120 



LECTURE VIII. 



rarely at definite points on the surface of the leaf, are distinguished from the ordinary 

 air-stomata as water-stomata : beneath them are the terminations of fibro-vascular 

 bundles which convey the water. Later on, when the movements of water in the 

 plant is being treated of, we shall return to these modified stomata. If, proceeding 

 from the vascular plants down towards the more simply organised groups, we trace the 

 formation of epidermis, we often find, especially in the Muscinece, that where it is a matter 

 not of simple tissue-layers but of solid masses of tissue containing chlorophyll, a com- 

 pletely typical epidermis, even with stomata, is present. In the true Mosses, the spore- 

 forming capsule, also histologically complex in other respects, possesses stomata in 



u;. 123.— Transverse section of tlie liorizontal flat slioot of Manlicintia 

 •orfha. A middle portion, witli leafy appendages (b) and roots (//) on tlie 

 lower surface (X 30); B part of margin of the shoot, more highly magnified; 

 / colourless parenchyma with reticulate thickenings ; epidermis of the upper 

 surface; chl chlorophyll cells; sp stomata; j partitions between the broad 

 intercellular spaces; u lower epidermis, with dark-coloured cell-walls. 



<-ia liygyometricn. Portion of a longitudinal section of the 

 I'hich develope later the inner and outer peristome. 



;; / parenchyma ; 



a highly organised epidermis ; and among the flat-shooted Liverworts, the shoot, com- 

 posed of several layers of cells, exhibits on the surface conspicuously large stomata, 

 deviating from the typical structure, and also differing in their development. In all 

 Algae and Fungi, on the other hand, these organs are completely wanting ; and in 

 most cases, the superficial tissue of these plants cannot well be termed epidermis, in 

 the narrower sense of the word, although wherever the tissue of a plant consists of 

 several layers, the outer layer, formed of one or more strata of cells, is so organised 

 that it sharply shuts off the internal tissue from the external world. The cells of 

 the outer layer especially fit together without inter-spaces, and are, like the 

 epidermis cells of the higher plants, generally somewhat smaller than those of the 



