i] REACTIONS TO STIMULI 31 



absence. In ( 'oprinus niveus, and ( oprinus curtus, both coprophilous species, 

 Buller found a well-marked positively phototropic response in young fruit 

 bodies so that they push up from between <>r beneath the irregularities of 

 the substratum. The stipe ceases to be phototropic when the pileus begins 

 to expand and develops instead a negatively geotropic reaction; by these 

 means the apex of the stipe is brought out into the light and the horizontal 

 expansion of the pileus is ensured. A similar succession of reactions takes 

 place in the development of the sporophore of Loitinus lepidcus; the young 

 stipe is positivel\- phototropic and in the absence of light grows straight 

 onwards without giving rise to a pileus; in a sufficiently strong illumination 

 the pileus soon appears and, as its development proceeds, the positively 

 phototropic response changes to a negatively geotropic one. Amanita phal- 

 loides and A. crenulata show, according to Streeter, a positively phototropic 

 reaction even after the appearance of the pileus. 



Corresponding reactions are to be expected in other pileate species 

 growing on irregularly shaped substrata; on the other hand Agaricus cam- 

 pestris, growing on ground, is quite insensitive to light, negative geotropism 

 being, under normal conditions, sufficient to secure the satisfactory adjust- 

 ment of its parts. 



A negatively phototropic reaction is very much less frequent than a 

 positive response; Robinson found, however, that germ-tubes from the 

 basidiospores of Puccinia Malvacearum and the conidia of Botrytis sp. turn 

 away from a unilateral source of light, while those from the aecidiospores of 

 Puccinia Poarum and from the conidia of Peronospora parasitica and Peni- 

 cillium glaucum are indifferent. 



Phototaxis. A phototactic reaction has been observed by Strasburger 

 in the zoospores of Chytridium vorax, and by Wager in those of Polypliagus 

 Euglenae. Both these species parasitize motile green organisms, Chytridium 

 infecting Chlawydococais pluvialis, and Polyphagus, Euglena viridis. The 

 host organisms react to light since they obtain their carbon supplies by 

 photosynthesis, and the phototactic reaction of the parasite brings it into the 

 region where the hosts are to be found. 



Formative Influence of Light. Irrespective of phototropic response the 

 formative influence of light is important. Buller found that the sporophores 

 of Polyporus squamosus, though quite irresponsive to the direction of light, 

 fail to give rise to pilei unless illuminated. The same is true of many other 

 Hymenomycetes, including those which, like Lentinus lepideus and the 

 species of Coprinus mentioned above, are positively phototropic. In many 

 •cases only a brief period of illumination during the early stages of develop- 

 ment is essential. Similarly, light appears to stimulate the development of 

 ascocarps in certain Ascomycetes, and cultures may remain sterile in dark- 

 ness or very dim light. 



