on the Sporidia of Puccinia mctlvacearum (Mont.). 339 
which was one of the species Fulton used, show conclusively that the direction 
of the growth of the germ tubes was influenced by light. Fig. 6 A 
represents one of these experiments with Botrytis and Fig. 6 B is the 
corresponding control which was kept in the dark. 
The effect of water on the germination of the sporidia is of some interest. 
In certain preliminary experiments where the sporidia germinated in the 
droplets of water which condensed on the cover of the cell, it was observed 
that the germ tubes were invariably directed towards the margin of the 
droplet and often grew out of it (Fig. 7). It was not decided how far 
this was an expression of negative hydrotropism or merely of the need of 
the growing germ tubes for oxygen. It may be mentioned that where the 
sporidia were germinated in an atmosphere saturated with water vapour 
they frequently produced germ tubes which formed a slight swelling at the 
tip and grew into the gelatine. 
The growth and direction of the germ tubes did not appear to be 
influenced in any way by gravity. 
Summary and Conclusion. 
The results of the experiments made on the effect of various stimuli on 
the germinating sporidia of Puccinia malvacearum may be briefly summarized 
thus : 
1. The sporidia of P. malvacearum have been shown to be negatively 
heliotropic, and this is also the case for the conidia of a species of Botrytis ; 
whilst with conidia of Penicillium , Alternaria , Peronospora , and aecidiospores 
of Puccinia poarum no irritability to light was apparent. 
2. Other influences shown to affect the germ tubes are moisture and 
contact. As regards moisture, the germ tubes tend to grow out of a drop of 
water into the moist atmosphere around. On the other hand, germ tubes, 
on the surface of gelatine in a moist atmosphere, tend to penetrate the 
gelatine. These influences are difficult to analyse satisfactorily. With 
respect to contact, the tip of the germ tube swells and becomes closely 
applied to the epidermal surface of both the host and non-susceptible 
plants. This may, perhaps, be a result of the contact. 
3. On the normal host-plant a very slender growth from the swollen 
end of the germ tube penetrates the cuticle and outer wall of the epidermis, 
and this brings about true infection. This has not been seen in any case on 
plants other than the normal hosts although numerous attempts at infection 
were made. 
4. No evidence of chemotropic influences radiating from a fragment of 
leaf laid on a drop of gelatine could be obtained. There were no indications 
of positive chemotropis.m of the germ tubes towards the normal host or of 
negative chemotropism of the germ tubes towards fragments of non-sus- 
ceptible leaves. 
