ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 319 



the tip, becoming nearly as large as the flower ; the buoyancy of the 

 enclosed gas, aided by the low specific gravity of the flower, overcomes 

 the weakened attachment, and the flower darts to the surface. The 

 bubble then disappears, and the sepals snap back quickly, forming three 

 floats, which support the sporangia above the water ; the pollen how- 

 ever is nearly all discharged at the moment the flower comes to the 

 surface, the snow-white tetrads being quite conspicuous floating on the 

 water. The pistillate flower is impervious to water and so produces a 

 depression in the surface film. Pollen-grains floating near are brought 

 into contact with the stigmas by means of gravity operating through 

 the declined surface film. The large pollen-tubes penetrate the long 

 floral tube and pass directly through the ovarian cavity to the upturned 

 micropyles. Tubes which have failed to enter ovules often swell up 

 into cyst-like enlargements in the ovary, in which the distinct male cells 

 can be seen. Fertilisation takes place in the usual manner, and the 

 second male cell unites with the endosperm nucleus. The primary 

 endosperm nucleus does not divide until a two-celled embryo has been 

 formed ; the pollen-tubes persist until the embryos are well developed. 

 The suspensor cell of the embryo becomes enormously enlarged, and 

 the synergid often increases in size. The primary root is probably 

 functionless, and secondary roots are developed in the seed from the 

 lower parts of the stem. 



Pollination of Buckwheat.* — P. P. Richer finds as a result of 

 experiment with Polygonum Fagopyrum, a heterostyled dimorphic plant, 

 that the flowers are always quite sterile when self -pollinated, or with 

 pollen from flowers of the same form on the same plant. They are 

 very slightly fertile after illegitimate cross-pollination between flowers 

 of the same form on distinct plants. They are, on the contrary, highly 

 fertile after legitimate cross-pollination between flowers of different 

 form on distinct plants. It seems evident, therefore, that the two forms 

 of flower owe their fertility solely to reciprocal crossing, and conse- 

 quently that almost all the seeds produced naturally are the result of 

 legitimate union. 



Physiology. 



Nutrition and Growth. 



Nitrogen-assimilation of Fresh-water Algae. t — J. Reinke de 

 scribes a symbiosis between Volvox and Azotobacter, associated with an 

 increase in the amount of combined nitrogen in the solution. He 

 suggests the importance of the latter as a means for obtaining nitrogen 

 from the atmosphere in the case of fresh water as well as of marine 

 organisms. The author adduces in support of his hypothesis the 

 observation of Gerlach and Vogel, that 10 to 12 p.c. of the dry weight 

 of Azotobacter consists of nitrogen ; it is therefore eminently qualified 

 to act in symbiosis as a nitrogen-assimilator for other plants. 



Irritability. 



Chemotropism of Roots.J — F. C. Newcombe and A. L. Rhodes find 

 that the roots of Lupinus alius are positively chemotropic towards solu- 



* Coroptes Eendus, cxxxviii. (1904) pp. 302-4. 



t Ber. Deutsch. But. Gesell., xxi. (1903) pp. 481-3. 



j Bot. Gazette, xxxvii. (1904) pp. -23-35. 



Z 2 



