366 DIVISION III. MODE OF LIFE OF THE FUNGI. 



germ-tube causes a solution of continuity in the membrane which is being perforated 

 in the line of the perforation. Where the perforation is a hole which is permanently 

 fitted by the tube, as for instance in the chitinous skin of insects and in many 

 plant-cell-membranes, it is natural to suppose that the hole is caused by partial 

 dissolution of the membrane, and that this dissolution is the result of a fermentation 

 which proceeds from the Fungus (see page 355). The case is somewhat different 

 where, as in the perforating germs of Uredineae and Synchytrieae, the holes soon 

 close up again. Here it may at least be asked, whether the perforating process from 

 the tube or spore attached to the epidermis of the host does not split the membrane 

 by purely mechanical means, much in the same way as a sharp needle divides the 

 plate of caoutchouc which is pierced by it, and whether the small split does not close 

 up again purely in consequence of the elasticity of the membrane, as is the case in the 

 plate of caoutchouc after the needle is withdrawn, when the turgescence in the 

 perforating process sinks to nothing in consequence of the growth of the germ-tube. 

 The deflection of Isaria by Melanospora can only be explained by assuming that 

 some substance is secreted by the germinating spore which exercises a specific 

 attraction on the growing hyphae of Isaria. 



On the other hand, the effects produced by the host on the parasite which is 

 germinating or about to germinate are more varied than those of the parasite on its 

 host, and a greater number of physiological questions are connected with them. We 

 do not know the cause why germ-tubes penetrate through stomata, why some spores 

 attach themselves to stomata, others to the surfaces of membranes, why the hyphae of 

 the Lichen-fungi clasp the cells of the Algae in their embrace, and so on. We may 

 make an attempt to explain the fact that certain parasitic germ-tubes perforate the 

 epidermis of one species of Phanerogams and not of others by assuming the secretion 

 of specific ferments and specific differences in the structure, firmness or cuticularisa- 

 tion of the membranes of the host ; but it is scarcely possible to explain from the 

 data before us why a germ-tube bends its extremity towards the membrane of the 

 proper host and not towards every membrane or moist surface, or even turns only 

 towards the outer edges of the lateral walls of the epidermal cells, as in the cases 

 mentioned above. Are specific physical irritations brought into play in these cases, 

 or chemical stimulations, -which may be supposed to operate through unknown 

 secretions from the surface of the host, with certain specific reactions on the part of 

 the parasite ? Questions of this kind present themselves here at every turn, as they 

 do in some similar phenomena which occur in the saprophytes, and offer very 

 promising subjects for experimental enquiry 1 . 



SECTION CIII. The parasite pursues its further development as soon as 

 it has attached itself to the host, while the living host reacts on the plant which 

 occupies it permanently and sometimes continues to spread through it ; this reaction 

 varies extremely in different cases, being everywhere determined by the specific qualities 

 of the symbionts. The chief phenomena attending these processes will be briefly 



1 Pfeffer's work on chemical stimuli, which appeared some time after the above words were 

 written, has shown more distinctly the way to the answering of these questions, and has made some 

 advance upon it. See Ber. d. Deutschen Botan. Ges. 1883, and Unters. d. Bot. Instil, zu Tubingen, I, 

 Heft 3 (1884). 



