THE STEM. 



21 



animals. When examined with the microscope, 

 it is seen to be composed of a single layer of 

 cellules, whose form varies exceedingly in differ- 

 ent plants. It is this cellular structure that has 

 led into error the authors who have thought the 

 epidermis to he formed of the outer wall of the 

 cellular tissue. But, were this the case, the 

 cellules which constitute the epidermis would 

 always have the same form as the subjacent 

 tissue, which, however, they are found not to 

 have. Thus, in the pink, the cellules of the 

 epidennis have a four-sided form, while the im- 

 mediately subjacent layer consists of a multitude 

 of tubes perpendicular to the epidermis. The 

 same occurs in many other vegetables; from 

 which it ^lay be concluded that the epidermis 

 is a cellular membrane, entirely distinct from 

 the subjacent tissue, upon which it is merely 

 applied. 



The epidermis presents numerous small open- 

 ings, named cortical pores, cortical glands, epi- 

 dermic glands, and lastly, stomata. Several 

 authors have denied their existence ; hut Amici, 

 by the aid of the microscope, has seen them in a 

 great number of vegetables, and has described 

 and figured them with the greatest accuracy. 

 They are a kind of small bags, situated in the 

 substance of the epidermis, and opening exter- 

 nally by a slit or elongated oval aperture, bor- 

 dered .with a kind of rim formed by particular 

 eeUules of the epidermis. This rim, or thick- 

 ened margin, which is very seldom wanting, 

 possesses the power of contracting or dUating 

 the aperture according to circumstances. They 

 are here represented as seen in the leaves, a b. 

 Thus, humidity or water 24 



closes the pores, while 

 drought, and the action of 

 the solar rays, keep them 

 open, and separate their mar- 

 gins. The motions of dUa- 

 tatiou and contraction are 

 not confined to the living 

 plant alone, hut also take 

 place in detached fragments 

 of the epidermis. These 

 pores or little bags always correspond by their 

 base to spaces filled with air only, and resulting 

 from the arrangement of the cellules or tubes 

 with respect to each other. These intercellular 

 spaces almost always communicate with each 

 other, and thus afford a means of communica- 

 tion to the aeriform fluids which exist in the 

 interior of vegetables. Some parts, however, as 

 the roots, the petioles which are not leafy, the 

 petals in general, the epidermis of old stems, and 

 that of fleshy fruits and seeds, appear to he desti- 

 tute of stomata. Certain leaves have them only 

 on one of their surfaces, while others have them 

 on both. 



Various conjectures have been formea regard- 



ing the use of these curious pores. They can- 

 not be destined for the absorption of moisture, 

 for we have already seen that they correspond 

 to internal spaces which are destitute of juices, 

 that they are closed by water, and that light 

 and drought cause them to open. Moreover, 

 they are wanting in all roots, as well as in plants 

 that live constantly under water. They do not 

 therefore serve for the absorption of water. Nor 

 are they intended for evaporation; for if we 

 allow a plant which has been detached from its 

 roots to die, although the pores close after some 

 time, evaporation still continues, so long as any 

 fluid remains in its interior. It has been ob- 

 served, moreover, that the coroUas and fruits, 

 which are destitute of cortical pores, yet produce 

 an abundant evaporation, M. Link supposed 

 them to be excretory organs, but this cannot be 

 the case, as they always correspond to empty 

 spaces. The real office of the cortical pores 

 seems to be to give passage to air. But it is not 

 easy to determine with certainty whether they 

 serve for inspiration more than expiration, or 

 for both these functions ahke. If we consider 

 that at night, when the large pores of the epi- 

 dermis are closed, leaves absorb carbonic acid 

 gas dissolved in the dew, which undoubtedly 

 penetrates into the cellules by passing through 

 their membrane; and if we reflect, moreover, 

 that these leaves decompose carbonic acid gas, 

 when the pores are open, that is, during the day, 

 we may suppose them to be solely destiped for 

 the exhalation of oxygen. This use becomes 

 still more probable, when we add that the 

 coroUas which, according to DecandoUe's obser- 

 vations, are destitute of pores, are equally desti- 

 tute of the faculty of disengaging oxygen. 



The surface of the epidermis sometimes pre- 

 sents certain organs named lenticular glands, or 

 lenticelles, which appear under the form of small 

 spots elongated in the longitudinal direction in 

 young branches, and in the transverse direction 

 in older branches. No traces of them have yet 

 been discovered in the monocotyledonous or 

 aootyledonous plants. They are also wanting 

 in the herbaceous plants of the dicotyledonous 

 class. They are very distinct on the epidermis 

 of the birch, and especially on that of euonymtis 

 verrucosus, where they are very prominent and 

 close. From these lenticelles spring the roots 

 which certain trees develope upon their stem, or 

 those which form when a branch is immersed in 

 the ground, as in the operation of propagating 

 by layers. They may therefore, in some measure, 

 be considered as root-buds. 



Prom the surface of the cuticle also spring the 

 hairs of various kinds which are observed on 

 many plants. 



The herbaceous envelope. Under the epider- 

 mis is observed a layer of cellular tissue, con- 

 necting the former with the cortical layers, and 



