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William Brown. 
cal investigation already alluded to, it is clear that the actual 
process of penetration of the cuticle takes place in a purely 
mechanical way. In investigating this problem several subsidiary 
problems arise. Mechanical pressure requires the existence of a 
fulcrum or attachment of the germ tube to the cuticle. The nature 
of this attachment, its presence or absence in different fungi requires 
investigation. Again, evidence has been brought forward that 
different fungi under the same conditions have different powers of 
mechanical penetration. A method, was devised for testing the 
penetrative power of fungi. This consisted of the use of a series 
of gelatine membranes of graded hardness (5). It was found that 
a membrane of greater hardness was required to prevent the 
passage of Botrytis than was required in the case of Rhizopus or 
Penicillium. It is hoped when the details of this method are 
worked out that the penetrative powers of fungi can be catalogued 
on the basis of this graded series of gelatine membranes. 
The question of penetration cannot satisfactorily be treated 
without consideration of the tropic factors which may come into 
play. The researches of Miyoshi (7 and 8) pointed to marked 
chemotropic influences in this connection, but these have been to a 
large extent discounted by the later work of Fulton (9). The recent 
work of Graves (10) has demonstrated the existence of a positive 
chemotropism exercised by cane sugar and, more especially, certain 
unknown constituents of turnip juice, together with that of a 
marked negative chemotropism of the fungus to its own excrete 
products. In the case of membrane penetration, positive chemo¬ 
tropic forces may conceivably play a part; negative chemotropism 
to the excrete products of its growth is physically impossible under 
the circumstances (If anything, there should, on the hypothesis of 
of such a force, be a tendency for the germ tubes to grow away from 
the epidermis into the spore-free region of the drop). While a positive 
chemotropism is thus not excluded, experiments have already shown 
that penetration of membranes may take place under circumstances 
which exclude such a factor. Fungi have been found to penetrate 
the following membranes:— 
(1) Membranes of paraffin wax, which were shown by col¬ 
lateral test to be impenetrable to the simplest electrolytes. 
(2) Plant epidermes which had been removed and repeatedly 
washed till all soluble substances had been removed. 
(3) Gelatine membranes which had been soaked in exactly 
the same nutrient as the spores were sown in. 
