1895- NOTES AND COMMENTS. 9 



<£ it agrees pretty well with the Cupuliferae and other dicotyledons." 

 This latter resemblance is of some importance in the light of recent 

 work (to which allusion was made in Natural Science, vol. iv., p. 333) 

 by Miss Benson on the reproduction of the Amentiferae ; an importance, 

 however, which is diminished by the occurrence of points of agree- 

 ment in the wood of Rosaceae, Saxifragaceae, and others. 



Geotropism in Plants. 

 In the same number Dr. Pfeffer describes an ingenious means by 

 which certain proof is obtained of Darwin's assumption that only the 

 tip of a growing root is sensitive to the stimulus of gravity. Darwin's 

 assumption was correct, but his investigations, like those of subse- 

 quent workers, were inconclusive, since they involved cutting off the tip 

 of the root, and the results were in a sense pathological. In Pfeffer's 

 experiments, the roots of a bean or lupin are allowed to grow into 

 short tubes of thin glass, bent at a right angle. The growing root 

 follows the bend, and pushes on to the closed end of the tube. The 

 terminal portion of the root, from one to five mm. long, thus becomes 

 bent at right angles to the remainder. Experimenting with roots 

 thus prepared, Pfeffer finds that, so long as the tip remains vertical, 

 no geotropic curvature takes place in the rest of the organ ; but 

 when the tip is directed horizontally or at an acute angle with 

 the normal vertical position, the rest of the root endeavours by a 

 curvature to re-establish the normal relation to the force of gravity. 



Rootlets Penetrating Living Tissues. 

 Mr. G. J. Peirce, who has previously studied the penetration of 

 living tissues by the modified roots or haustoria of the Dodder, has 

 also shown (see Botanische Zeitung, Hi., part i., p. 169) that this power 

 of making a way through plant-tissue is also present in the roots of the 

 pea, bean, turnip and white mustard. Seeds of the two latter enclosed 

 between halved potato tubers forced their roots in twelve days through 

 the substance of the potato, and even pierced the external cork layer. 

 No root-hairs were developed, and the passage made by the root was 

 surrounded by torn, dead, brown-walled cells. As no corroded starch 

 grains were found, the author concludes that the penetration is due to 

 mechanical pressure and not to the action of a diastatic ferment. 

 These experiments recall an account of the production of internal 

 tubers in a potato (Rendle in Jouru. Bot., 1893, P- J 93' *• 33^)- ^ n 

 the case described, shoots had grown inwards from the base of the 

 eyes or buds, pushing their way through the substance and giving rise 

 to a number of thread-like roots as well as to small tubers. The latter, 

 by their growth, eventually burst the skin of the mother-tuber, which 

 was thus seen to be actually bringing forth young from its interior. 

 It was also pointed out that the layer of cells lining the channels and 

 cavities in the old potato were losing their contents, and their walls 



