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 ailached 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plate of caoutchouc after the needle is withdrawn, when the turgescence in the 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’. 
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. Déutschen Botan. Ges. 1883, and Unters. d. Bot. Instit. zu Tiibingen, I, 
Heft 3 (1884). 
