902 
Journal of Agricultural Research 
Vol. XXIV, No. II 
Lihertiana by Boyle (2), we may be greatly misled. This would apply 
also to the conclusions of Hawkins and Harvey (7) regarding the pene¬ 
tration of cellulose walls. Fungi produce a variety of enzyms by means 
of which they decompose substances and obtain food. Moreover, it is 
quite likely that they produce many more enzyms than have been 
isolated. Failure to isolate an enzym therefore does not disprove its 
existence, especially as the physiological factors involved in the produc¬ 
tion of enzyms and the activators and inhibitors controlling their activi¬ 
ties are little understood. Moreover, the observation by Hasselbring 
(6) that the cavities made in the waxy covering of Berber is Thunbergii 
berries by the anthracnose fungi are much larger than the germ tubes 
and the conclusions by Ward (jo, ii) from an exhaustive study of the 
brownrust of bromes that there is absolutely no relation between thick¬ 
ness of walls, number and size of stomates, hairs, and other mechanical 
structures and immunity to rust, present a striking contrast to the 
pressure theory of parasitic invasion. That growth pressure often ac¬ 
companies enzymic action is obvious. It may also speed up the process 
of penetration and in certain cases serve as the chief, if not the sole, 
means of invasion, but it plays only a minor and nonessential part in 
the invasion of the tomato fruit cells by Oospora laciis and no doubt acts 
only in a secondary capacity in most cases. This is fortunate, for if the 
development of disease-resistant fruits depended upon thickness of walls, 
quality and resistance would often be diametrically opposed. 
Since Oospora passes through the cellulose walls of two adjoining fruit 
cells without attaching itself to the wall or protoplasm and not infre¬ 
quently makes an opening larger than the filament without causing a 
depression in the wall of the occupied cell, nor lateral cracks, or ruptures, 
in the wall of the invaded cell, its means of penetration can not be ascribed 
to pressure. The only other known means by which a fungus can 
make an opening of this description is by the use of an enzym, such as 
cellulase, which has been isolated from fungi. 
There are features in the penetration of cell walls by some fungi, 
especially species of the genus Pythium, which cast considerable doubt 
on the penetration of cell walls by pressure. In his study of Pythium 
gracile, Ward observed that an oospore which had germinated at some 
little distance from a cress seedling, produced as it grew several bends in 
its germ tube and passed around a small algal cell at right angles before 
reaching a cress cell. On coming into contact with this cress cell its 
‘'apex became closely pressed against the cuticle, apparently lifting the 
whole hypha slightly in the process,” evidently a result of pressure, but 
thereafter made no further movement nor change in its position as it 
produced a small hole in the cuticle and cellulose wall, passed through, 
and enlarged to normal size within the cell. As this fungus filament con¬ 
sisted of a single cell, the pressure within it was equally distributed. 
If the pressure had been sufficient to penetrate the host cell wall, which 
was many times as thick as the fungus wall, it would have straightened the 
fungus filament. As a matter of fact, however, it did not straighten 
a single bend nor change its position in the least, although such fila¬ 
ments bend easily, even by the motion of delicate water currents. More¬ 
over, if pressure had caused the penetration of the wall, the fungus would 
have made a hole as large as its filament instead of a small hole, for the 
pressure on every unit area of its wall surface was equal. A more prob¬ 
able cause of this type of penetration is that cellulase is formed solely at a 
single point on the tip of the filament. 
