68 GARKEAU'S BXPEEIMENTS. 



which a plant grows. Submerged in water, perspiration is 

 necessarily arrested; in an ordinary atmosphere, it wiU he m 

 proportion to the quantity of elastic vapour the atmosphere may 

 contain ; and it is probable, although there are no experiments 

 upon the subject, that it is increased in proportion to the 

 rarefaction of the air. 



Among tie experiments of M. Garreau to wMoi allusion has been 

 already made, was one on the relative amoimt of perspiration hy the 

 two surfaces of ordinary foliage. Leaves growing on healthy plants 

 were selected, and a circular portion inclosed between two closely 

 fitting glass receivers, so arranged that the leaf formed the division 

 between the two glasses — ^the upper surface was in the one glass, whilst 

 the under surface of the leaf was in the other. The quantity of 

 moisture given off was ascertained by placing in each glass a weighed 

 .portion of dry chloride of calcium, which, being very greedy of 

 moisture, would absorb aU the vapour as fast as the surface of the 

 leaves gave it out. The result of this experiment was that the lower 

 surface of leaves gives off, irom an equal quantity, three times as much 

 as the upper surface ; sometimes the proportion was as high as five to 

 one ; and the ratio was independent of the position of the leaf itself. 

 This exhalation of water has some connection with the number and size 

 of the stomates, but is by no means wholly dependent on it, as there is 

 evidently a large quantity of water given off independently of them. 

 The evaporation is most abundant along the course of the nerves, and 

 in those parts of the epidermis, on which there is the least quantity of 

 oily matter. Hence it is apparent why carefully washing with soap 

 and water proves so beneficial (see- p. 68). The operation increases 

 greatly the power of evaporation, 



AU such experiments teach us that under ordinary circumstances the 

 growth of a plant causes the formation and development of certain 

 substances, which in time fiU up its pores, check perspiration, and 

 consequently interfere with the nourishment and farther growth of the 

 plant. But, on the one hand, it is possible that in hot weather these 

 matters may be useful in checking extreme perspiration, and in 

 diminishing for the time the powers of the plant to absorb too much 

 food from the air, or to part with water and oxygen too rapidly. On 

 the other hand, the effect of rain must be to wash away a portion of 

 these deposits, and so to favour the perspiration and consequent growth 

 of the plant. Moreover, as the more heat a plant is exposed to, the 

 more it perspires, and the faster it grows, the greater will the tendency 

 be to fiU up its pores ; so it foUows that when plants are exposed to 

 great heat in a close house, and are not washed or syringed, they are 

 placed in an unnatural condition, where the care of the gardener 

 defeats, to some extent, the object which he has in view. 



