188 



PHYSIOLOGICAL BOTANY. 



petiole ; and upon removing the receiver, the water will 

 then find its way into the leaf, and occupy the in- 

 terstices which were originally filled with air. This 

 effect is rendered particularly striking in those leaves 

 whose under surfaces are of a paler colour than their 

 upper, in consequence of the larger dimensions of the 

 intercellular passages in those parts. When the water 

 is introduced and occupies the whole of these passages, 

 the two surfaces become equally coloured. 



(1?4.) Air Cells. Besides the air in the leaves, some 

 also is found in the stems and other parts of plants, where 

 its precise use has not been fully ascertained. In many 

 aquatics, indeed, it is contained in large cavities, termed 

 "lacunae," as we have stated (art. 21.). The obvious use 

 of such reservoirs as these, is to float the leaves and other 

 parts in which they exist. The Pontiih-rw mm/>!# 

 has its petioles (fig. 1 53 rt.) remarkably distended with 

 air. The roots of the Utri- 

 cularia? are furnished with a 

 multitude of little bladders 

 (fig. 32.) by which they are 

 floated to the surface during 

 the season of flowering ; and 

 a number of other instances 

 might be mentioned where 

 some provision or other of 

 this kind exists. But, be- 

 sides the mere mechanical 

 effects which are thus pro- 

 duced, it is probable that 

 the air introduced into the 

 system may in many instances serve some physio- 

 logical purpose. It seems to be sufficiently ascer- 

 tained, that some portions at least of the vascular system 

 are destined to convey air from one part of the plant to 

 another. The spiral vessels and some ducts are often 

 found filled with it; and in these positions, according to 

 some experimenters, it contains rather more oxygen 



