November 1, 1890.] 



KNOWLEDGE 



247 



the under surface of leaves. By a comparison of the dia- 

 grams of Fig. 1 with those of Figs. 2 and 3 a tolerably 

 correct idea of the structure of the ordinary stoma may 

 be obtained. In each case tira cells, termed the (juard- 

 crlls, surround the stoma ; and these are contiguous at 

 each end of the slit. The whole apparatus is elliptical in 

 outline, and the longer axis of the ellipse generally assumes 

 the same direction as that of the structure on which the 

 stoma arises. Each of the stomata depicted in the figures 

 has arisen by a splitting of a cell of the epidermis ; but 

 nevertheless they differ in many important respects from 

 the cells which surround them, and also exhibit differences 

 among themselves chiefly as regards their relative posi- 

 tions. In the first place it. is plainly evident that, with the 

 exception of the Fern (Scolopendrium vulgare), chlorophyll 

 or green-colouring matter is absent from all the epidermal 

 cells except the guard-cells, which are densely packed with 

 chlorophyll granules. In the second place, the walls of 

 the guard-cells exhibit a difl'erence in thickness, those 

 portions next the apertures, and those contiguous to the 



cuticle 



,e}3(.ae 



(quard- 

 l-'ceU 



'ha-rench^yna. 



]»rne%o^1rtt^l\ 



Tfsbiratory cavitu 

 Stoma 'sclereMcViiirna.' 



fijl 



Fl(i. 1. — TllANSVERRK SECTION OF THE LeAF OF Osmcmfhus. 



The cuticle is the continuous outer coat of the epidermal cells. The 

 xt/lein (^uXor, wood) is the technical name applied to the wood, the 

 tissue which is the chief conductor of the sap up the stem. Phlnem 

 (<^X£a), to overllow) is the tissue which carries down the elaborated 

 sap. Schrenr.hiiiiiii ((TKA/jpos, hard)is thick-walled supporting tissue. 

 The menophjill (|U.€(tos, middle ; and t^i'XXov, a loaf) is the tissue 

 which occupies the middle of the leaf : it consists of a loosely arranged 

 tissue with many air-spaces amongst the cells. 



epidermal cells which may be termed the lateral walls, 

 are very thin ; while, on the other liand, the walls facing 

 the outside of the leaf, and in part lining the respiratory 

 cavity, have become much thickened. Furthermore, the 

 position of the whole stomatal apparatus is subject to 

 variation. In the C'rassulaand White Lily (Fig. 8, a, v) the 

 guard-colls are almost on a level with the other epidermal 

 cells, whereas in the Iris and Horse-tail (Fig. 3, h, d) they 

 are in the one case half, and in the other wholly, sur- 

 mounted by these cells. The two lateral adjacent cells of 

 the epiilcrmis in the case of the Seduni form part of the 

 stomatal apparatus ; they are termed the siihsiilKiri/ ccZ/.s- af 

 the stoma. The stomata of Nerium, the Oleander, and 

 a climbing plant common in our greenhouses, occupy a 

 peculiar position. On the surface of the leaves arc 

 numerous pits ; each of these is liali filled with hair-like 

 appendages, projections of the cells lining the pit, and at 

 the base arc situated the stomata, two, three, or more in 

 number. In all cases the stomata open into an inter- 



cellular space, the so-called " respiratory ca\dty," which is 

 enclosed by thin-walled cells containing chlorophyll. 

 Figure 2 and its description give fuller information on the 

 stomatal surroundings. There are many anomalous forms 

 of stomata, which the reader can easily find out for him- 

 self by adopting the simple methods described in the foot- 

 note.* 



With these we will not at present deal, but will proceed 

 to discuss the mechanism of the stomata and the use of 

 the stomata to the plant. 



The mechanism of the stomata is a subject which has 

 received the attention of many botani.sts, but no satisfac- 

 tory solution of the problem has yet come to hand. The 

 following may be taken as a general statement of the 

 present views on the case. In the guard-cells are many 

 granules of the green colouring matter, chlorophyll. The 

 presence of these induces during daylight carbon assimila- 

 tion. In other words, in these, as well as in the other 

 chlorophyll-bearing cells, the carbon dioxide of the air, 

 commonly but wrongly spoken of as carbonic acid gas, is 

 decomposed into its constituent elements — carbon and 

 oxygen. The carbon and part of the oxygen are re- 

 tamed by the cells, and the rest of the oxygen is given 

 out again to the atmosphere. The carbon and oxygen 

 then unite with water which is present in the ceDs. The 

 using up of this water causes a fresh supply to be drawn 

 in from the surrounding cells. This supply probably 

 comes, in the case of the guard-cells, from the surrounding 

 epidermal cells, and the process goes on continuously 

 during sunlight, so that the guard-cells are during that 

 period always kept filled with a watery fiuid, and thus 

 rendered turgid. Turgidity causes a straightening of the 

 thin lateral walls of the guard-cells, and a simultaneous 

 curving of each of these cells in such a manner as to open 

 the orifice. On the other hand, when the guard-cells 

 become flaccid, as they do at night, the orifice is closed. 



A few general remarks on vegetable physiology form 

 a necessary introduction to a discussion on the function of 

 the stomata. As we stated above, protoplasm is the 

 essential portion of the plant ; it is the vital substance 

 by whose activity all the other portions of the organism 

 are built up. Its composition is extremely complex ; no 

 chemical formula can be assigned to it, but it is known 

 that the elements carbon, hydrogen, oxygen, nitrogen. 



* As many of oar readers are doubtless the possessors of a micro- 

 scopo, perhaps the following hints on the preparation of slides for 

 microscopic use will be appreciated. If it bo wished to make an 

 examination of the epidermis of the leaf, the leaf should be steeped 

 for some time in water and then folded round the fore finger of the 

 left-hand. The operator can then peel off a portion of the epidermis 

 by inserting a needle mounted in a handle into the leaf near the 

 thickest vein or midrib, and then working gently imder the epidermis 

 in a direction away from the midrib. To obtain a cross or transverse 

 section, that is one at right angles to the blade of the leaf, requires 

 more skill and practice. A part of the leaf should be placed in a slit 

 made in a ])iece of carrot or pith, and then with a very s/iar/i hollow 

 ground razor, thin sections of both carrot and loaf should bo cut off. 

 The razor should not be forced right across the object, but should be 

 worked with a sliding motion fr mi hoel to tip. Both the object and 

 the razor must bo kept wet with dilute methylated spirit. The 

 sections obtained should be transferred to a watch-glass containing 

 water and methylated spirit, and then the thinnest of these having 

 been selected, they can bo perm.anently mounted by placing them, one 

 in each case, by means of a camel-hair brush, on a glass sli c mea- 

 suring 3 inches by 1 inch. The supertluous water must now be drained 

 off with a piece of blotting-paper : then some glycerine jelly must be 

 gently lowered on the section with a glass rod. The whole must ihen 

 be covered with a thin glass cover-slip of J-inch diameter. The 

 epidermis may be mounted in the same manner. 



N.B. — The needles in handles, glycerine jelly, slides, and cover- 

 glasses, can all be obtained from any dealer in microscopical 

 appliances. They are quite inexpensive. The glycerine jolly 

 requires to bo heated before use. 



