33 2 Robinson . — Experiments on the Effect of External Stimuli 
of chemotropism will explain the entrance of the germ tubes of parasitic 
Fungi into the tissues of the host-plant. He also found that various Fungi 
show positive hydrotropism, but that an over-abundance of moisture may 
cause a negative reaction in certain Fungi. 
From a large number of experiments on the effect of tannin upon the 
germination and growth of many Fungi, Cook and Taubenhaus 1 in 1911 
found that this substance has a tendency to retard the growth of Fungi. 
They found also that the parasitic forms are more sensitive to the action 
of tannin than the saprophytic forms they used. It should be noted, 
however, that the parasites employed were not of the obligate class, as 
they could be cultivated upon nutrient media apart from the living host- 
plant. These investigators suggest also that tannin may, to some extent, 
serve as a protective agent against the attacks of parasitic Fungi. 
Up to the present the Fungi used in this kind of work have, for the 
most part, been saprophytes or facultative parasites. We have, therefore, 
very little information respecting the influence of external stimuli on the 
germ tubes of obligate parasites. The causes which determine the penetra- 
tion of the epidermis of the host-plant by the germ tube of the sporidium 
c 
Fig. 1. A sectional elevation of a Van Tieghem cell as used in these experiments, a , fragment 
of a sorus with ripe teleutospores ; b, a small drop of water ; c , a drop of gelatine. 
of Puccinia malv ace arum are obviously somewhat different from those 
operating in the case of infection by uredospores or aecidiospores. I11 
infection by the latter, entrance is effected through the stomata, and a very 
considerable amount of growth can occur at the expense of the reserve 
material in the relatively large spore before the fungus enters into its 
normal relationship with the host-plant. On the other hand, the sporidium 
is a small thin -walled spore with very little reserve material, and conse- 
quently, in order to succeed, its germ tube must promptly enter the tissues 
of the host. What determines this entry is unknown. In order to obtain 
some insight into the stimuli affecting the germinating sporidia, experiments 
were designed to test the effects of fragments of various leaves, of light, of 
gravity, and of water upon them. 
Most of the experiments were carried out in Van TieghenTs cells, and 
large pure sowings of sporidia were obtained from germinating teleutospores 
by the device shown in Fig. 1. A small fragment of hollyhock leaf 
bearing a portion of a teleutospore sorus (a) was fastened to the floor of the 
cell by a small quantity of vaseline. Near this, but not in contact with it, 
1 The Relation of Parasitic Fungi to the Contents of the Cells of the Host-plants (Toxicity of 
Tannin). Bull. 91, Delaware Coll. Agri. Exp. Station, Feb. 1911. 
