266 THE POWER OF RESISTANCE TO EXTREMES 



innocuous. A poison, may be converted into a harmless compound during 

 its absorption by the protoplasm, or during its passage through the cell- 

 wall. Hence it arises that many flowering plants can accumulate large 

 quantities of zinc, although the diosmosing salts of this metal are highly 

 poisonous. The accumulation of a large quantity of a poisonous substance 

 in a plant does not in fact necessarily show that the protoplasts are 

 indifferent to this poison. Furthermore, as in the case of the cuticle, a 

 relatively impermeable plasmatic membrane may hinder the absorption 

 and injurious action of a soluble poison. 



The influence of a particular poison is not merely a question of 

 absorption or non-absorption, but is probably mainly determined by the 

 specific nature of the protoplasm. Thus different species are unequally 

 resistant to free acids, although these readily penetrate the protoplasm. 

 The same applies to alcohol, ether, chloroform, and other substances which 

 are unequally poisonous to different plants 1 . 



On the other hand, the resistance of Penicillium glancnm to copper 

 salts is due to their non-penetration of the protoplasm. Pulst found so 

 little copper in mycelia grown on strong cupric solutions, that the small 

 amount was probably due to mere adherence, or to the absorption of 

 copper by the cell-walls and dead protoplasts. In any case the amount 

 was less than if the copper had penetrated to the cell-sap, for the slowest 

 penetration would result on prolonged cultivation in the equalization of 

 the internal and external percentages, even in the absence of any passive 

 secretion. 



The ectoplasmic membrane of Penicillium is therefore so constituted as to 

 be able to retain its living properties when in contact with a solution of copper, 

 for if it were killed the poison would at once penetrate to the protoplasm. This 

 occurs in the case of most poisonous metallic salts, and it is uncertain whether 

 the materials of the plasmatic membrane in Penicillium do not react to copper 

 salts, or whether they become covered by a precipitation membrane impermeable 

 to salts of copper 2 , 



1896, Bd. XLI, p. 403; 1899, Bd. XLIV, p. 108) states that the salts of alkaloids penetrate more 

 slowly than the free bases, and hence are less poisonous. [This might also be the effect of 

 neutralization. Thus a solution of veratrine is alkaline; its nitrate is neutral and hence less 

 poisonous. Cf. Ewart, On the Physics and Physiology of Protoplasmic Streaming in Plants, Clar. 

 Press, 1903, pp. 83, 84.] 



1 Overt on (Vierteljahrsschrift d. Naturf. Ges. zu Zurich, 1899, Bd. XLIV, p. 106) has recently 

 shown that various substances, including carbon monoxide, rapidly penetrate the protoplast. 



3 Overton (1. c. ; Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 670) concludes that only those bodies 

 are absorbed which are soluble in oil or fatty substances, such as cholesterin and lecithin, which 

 impregnate the plasmatic membrane. Such impregnation may play an important part in 

 regulating absorption, but nevertheless the proteid components of the plasmatic membrane are 

 also of importance, and their existence is shown by the rigor induced by dilute acids and by 

 mercuric chloride. Further research will undoubtedly show that many substances will readily 

 penetrate which are insoluble or nearly so in fats and oils. Indeed some of the salts of copper 



