K.— BOTANY. 239 
into the epidermis of the host. This response to contact with a solid 
substratum is usually termed thigmotropism, though stereotropism would 
seem to be the more satisfactory term. That the germ-tubes and hyphee 
of many fungi (such as Botrytis, Colletotrichum, Sclerotinia Libertiana) 
exhibit a stereotropic response is, of course, easily demonstrated by growth 
of such fungi in hanging drops on a glass surface. The question then arises 
as to whether stereotropism is the sole or main cause of the growth response 
which leads to entry. If a tropism of this kind be the main factor, one 
would expect the penetration of any surface (such as a leaf) of not too 
great resistance by any germ-tube responding to a contact stimulus, 7.e. 
an entry quite non-specific.® At present the data available do not seem 
sufficient to answer this question. The problem of the mechanism of 
infection requires investigation from this particular angle. Some light 
on the matter could no doubt be obtained by germinating together the 
spores of two parasitic fungi, say A and B, first on the host of A (which 
B does not infect), and then on the host of B (which A does not infect), and 
comparing accurately their responses on the two substrata. It is the 
melancholy experience of physiological work that a simple explanation 
of any process is almost certain to be wrong. It would therefore seem 
_ unlikely that stereotropism alone is responsible for penetration. How 
complex is the relationship is shown by another observation of Dr. Brown’s 
that germ-tubes of Botrytis cinerea are unable to penetrate the epidermis 
of an uninfected leaf of Hucharis amazonica, but they will bore through it 
when the mesophyll tissue below is cut away, even when the leaf so 
treated is “ backed ’ with agar. 
The question of the actual mechanism of entry is of considerable 
physiological interest. A number of studies by Brown, Blackman and 
Welsford, Boyle, and Dey !° have been published which bring forward 
evidence for the view that the entry of a germ-tube through the cuticle 
of the host is a purely mechanical process in which enzymes play no part. 
The evidence for this is in part directly observational. When the process 
of entry is carefully followed in such forms as Botrytis cinerea, Colletotrichum, 
Sclerotinia Libertiana, the sporidia of Puccinia graminis, it is found that 
the germ-tubes or appressorea become firmly attached to the surface of 
the host before entry, and no swelling of the cuticle can be observed prior 
toentry. Furthermore, entry is usually by a very fine infecting hypha 
(fig. 2), and at the actual point of entry of such hypha there is no rounding 
of the contours of the cuticle as we should expect if enzymes were at work. 
There is also the additional point that no enzyme is known that is able to 
dissolve cuticle. The injection into an organ, such as a leaf, of an extract 
of the germ-tubes of Botrytis is a very convenient way of preparing sheets 
of cuticular material. The resistance of cuticle to bacterial attack is well 
shown by the composition of brown coal, which often consists very largely 
of cuticular material. 
If the germ-tube is to exert sufficient force to bore its way through the 
| 
: 
' 
 °In the case of the Uredinex the entry through a stoma is quite non-specific, for, 
as Miss Gibson showed, the germ-tube of almost any Uredine will enter the stoma of 
ae any leaf, but the establishment of parasitism depends upon the suitability of 
the host. 
10 Studies in the Physiology of Parasitism,’ Annals of Botany, xxix.-xxxv., 
1915-1921, 
