ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 437 



of potato on another, tbe stock exercises no visible influence on the 

 form or coloration of tbe tubers produced by tbe graft. 



Non-sexual Propagation of Opuntia.* — In Opuntia arbuscula Carle- 

 ton E. Preston finds a metbod of non-sexual propagation analogous to 

 layering, decumbent branches being borne down to tbe ground, where 

 they root, by the weight of the fruit. The same species also displays 

 another mode of non-sexual propagation by the formation of adventi- 

 tious shoots on the roots. 



(3) Irritability. 



Perception of Gravitation by Plants, f — B. Nemec gives the result 

 in detail of observations on the effect produced by gravitation on the 

 position in the cell of the nucleus and of other bodies of less and of 

 greater specific gravity. He regards these observations as proving in- 

 contestably tbe presence in plants of a " sense-organ " which has for 

 its function the perception of the irritation of gravitation (geotropism). 

 The unicellular geotropic organs of multicellular plants often contain 

 in their apex a dense protoplasm in which are a variety of solid bodies?, 

 the position of which is not directly determined by gravitation, but 

 which, nevertheless, enable the protoplasm in which they are imbedded 

 to exercise pressure in a definite direction. Gravitation may be re- 

 garded as the pressure exerted by bodies of a higher (or of a lower) 

 specific gravity on sensitive protoplasm. 



Geotropic Function of the Root-tip. J — Prof. F. Darwin describes a 

 " root-lever," which has for its object the support of the weight of the 

 cotyledons during germination, while they are free to move in any 

 direction in obedience to the curvature of the root. Experiments made 

 with this instrument confirm the author's previous conclusion that in 

 ■certain geotropic parts of plants the apex is a percipient organ, while 

 the more basal motor region is set in action by an influence transmitted 

 from the sensitive region. 



Motile Cushions of Robinia and Psorliera.§ — E. Fantanelli con- 

 firms, in the case of Robinia pseudacacia, the statement of Pfeffer that, 

 in the movements of the motile cushions, the two halves of tbe cushion 

 display similar properties ; while iu Mimosa, Oxalis, Phaseolus, and 

 Desmodium, the two halves of the cushion react differently. In Psorliera 

 hygrometrica, great moisture of the air affects the nyctitropic move- 

 ments by promoting the closing movement in the evening and the 

 opening movement in the morning. The leaves close during rain. 



Horizontal Nutation of the Stem of Pisum sativum, &c.|| — Ex- 

 periments made by D. Neljubow on Pisum sativum and some other 

 Legurninosae (Vicia sativa, Ervum lens) led to the same results as those 

 obtained by other observers, viz. a tendency, when grown in tbe dark 

 in the open air, for the shoots to assume a horizontal position. From 

 control experiments made under cover (in the laboratory or orangery) 



* Bot. Gaz., xxxi. (1901) pp. 127-8. 



t Pringsheim's Jahrb. f. wiss. Bot., xxxvi. (1901) pp. 80-178 (36 figs.). Cf. this 

 Journal, ante, p. 179. 



X Proc. Cambridge Phil. Soc, xi. (1901) pp. 134-6. Cf. this Journal, ante, p. 179. 

 § Atti soc. nat. e mat. Modena, ii. See Bot. Ztg., lix. (1901) 2 le Abt., p. 122. 

 || Beih. z. Bot. Ctntralbl., x. (1901) pp. 128-39 (2 figs.). 



