EXTERNAL CAUSES OF GROWTH AND FORMATION. II 317 



are equally poisonous to plants and animals, while others affect even closely- 

 related organisms in entirely different ways. This is explained in part by the 

 fact that protoplasm is not identical in all organisms, but more especially that 

 the relative rate of entry of the poison into the protoplasm is most varied. Thus 

 PuLST (1902) has shown that sulphate of copper, which is generally a virulent 

 poison, like most of the salts of the heavy metals, has no e^ect on Penicillium, 

 only because it is not absorbed by this fungus. It is quite inexplicable, how- 

 ever, why sugar and peptone, which are valuable nutrients to the majority 

 of plants and not in themselves in any sense injurious, should be extremely 

 poisonous to nitro-Bacteria (p. 229). 



It is of special interest to recall the fact already mentioned that many 

 poisons have not only no injurious effect in dilute solution, but are actually of 

 service to the organism by stimulating its respiratory and metabolic activity. 



We may also recognize that many substances act as chemical 'stimuli' which 

 are not poisonous ; even nutrients may act as such, though that is not their 

 chief function ; at all events this aspect of their activity is not to be associated 

 with their nutritive value. Examples of these phenomena are seen in cases 

 where growth is initiated by the presence of substances whose nature is in some 

 cases known, in some cases unknown, and more especially in the germination 

 of spores and pollen-grains. For example, the pollen-grains of certain species 

 of Mussaenda germinate in distilled water, according to Burck (1900), but only 

 when a small portion of the stigma is added to the water. Apparently the 

 stigma contains levulose, for of all the materials experimented with, and especi- 

 ally sugars, this was the only one which acted in this way even when merely very 

 minute traces were present. It is not easy to understand why dextrose should 

 not act in the same way if it be merely the addition of some material needed 

 for growth that was wanted. If, however, levulose be considered merely as 

 a stimulant to growth, then the extreme specialization becomes intelligible. 

 Further, closely-allied forms show great differences in this respect ; the pollen of 

 Pavetta javanica germinates only in an extract of the stigmas of that plant or of 

 Pavetta fulgens, but not in that of other species. Finally, it may be noted that 

 according to De Bary (1884) spores of Completoria, Protomyces, a.nd Synchytriiim 

 germinate as a rule only on their host-plants, and that OrobancJie and Lathraea 

 develop their haustoria only in the neighbourhood of the roots of their hosts. 

 It cannot be doubted that in these cases also some growth stimulant of a definite 

 chemical nature is given off from the host-plant, but such substances have not 

 as yet been isolated. 



That chemical stimuli are also able to act in a formative manner need not be 

 further exemplified, since we have already seen (p. 249) how Basidiohohts behaves 

 under such conditions. Later on (galls, p. 320) we shall have an opportunity 

 of studying some morphogenetic results of chemical stimuli. 



We may conclude this account of the effect of materials on growth by 

 considering water, which in addition to its chemical effect has also undoubtedly 

 an important physical influence on imbibition and turgescence, and hence on the 

 elasticity and pressure phenomena of the plant. When water is withdrawn, 

 as a rule all vital activities, including growth, come to an end, but certain plants 

 retain capacity for life even in the desiccated condition. Many mosses, lichens, 

 and even species of Selaginella are able to endure air-drying without suffering 

 permanent injury, and to start growing again as soon as water is once more 

 supplied. The majority of plants, however, die when once dried in the vegetative 

 state. The capacity for withstanding desiccation is very general in the resting 

 stages of plants, e. g. ' spores ' and seeds ; and in many of the lower plants the 

 actual formation of such bodies is dependent on the withdrawal of water. These 

 resting stages can frequently withstand much higher degrees of desiccation than 

 can be obtained by simple air-drying ; many seeds, for example, can withstand 



