we may present to the cell ; whether we obtain results or not 

 will depend solely on our ability to establish a definite criterion for absorption or 

 non-absorption. Such a criterion of protoplasmic impermeability, indeed the 



Fig. 4. Young cells from the cortical parenchyma of the peduncle of Cephalaria leucantha. m, cell-wall ; 

 pi, protoplasm ; z, vacuole. /, in water ; II, in a 4% solution of potassium nitrate ; Iff, in a 6% solution of the 

 same; IV, in a 10% solution of the same. (After DE VRIES, 1877.) 



best one available, is plasmolysis, a phenomenon first enunciated by NAGELI 

 (1855), and afterwards so thoroughly worked out by DE VRIES (1877) and 

 PFEFFER (1877), that at the present day it is perhaps not only amongst the most 

 fully investigated phenomena in plant physiology, but, far beyond the special 

 branch of science concerned, has become a subject of the greatest interest to 

 students of general chemistry. 



Suppose we revert to our alga cell and assume that we know the actual 

 contents of the fluid in the vacuole, that it contains a 10 per cent, solution of 

 cane sugar ; what happens when this cell is placed in pure water we have 

 already seen, but what will happen if we place such a cell in a solution of cane 

 sugar which has, let us say, a lower concentration than that presented by the 

 cell-sap ? From such a solution the vacuole will obviously withdraw less fluid 

 than from pure water, less and less in fact as the fluid without becomes more 

 and more concentrated. When the concentration of the fluid outside is equal 

 to that of the fluid in the vacuole, no water will be taken up at all from the 

 surroundings ; when, on the other hand, the fluid outside has a higher concen- 



