454 SUMMARY OF CURRENT RESEARCHES RELATING TO 



relative position of the florets, the existence of which has been denied 

 by C. de Candolle, Schumann, and Jost, do actually exist. By aid of 

 photography he has been able to demonstrate in a given flower-head, 

 between times of flowering and ripening, an alteration in the angle of 

 imbrication amounting to 34°. The angle at first decreases, the organs 

 becoming pushed in towards the centre, causing the head to be bent in 

 a convex manner at the beginning of the flowering period ; later the 

 angle increases again, and the head gradually reassumes its flat form. 

 Evident alterations in divergence accompany these changes, which may 

 even cause contact alterations in the florets. Such photographic results 

 set beyond dispute the truth of Schwendener's theory of " shifting." 



On the Separation of Characters in Hybrids of the Pea Type.* — 

 C. Correns discusses the highly theoretical question of the method and 

 time of separation of the " Anlagen " of characters which obey Mendel's 

 law (of which the best known case is that of pea-hybrids), especially 

 in relation to the criticism passed on his views by Strasburger. Correns 

 believes that the separation of characters in the gametes of the hybrids 

 is brought about by a nuclear division, physiologically of the nature of 

 Weismann's reducing division, though not necessarily by a cross division 

 of the chromosomes. This, in the case of the female gametes, is the 

 first division of the embryo-sac mother-cell, and in the case of the male 

 gametes, probably the division which gives origin to the vegetative and 

 generative nuclei. 



Irritability. 



Rheotropism of Roots.f — F. C. Newcombe has experimented with 

 thirty-two species of plants belonging to widely different families ; 

 twenty have proved positively rheotropic, and fourteen were insensitive. 

 Rheotropism cannot therefore be regarded as a general phenomenon. 

 Sensitive species differ greatly in degree of sensitiveness to a stimu- 

 lating water current. The fourteen insensitive species are distributed 

 through nine families, and the twenty sensitive through six ; there is 

 an indication that the roots of genetically related plants behave alike 

 toward a water current. The watercress, and four other aquatic plants 

 tested, were not sensitive ; hence it is improbable that rheotropism stands 

 in any biological relation to water-plants. As regards the effect of 

 velocity of water-current on the response, the author finds that velocities 

 above 1000 cm. per minute give general negative (mechanical) curves, 

 that the optimum velocity lies between 100 cm. and 500 cm. per minute, 

 and that velocities below 50 cm. bring fewer and slower responses with 

 smaller angles. As regards latent period, species differ greatly in time 

 of response, though the latent period in every case is relatively long 

 as compared with that for geotropism. In the competition between 

 geotropism and rheotropism some roots attain only a small deviation 

 from the vertical, some reach 45°, and some 90°, the last seeming to 

 overcome entirely their geotropism. The author finds that the rheotropic 

 sensitiveness occurs over at least 15 mm. of the length of the root-apex, 

 and also that the extreme apex is rheotropic. Rheotropism is not confined 



* Bot. Zeit., lx. (1902) pp. 65-82. 



t Bot. Gaz., xxxiii. (1902) pp. 177-98, 263-83, 341-62 (15 figs.). 



