PLEASURES OF SCIENCE. 33 



window or the ceiling, are performed exactly by the same po\ver, the 

 weight of the atmosphere, which causes the quicksilver to stand in the 

 weather-glass, the wind to whistle through a key-hole, and the piston 

 to descend in a steam -engine. 



Although philosophers are not agreed as to the peculiar actions winch 

 light exerts upon vegetation, and there is even some doubt respecting 

 the decomposition of air and water during that process, one thing is 

 undeniable, the necessity of light to the growth and health of plants ; 

 and accordingly they are for the most part so formed as to receive it 

 at all times when it shines on them. Their cups, and the little assem- 

 blages of their leaves before they sprout, are found to be more or less 

 affected by the light, so as to open and receive it. In several kinds of 

 plants this is more evident than in others ; their flowers close entirely 

 at night, and open in the day. Some constantly turn round towards 

 the light, following the sun, as it were, while he makes or seems to 

 make his revolution, so that they receive the greatest quantity possible 

 of his rays. Thus clover in a field follows the apparent course of the 

 sun. But all leaves of plants turn to the sun, place them how you 

 will, light being essential to their thriving. 



The lightness of inflammable gas is well known. When bladders, 

 of any size, are filled with it, they rise upwards, and float in the air. 

 Now, it is a most curious fact, ascertained by Mr. Knight, that t ie fine 

 dust, by means of which plants are impregnated one from another, is 

 composed of rery small globules, filled with this gas in a word, of 

 small air balloons. These globules thus float from the male plant 

 through the air, and striking against the females, are detained by a 

 glue prepared on purpose to stop them, which no sooner moistens the 

 globules than they explode, and their substance remains, the gas flying 

 off which enabled them to float. A provision of a very simple kind is 

 also, in some cases, made to prevent the male and female blossoms of 

 the same plant from breeding together, this being found to hurt the 

 breed of vegetables, just as breeding in and in does the breed of animals. 

 It is contrived that the dust shall be shed by the male blossom before 

 the female is ready to be affected by it, so that the impregnation must 

 be performed by the dust of some other plant, and in this way the 

 breed be crossed. The light gas with which the globules are filled is 

 most essential to this operation, as it conveys them to great distances. 

 A plantation of yew trees has been known, in this way, to impregnate 

 another several hundred yards off. 



The contrivance by which some creeper plants are enabled to climb 

 walls, and fix themselves, deserves attention. The Virginia creeper has 

 a small tendril, ending in a claw, each toe of which has a knob, thickly 

 set with extremely small bristles ; they grow into the invisible pores of 

 the wall, and swelling stick there as long as the plant grows, and prevent 

 the branch from falling; but when the plant dies, they become thin again, 

 and drop out, so that the branch falls down. The Vanilla plant of the 

 West Indies climbs round trees likewise by means of tendrils; but when 

 it has fixed itself, the tendrils drop off, and leaves are formed. 



It is found by chemical experiments, that the juice which is in the 

 stomachs of animals, (called the gastric juice, from a Greek word 

 signifying the belly,) has very peculiar properties. Though it is for the 



