678 SUMMARY OP CURRENT RESEARCHES RELATING TO 



seated higher on the axis than the female, which are mature about 

 the same time ; the male flowers being very much the more nume- 

 rous. When the anther is ripe, the pollen-mass, in which the last 

 ordinary stages of development are suppressed, becomes free ; and the 

 elliptic-cylindrical pollen-grains, which are completely filled, except 

 at the two polar ends, with starch-grains, sink in the water in conse- 

 quence of their greater specific gravity, and are caught by the 

 detaining apparatus of the female flowers. In dia»cious species the 

 process varies only in the pollen having to be carried to the stigma 

 of another plant. When once the pollen-grain is detained, the 

 pollen-tube passes into the canal of the style, by the wall of which 

 it is attracted and hindered in its growth, in consequence of which 

 it becom'^s separated by coUenchymatous septa, so that the rapid 

 access of lood-material is prevented. From the conducting tissue 

 at the mouth of the canal, the pollen-tubes usually find their way 

 direct to the conducting tissue at the mouth of the micropyle, whence 

 they reach the wall of the embryo-sac. 



In Callitriche autumnalis the pollen-grains are round, filled with 

 oily protoplasm, and lighter than water. They are carried actively 

 by the water to the stigma, whence they reach the canal of the style. 

 The difference in the mode of fertilization in Naias and Callitriche 

 corresponds to the difference in their habit, the former preferring 

 still, the latter running water. 



Influence of direct Sunlight on Vegetation.* — M, Buysman calls 

 attention to the influence of direct sunlight on vegetation, tracing in 

 the first place the effect of the sun's rays in the tropical regions and 

 afterwards in the temperate and arctic zones. 



The constant high temperature within the tropics is the cause of 

 the plants being less dependent on the direct solar heat than is the 

 case in the greater part of the temperate and cold zones. Plants in 

 the high northern regions when they vegetate receive more warmth by 

 insolation than is often supposed — 1st by the direct solar light itself, 

 and 2nd by the heated surface of the ground. The snow and ice being 

 melted by the sun, the necessary water and humid atmosphere never 

 fail ; this is the cause of the luxuriant growth of grass on some places 

 of the Tundra. The flowing water gradually communicates its 

 warmth to the soil, and prevents also nightly radiation. 



In the temperate regions vegetation commences in spring when the 

 difference in temperature between night and day is greatest ; in the 

 high north this difference is often insignificant because the sun 

 remains above the horizon ; but the temperature of the soil being at 

 this time very much lower than that of the objects exposed to the 

 sun's rays, even this great difference is the cause of the very rapid 

 vegetation in sheltered localities and under the influence of the solar 

 light. 



Absorption of Oxygen and Evolution of Carbon dioxide in Leaves 

 kept in Darkness.f — MM. P. P. Deherain and L. Maquenne, repeating 



* Nature, xxxi. (1885) pp. 324-6. 



t Comptes Rendus, c, (1885) pp. 1234-6. 



