742 PHYSIOLOGY. 



while in the latter state, the exhalation is more or less prevented/ 

 In all the above cases of adaptation of the epidermis and sto- 

 mata to the conditions under which plants are placed, it is 

 impossible not to be struck with the wonderful evidences of 

 Design exhibited. 



It is also through the cells of the epidermis, and more espe- 

 cially through the stomata, that certain gaseous matters are 

 absorbed from, and exhaled into, the atmosphere, in the processes 

 of Respiration and Assimilation. (See p. 752.) 



It is still a disputed question whether the epidermal tissue 

 and its appendages have the power of absorbing liquids, such 

 as water. Some authors, as Unger and Dachartre, not only deny 

 the possession of such a power, but also that of taking up watery 

 vapour ; and Prill ieux has lately repeated their experiments with 

 the same resiilts and conclusions. It is, however, very difficult 

 to account for the immediate recovery of drooping plants in a 

 greenhouse, when water is sprinkled upon the floors, or the 

 revival in nature of vegetation, when a mist follows a long 

 succession of dry weather — except on the supposition that 

 watery vapour is taken up by the epidermal tissue and its ap- 

 pendages. Epiphytical species seem also to obtain nourishment 

 from the atmosphere by absorption through the epidermis. 

 Whether water itself is absorbed by the epidermal tissue and 

 its appendages is very doubtful. Various experimenters have 

 endeavoured to show that they have this power. The researches 

 of Garreau led him to the following conclusions: — 1. That the 

 cuticle possessed an evident endosmotie property, the intensity 

 of which was in proportion to the age of the tissues which it 

 invested ; thus it was greatest when they were young, and gra- 

 dually diminished as they approached maturity, and was alto- 

 gether lost when they became old. 2. The absorbing power of the 

 cuticle was greater in proportion to the absence of waxy or fatty 

 matters. 3. The cuticle covering the upper surface of the ribs, 

 and especially of that of the petiole where it joins the stem, is 

 that part of the leaf surface which presents the most marked 

 power of absorption. 4. In certain instances in which the 

 cuticle is absorbent, the epidermis presents impediments to ab- 

 sorption. 5. Simple washing with distilled water, and more 

 especially with soap and water, augments the absorptive power. 

 6. When the epidermal tissues of leaves have lost their power 

 of absorbing water, they can still absorb carbonic acid. 



Origin and JDevelo'pmcnt of Stomaia. — The exact origin and 

 mode of development of stomata is not clearly ascertained. 

 By Mohl and other authors, the stomatal cells are described as 

 originating from one of the cells situated below the epidermis 

 rising into a space formed by the separation of the epidermal 

 cells at the points where stomata occur. Nageli and others, 

 again, describe the stomatal cells as being formed originally out 



