370 PLANT PHYSIOLOGY 



example is that of the coprophilous fungus, Pilobolus crystallinus, 

 which grows on cow or horse dung and produces numerous fruiting 

 bodies called sporangiophores. If these plants are grown in a 

 darkened chamber with a small window at one side, the sporangio- 

 phores all point toward it, and when the sporangia are ripe, they 

 are shot away from the plant directly towards the light. The 

 sporangia stick to the pane of glass, showing by their location 

 with what fine precision they were aimed. 



Other Tropisms. — Geotropism, or the movement in response 

 to gravity, has been discussed in the last chapter. Chemotropism 

 (turning in relation to chemicals) is of very common occurrence. 

 Fungi are negatively chemotropic to their own metabolic prod- 

 ucts. Root hairs are very sensitive to nutrient elements and to 

 toxic substances, going towards the one and away from the other. 

 Pollen tubes are very sensitive to sugars and grow in their direc- 

 tion. It seems probable that the growth of the tubes down through 

 the style is determined largely by chemical compounds found in 

 the pistil and ovules. Hydrotropism (turning in relation to water) 

 is a form of chemotropism, seen especially in roots and root hairs, 

 which are positively hydrotropic. Aerotropism, which is important 

 in the case of roots growing in swampy places and in the growth of 

 pollen tubes, is another form of chemotropism. The former are 

 positively aerotropic, while the latter are negatively so and grow 

 away from the air towards the interior of the style, where oxygen 

 is less abundant. Fungi are very sensitive to many varieties of 

 materials and serve as excellent subjects for the study of the 

 various kinds of chemotropism, including hydrotropism and aero- 

 tropism. 



Thermotropism is of slight importance in nature. Plant parts 

 require an optimum temperature and will grow in the direction 

 of a source of heat which is nearer the optimum than the one to 

 which they are subjected; if the temperature is not as favorable, 

 they will grow away from the source of heat. 



When roots are grown in a current of water of a moderate ve- 

 locity, they set themselves against the current. The precise na- 

 ture of the response is not known, but it seems to be due to the 

 pressure of the water, which sets up resisting or counteracting 

 strains in the plant. This reaction, which is known as rheotropism, 

 has little importance and, like electrotropism (growth in response 

 to electricity) or galvanotropism (turning in response to an elec- 



