30 INTRODUCTION [CH. 



the Mucorales and, together with transpiration, has been held responsible for 

 their divergence from one another when arising from a common point. 



Aerotropism and Osmotropism. These factors have not so far been 

 shown to play any important part in the directive growth of fungi. 



Phototropism. A considerable number of fruit bodies are sensitive to 

 light and by means of this reaction are able to adjust themselves in a posi- 

 tion favourable to the distribution of their spores, the direction of light 

 indicating the direction of open space. 



Among the MucoraleS the sporangiophores of various species oi Pilobolus, 

 Mucor, Phyconiyces and no doubt of many other genera bend towards the 

 light. In the genus Pilobolus the hemispherical sporangium is borne on an 

 aseptate sporangiophore which develops a swelling or bulb just below the 

 sporangium and another at its base. A new set of sporangia matures daily 

 and is discharged in the morning or early afternoon. The young sporangio- 

 phores of the species studied by Jolivette' showed a recognizable phototropic 

 curvature about 30 minutes after their exposure to light from a new direc- 

 tion; growth is apical and the tips, as they grow, bend towards the source 

 of light. Curvature is arrested during the early stages of the formation of the 

 sporangium and is resumed again when the subsporangial bulb is beginning 

 to form; during the later stages of development curvature takes place just 

 below the bulb. In this way the bulb and the terminal sporangium are 

 pointed in the direction of the light and some accuracy of aim is secured. 

 In a series of experiments, involving some 20,000 specimens, in which light 

 reached the culture through apertures i cm. in diameter, nearly 90°/^ of the 

 sporangia hit either the aperture itself or the walls within i cm. on each side 

 of it. When illuminated by two equal sources of light the sporangiophores 

 point to either one or the other; this is obviously a useful adaptation as an 

 intermediate aim would fail of its object. When the sources of light differed 

 the sporangia were found to be shot off in larger numbers towards the light 

 in which the proportion of blue rays was greater. The species" studied by 

 Parr also responded more readily to blue or violet than to other rays, the 

 presentation time gradually decreasing from red to violet although the 

 sporangiophores were responsive to light from all regions of the spectrum. 



Among Ascomycetes a positively phototropic response is found in the 

 necks of the perithecia in Sordaria and many other Pyrenomycetes, and is 

 sufficiently delicate to induce a zig-zag development of the neck if the 

 direction of light is repeatedly changed. The asci of Ascobolus immersus 

 and A. furfur aceiis are also positively phototropic so that an appropriate 

 direction is obtained for the ejection of their large spore mass. 



As early as 1877 Brefeld recorded sensitiveness to light in the stipe of 

 various Coprini, and found that normal pilei failed to develop in its 



^ Specific name not given. 



