8 PLANT PHYSIOLOGY 



18, 1. 14, for had best be avoided read cannot be discussed here (see Lecture 

 XXXI). 



1. 27, for atmospheres in read atmospheres even in 



I. 28, after onion read and as much as forty atmospheres in the pulvini 

 of grasses. 



II. 35-6, for manifestly . . . accumulated read merely an incidental effect of 

 the nature of the stored 



19, 11. 52-3, for alteration ... a part read effect is due solely to a change in 

 the acidity of the cell-sap. 



21, 11. 12-30, for On this question . . . physical manner read NATHANSOHN'S 

 osmotic researches (1902-4) have made us acquainted with a series of pheno- 

 mena which are not to be interpreted in a purely physical manner. He found 

 that many protoplasts are permeable to salts occurring in the environment, 

 but that these do not go on entering until the concentration inside is the same 

 as that outside ; he found, on the other hand, that osmotic equilibrium ensued 

 when the concentration inside reached a certain definite fraction of the con- 

 centration outside. Should the concentration of the fluid outside increase, 

 further entry takes place ; if it decreases the salt undergoes exosmosis until 

 this definite relationship between the solutions inside and outside is re-estab- 

 lished. Under these circumstances, it would be quite possible to induce per- 

 manent plasmolysis with a substance, although it continued to enter the cell. 

 In this way we might explain the contradiction previously noted, namely, 

 that all nutrients (salts, sugars) which necessarily reach the interior of the 

 cell must induce plasmolysis. It is very desirable that these observations of 

 NATHANSOHN'S should receive confirmation by further research. 



It should be noted here that all the substances required by plants do not 

 behave like salts and sugar. The essential gases, oxygen and carbon-dioxide 

 more especially, may be shown to enter the protoplasm readily (J. K. GOEBEL, 

 1903). Protoplasm is also permeable to nitrogen and hydrogen, as numberless 

 instances prove. Since, however, many lower organisms contain ' gas-vacuoles ', 

 the surrounding protoplasm must be impermeable to the gases in them. 



1. 52, for So far as purely physical . . . to the end of the lecture, read OVERTON 

 (1895-1900) has made several attempts to refer the absorption of the materials 

 by the plasma to the principle of ' selective solubility ' ; viz. only those sub- 

 stances can enter the cell that are soluble in the plasmatic membrane. OVER- 

 TON'S researches indeed show that the substances that enter most rapidly 

 are those, such as alcohol, ether, chloroform, chloralhydrate, &c. (for a complete 

 list see OVERTON, 1899), that are characterized by their ready solubility in 

 fatty oils. Since, however, there are several reasons for believing that the 

 plasmatic membrane cannot consist of oil, OVERTON puts forward the hypo- 

 thesis that it might be, in great part, composed of cholesterin. In fact, he has 

 shown that solubility in cholesterin agrees much better with absorption by 

 protoplasm than solubility in oil, and that this is especially true of aniline 

 dyes. Hence, according to OVERTON (1899) the absorption of a fatty oil 

 (Lecture XIII) and of xylol through a cholesterin layer would be just as in- 

 telligible as the absorption of water. In fact, cholesterin can absorb water, 

 but NATHANSOHN (1904) has shown that its capacity for dissolving becomes 

 completely altered by this absorption. For this and other reasons (comp. 

 PFEFFER, Phys. I. no, note) NATHANSOHN rejects the idea of a homogeneous 

 external layer, and believes that the periphery of the plasma consists of a kind 

 of mosaic of alternating cholesterin and living protoplasmic particles. The 

 cholesterin would thus determine the permeability of the membrane for sub- 

 stances soluble in oil, while the protoplasmic particles would permit the passage 



