414 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of plants. To this cause lie attributes the motion of the plasmodia 

 of the Myxomycetes. If a plasmcdium is placed on a piece oi 

 blotting-paper which dips into water at one end, it moves towards 

 the source of water, attracted by the current of water caused by the 

 capillarity of the blotting-paper. Plasmodia are therefore positively 

 rheotropic. Spores of Phycomyces and Mucor sown on blotting-paper 

 and fed by a current of a nutrient fluid, put out hyphse which grow 

 with the stream, and which are therefore negatively rheotropic. 

 Botrytis cinerea is, on the other hand, positively rheotropic. Eoots 

 of seedlings of maize and other cereals which hang down into a 

 free current of water grow towards the stream ; they are, like other 

 roots, positively rheotropic. 



Transpiration.* — A. Leclerc has performed a series of experi- 

 ments to determine the laws which regulate the amount of transpira- 

 tion from the surface of leaves. In a perfectly saturated atmosphere 

 he asserts that leaves do not transpire; they may even acquire a 

 not inconsiderable increase in weight. If the figures obtained from 

 experiments are represented in a system of rectangular co-ordinates, 

 the curve of transpiration is found to correspond much more with the 

 psychrometric than with the actinometric curve. The following are 

 the general conclusions arrived at by the author : — 



1. Transpiration is independent of light. 2. It falls to zero in 

 an absolutely moist atmosphere. 3. It is a function of the hygro- 

 metric condition of the air, and may be expressed with sufficient 

 accuracy by the equation E = a (F — /) db c ; where a is a coefficient 

 varying for each plant, but invariable for plants in the same series of 

 experiments ; / the tension of the aqueous vapour existing at the time 

 in the air; c a positive or negative constant. 4. When the trans- 

 piration of a plant is more active in the sun than in the shade, this 

 depends (a) on the rays of heat, which always accompany the rays of 

 light, and warm the tissues ; and (h) on the activity of assimilation of 

 the leaves in the light. 



The yellowing of leaves is often due to the transpiration being 

 checked. The disease of the vine known as " folletage," is due to the 

 leaves withering and dying in consequence of excessive transpiration. 



Transpiration-current in Woody Plants.f — J. Dufour continues 

 the discussion on this subject, adducing fresh arguments in favour 

 of Sachs's theory of imbibition ; the currents also being assisted by 

 filtration from cell to cell, especially through the agency of pitted 

 vessels. Experiments are described carried on for the purpose of 

 proving that the transpiration-current can only take place through 

 the walls of the wood. 



To these arguments E. Hartig replies,^ maintaining that the 

 passage of water through the wood does not ordinarily take place 



* Ann. Sci. Nat. (Bot.), xvi. (1883) pp. 231-79. 



t Dufour, J., ' Ueber den Transpirationsstrom in Holzpflauzen,' 1883. See Bot. 

 Ztg., xli. (1883) p. 843. 



I Hartig, E.., 'Die Gasdrucktheorie u. die Sachssche Imbibitions-theorie,' 

 Berlin, 1883. See ibid., p. 844. 



