TRANSMISSION OF THE FOOD-GASES. 369 



rapidly towards these parts, penetrates into the epidermal cells, is changed into 

 carbonic acid, and reaches the green tissue by means of the aqueous contents of the 

 epidermal cells. 



According to the previous statement, which has been discussed in detail, the 

 epidermis has also to provide for the transmission of the carbonic acid to the places 

 of consumption, viz. to the green tissue. 



In accordance with climatic and other local conditions, and corresponding to the 

 individuality of separate species, the epidermis presents, as is well known, endless 

 variations in structure. This variety of formation is concerned chiefly with the 

 part which it has to play as a protective covering, as strengthener, and the like. 

 As a conducting apparatus for carbonic acid, that is, in the form of a water mantle 

 or outer aqueous tissue, it exhibits comparatively little variation. In evergreen 

 plants which grow in warm, damp situations where transpiration is limited, and 

 where the water of the soil is often conducted by root -pressure to the large 

 transpiring leaf-surfaces, as, for examples, in tropical bananas, palms, mangroves, 

 figs, and peppers, the aqueous cells which lie above the green palisade tissue are 

 always arranged in several layers. In all those plants also whose outermost cells 

 in contact with the air have much thickened walls, and consequently a restricted 

 lumen, as, for example, in the Oleander, which grows on the sides of brooks (see 

 fig. 73 3 ), and in the proteaceous Dryandra fioribunda growing in the Australian 

 bush (see fig. 68), the water mantle consists of a double layer of cells. When the 

 green tissue is penetrated by vascular bundles and groups of strengthening cells 

 without chlorophyll, the aqueous epidermal layer is also interrupted, and is usually 

 only co-extensive with the palisade cells. In the leaves of grasses the colourless 

 aqueous cells form rows which are placed above the green assimilating tissue, and 

 surround this tissue as an actual mantle. 



The demand of the green tissue for carbonic acid regulates itself to the 

 consumption in the formation of organic substances. But the consumption is at a 

 maximum at the time of strongest illumination and greatest warming of the green 

 tissue, and therefore coincides with the most abundant transpiration. At such a 

 time the carbonic-acid-bearing sap is drawn by the active protoplasm in the green 

 tissue with great eagerness from the epidermal cells lying above, often so 

 abundantly that a quick replacement is impossible. But in consequence of this 

 the epidermal cells lose their turgescence; they collapse, and the hitherto tense 

 epidermis presents a flaccid appearance. In order that this collapse may take place 

 without injury, the following contrivance has been devised. The side-walls of those 

 cells which form the epidermis, i.e. the outer aqueous tissue, are delicate, thin, and 

 flexible, and as these cells give up a portion of their sap, their side-walls are folded 

 together just like a bellows from which the air has been expelled. When the cells 

 become again filled with fluid, the folds are straightened out as in a bellows filled 

 with air, and the cells regain their former tenseness. 



In the course of the foregoing representation we have only described the 



transmission of carbonic acid through the epidermal cells rich in watery cell-sap on 

 vol. i. 24 



