1870.] Botany. 103 



M. Van Horen finds that two only, the L. polyrrhiza and gibba, 

 produce leaves of a different form in winter ; while with the three 

 other species, L. minor, trisulca, and arrhiza, the ordinary leaves 

 live through the winter, remaining on the surface. In L. jpolyrrhiza 

 these winter-leaves first make their appearance in August or Sep- 

 tember. They are much smaller than the ordinary leaves, reniform 

 or sometimes elliptical, olive-brown on both sides, and not gibbous 

 beneath ; their roots are exceedingly minute, and at first hidden 

 w T ithin the leaf. The aeriferous cells which serve to support the 

 ordinary leaves on the surface do not exist, causing the winter- 

 leaves to resemble an undeveloped bud. In consequence of the 

 absence of these vessels they are heavier than the water, and fall to 

 the bottom as soon as any agitation of the water detaches them 

 from the parent-leaf, which perishes with the first frost. At the 

 ordinary period of the revival of vegetation, a small bubble of oxygen 

 appears on the upper surface of these submerged leaves, which carries 

 them to the surface, from which they again descend should the 

 temperature fall below a certain point. In Lemna gibba, leaves of 

 a similar character, were observed hibernating beneath ^he water, 

 differing in shape, size, and structure from those developed during 

 the summer. 



Evaporation of Water and Decomposition of Carbonic Acid by 

 Plants. — An interesting and important series of experiments has 

 been made by M. P. P. Deherain, of Paris, for the purpose of 

 determining the natural agents most efficacious in promoting the 

 physiological functions of- the leaves of plants, the evaporation of 

 water, and the decomposition of carbonic acid. The results arrived 

 at are as follows : — 1. These two processes are carried on simulta- 

 neously and with corresponding intensity, the same agents which 

 facilitate the one being operative also with the other. They proceed 

 more rapidly from the upper smooth and hard surface of the leaves 

 than from the under-surface. 2. The principal agent in deter- 

 mining these changes is not temperature, but light. While the 

 amount of water given off was hardly affected by any changes of 

 temperature, proceeding almost with equal rapidity even when 

 surrounded by ice ; in bright sunshine leaves were found to give 

 off, in long exposure, more than their own weight of water, while 

 in diffused light it amounted to only six to eighteen per cent., and 

 in total darkness was scarcely perceptible. The evaporation and 

 condensation of water proceeded with equal rapidity when the air 

 was perfectly saturated with moisture. 3. The different rays of 

 light are not equally efficacious in promoting these actions. M. 

 Deherain found, as the result of a number of experiments, that, 

 with an equal intensity of light, the red and yellow rays, which 

 have little photographic power, cause in the same time the decomposi- 

 tion of about five times as much carbonic acid, and the evaporation 



