the Physiology of Parasitism. II. 3 93 
been infected in this way at other points and the leaf cells have been in part 
killed, that germ tubes or hyphae pass into the stomata. 
The germ tubes produced from spores sown sparsely on a leaf do not 
usually start penetrating the leaf immediately, but grow for a short time 
along the surface of the leaf and then turn down and press the point of the 
tube firmly on the cuticle. The germ tube is usually curved just behind the 
tip (Figs. 7, 17). The growing tip has a very tense appearance, being full 
of protoplasm which stains deeply. It is firmly anchored to the cuticle by 
mucilaginous material which, as described above, appears in fixed prepara- 
tions as numerous threads running from the tip to the surface of the leaf 
(Fig. 6). 
The first indication of penetration is a slight indentation of the cuticle 
and outer epidermal wall which can be observed where the germ tube 
presses closely upon the cuticle (Figs. 8, 10, 15, and 16). The next step is 
the actual penetration of the cuticle. The germ tube does not usually break 
through as a whole, but, if the epidermal wall has not been affected by 
the presence of neighbouring hyphae, a narrow projection is commonly put 
out which passes through the cuticle and enters the subcuticular layers 
of the epidermal wall (Figs. 14, 17, 19). A very close examination was 
made of these stages, but neither before nor after penetration did the 
appearance or the staining reactions of the cuticle give any evidence of its 
being softened or swollen or in any way altered chemically. There thus 
seems no doubt that the cuticle is ruptured mechanically by the pressure of the 
tip of the germ tube. 
After penetration of the cuticle the intruding germ tube either grows 
straight on and enters the cavity of an epidermal cell (Fig. 21), or else it pro- 
ceeds to grow more or less horizontally beneath the cuticle (Fig. j 8). In 
either case the subcuticular layers of the wall usually soon swell up ; but in 
a few cases, as in Fig. 21, penetration appears to have occurred without such 
swelling of the subcuticular layers. 
No swelling of the subcuticular celhdose layer was observed before the 
Passage of the invading hypha through the cuticle. This is in agreement 
with the results obtained in the third of these studies (Brown, 2 ) when 
it was shown that the active extract of B. cinerea had no effect on the 
underlying tissues when placed upon the surface of so sensitive a structure 
as a rose petal, though its effect was marked directly the continuity of the 
cuticle was broken. 
When once the subcuticular layer of the walls has swollen, hyphae which 
enter later usually grow horizontally in this layer. Such a direction of 
growth is no doubt the path of least resistance, and the layer itself doubtless 
supplies the fungus with suitable nutriment. 
It is to be noted that in the cases described above the germ tube enters 
without the development of any appressorium. Many cases, however, can 
E e 2 
