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 10 : 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 pollen 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.) 
Aerotropism. A special form of chemotropism has been called aerotropism, 
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 C>2 and COj, 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. 
