Vegetable Physiology. 243 



A more effectual provision for restraining the perspiration of 

 I lV es within due limits, is found in the epidermis or skin, that 

 • vests the leaf, as it does the whole surface of the vegetable, 



(1 which is so readily detached from the succulent leaves of 

 -uch pla ,lts as tne Stone-crop and the Live-for-ever of the gar- 

 dens- The epidermis is composed of small cells belonging to 

 t |ie outermost layer of cellular tissue, with the pretty thick- 

 «jde<l walls very strongly coherent, so as to form a firm mem- 

 brane. Its cells usually contain no chlorophylle. In ordinary 

 herbs that allow of copious evaporation, this membrane is made 

 u p of a single layer of cells ; as in the Lily and the Balsam. 

 It is composed of two layers in cases where one might prove 

 insufficient ; and in the Oleander, besides the provision already 

 described, the epidermis consists of three layers of very thick- 

 sided cells.* All these means of preventing evaporation might 

 be injurious to the plant, by entirely checking the necessary 

 evacuations; but Nature has provided against such a mis- 

 chance by the stomata already mentioned. 



Most leaves which grow in the usual position, are constructed 

 similarly to those just described. There are, however, many 

 leaves which grow upright, exposing each side equally to the 

 light. In these both sides are generally alike, and their colors 

 are the same. Each surface is equally furnished with inter- 

 cellular surfaces and stomata. This is found to be the case in 

 the common Iris. In other instances, the former plan is com- 

 pletely reversed, the stomata are only found on the upper sur- 

 face, and the upper parenchyma is much looser in texture than 

 the lower. In plants whose leaves float on the surface of the 

 water, this is usually the case. In the White Water Lily, for 

 example, the leaf is thick, and spongy, and contains large 

 reservoirs of air which give it buoyancy; but these are all just 

 beneath the upper surface, while in the lower surface which 

 lies on the water, and therefore performs none of its usual 

 functions, the cells are closely packed as in the upper surface 



* Thes* remarks, together with the figure, are extracted from Dr. Gray's " Botan- 

 ical Text Book," a work of which too much cannot be said in commendation, and 

 the principal inconvenience attending the possession of which is the overwhelming 

 temptation to appropriate its contents. 



