474 PHYSIOLOGY 



occur when the central layer contained food provided the outer layei 

 had enough more of the same to act as a stimulus. (In this case the ratio 

 had to be about lo : i. See Weber's law, p. 448.) Likewise the hyphae 

 grew through fine perforations in thin plates of mica or celluloid, when 

 the nutritive gelatin was thus separated from the other, suggesting the 

 way in which fungus hyphae, arising from spores on a leaf, turn into a 

 stoma and so find their way into the interior of a leaf of their host. In 

 fact, when leaves were injected with a solution of food, like sugar, fungus 

 hyphae of many kinds are reported to turn into the stomata, though they 

 do not naturally grow on the leaves used. A great variety of substances 

 were tested in similar ways. Some proved to be attractive, some repel- 

 lent; and the reaction varied according to the concentration of the solute, 

 though generally the hyphae were injured before the limits of concen- 

 tration for repelling effects had been reached. 



On the other hand, an apparently careful repetition of many of these 

 experiments gave negative results, in that the numbers of hyphae reacting 

 positively is so slightly in excess of the number indifferent or negative, 

 that the results seem scarcely more than chance, or ascribable to other 

 than the cause assigned heretofore. A complete restudy of the matter 

 will be necessary. 



Of poUen tubes. — When pollen tubes are developed under a cover 

 glass in company with a bit of the stigma of the same plant, they turn 

 toward it, from whatever direction they first issue. An ovule or a bit 

 of the wall of the ovary is likewise attractive. Investigation shows that 

 soluble carbohydrates and proteins are here the attractive substances. 

 It seems likely, therefore, that the growth of the pollen tube toward the 

 ovules is directed by the diffusion of such substances, which are always 

 found in these organs. (See the chemotaxy of sperms, p. 448.) 



AerotTopism. — A special form of chemotropism has been called aerolropism, 

 and was first ascribed to roots. When certain gases, especially oxygen, diffuse 

 against young roots from one side, it is reported that the root curves toward the 

 source of the gas. These results also have fallen under suspicion. Recent investi- 

 gations are conflicting ; and one is left in some doubt whether to ascribe the curva- 

 tures to a true reaction to gases, in accordance with the weight of evidence, or to 

 moisture, in which case they belong to the following special category of chemotropic 

 response. 



Stems also have shown sensitiveness to Oz and CO2, and it may be that aero- 

 tropism is more general than has heretofore appeared. It is not evident that it 

 can be of any great advantage to either roots or stems, except, perhaps, those of 

 swamp plants. 



