CHEMOTROPISM 483 



determined exactly. How great must this difference be so that perception may 

 follow, and how does this value vary with the absolute concentration ? In 

 order to obtain a definite but at the same time constant difference in concentra- 

 tion on opposite sides, Miyoshi sowed spores of a fungus on a collodium mem- 

 brane perforated in the middle, and laid it between two strips of filter paper 

 crossing each other at right angles. If streams of different concentration were 

 allowed to pass through the filter paper on both sides, the difference in con- 

 centration affecting the germ tubes was kept approximately constant. When 

 Saprolegnia was made to grow between a o-i per cent, and 0-3 per cent, solution 

 of sugar no curvature of the hyphae took place ; the same result was obtained 

 on using a o-i per cent, and 0-5 per cent., but when the solution on one side was 

 o-i per cent, and on the other i per cent., positive chemotropic curvature took 

 place. The same relative percentages must be maintained at higher concentra- 

 tions if perception is to follow ; thus a 0-5 per cent, solution of sugar must be 

 opposed to a 5 per cent. Miyoshi thought he was entitled to conclude from 

 these experiments that in general the solution must be ten times as strong on one 

 side as on the other if curvature was to take place (Weber's Law ; compare pp. 

 473 and 543). Investigations must, however, first of all be made as to whether 

 this relation holds good near the critical concentrations, and whether it is 

 effective at high concentrations, where negative curvature appears. It is very 

 probable that it is not so ; moreover the repellent results obtained at higher 

 concentrations are in all probability due, at least in part, to osmotic and not to 

 chemical activity (Massart, 1889 ; compare Lecture XLIII, p. 542). 



In pollen-tubes as well as in Fungi, well marked capacity for responding to 

 chemotropic stimuli has been established (Molisch, 1889, 1893 ; Miyoshi, 

 1894 b ; LiDFORS, 1899). 



If we place a portion of a stigma, a style, or an ovule of Scilla patula on 

 sterilized gelatine and sprinkle pollen of the same plant over the gelatine, keeping 

 the whole preparation moist and in the dark, we find that the pollen-tubes in- 

 variably grow towards the tissue and finally pierce it. The fact that Fungi 

 behave in the same way makes it very probable that what attracts the pollen- 

 tube is nothing out of the common but merely some kind of sugar or other body 

 commonly found in the plant. Since the stigma contains glucose, and since the 

 ovule has been shown to contain a polysaccharide, we naturally think at 

 once of cane or grape sugar as the exciting agent. As a matter of fact Miyoshi 

 (1894 b) has shown that pollen-tubes react vigorously to cane sugar and other 

 soluble carbohydrates, such as levulose, dextrose, dextrine, and lactose, while 

 the other substances which are active in the case of Fungi are in this case in- 

 different or repellent. Miyoshi was also able to determine the liminal differ- 

 ence in concentration for pollen-tubes by the same method as he adopted in his 

 experiments on Fungi. In the case of Agapanthus, chemotropic curvature 

 always took place when the concentration of the stimulant was at least five times 

 greater on one side than it was on the other, a fact which was established for 

 percentages of 0-5, i and 2. Starting from this basis Miyoshi was able to 

 deduce the degree of concentration of the cane sugar solution which escaped 

 from the ovules of Hesperis matronalis, assuming that the sensitivity of the 

 pollen-tubes of this plant was exactly the same as of those of Agapanthus. 

 When pollen-tubes and ovules were both placed on gelatine containing a known 

 amount of cane sugar, and whose surface was also moistened with a sugar 

 solution of the same concentration, approximation of the pollen-tubes to the 

 ovules took place only when the secretion from the ovule was at least five times 

 as concentrated as that in the gelatine medium. Growth towards the ovule took 

 place only if the substratum contained 0-25 per cent, to 2 per cent, of sugar but 

 not at higher concentrations, hence the concentration of the cane sugar solution 

 in the ovule must have been at least 10 per cent. 



It is well worthy of note that Miyoshi has determined in a large number of 



I i 2 



