LEAVES 



533 



In most of the lower plants the chlorophyll-bearing cells make up the body of 

 the plant and are not delimited into special tissues. In the red algae there are 

 elongated cells arranged somewhat as are palisade cells. Most moss and liver- 

 wort leaves consist of a single layer of green cells, but in Sphagnum colorless cells 

 alternate with the green cells (fig. 899) ; the leaves of Leucobryum are three cells 

 thick, the chlorophyll-bearing cells being centrally placed (fig. 900). In the air 

 chambers of some thalloid liverworts (as Marchantia, fig. 799) there are loose cell 



Fig. 766. — A cross section of a 

 succulent leaf, that of Begonia, show- 

 ing centrally placed chlorenchyma (c), 

 consisting of funnel-shaped palisade 

 cells (p) whose chloroplasts are grouped 

 chiefly at the basal end, and loosely 

 arranged sponge cells (/); note the 

 heavy epidermis («), averaging three 

 cells in thickness above and two be- 

 neath, and quite without chloroplasts; 

 such an epidermis represents a peripheral 

 water tissue ; considerably magnified. 



Fig. 767. — A cross section of the 

 upper part of a succulent xerophytic 

 leaf, that of the purslane {Porlulaca 

 oleracea), showing a festoon (t) of 

 funnel-shaped cells with large basal 

 chloroplasts grouped about a small vein 

 (v), and an outer ring of palisade cells 

 (p), containing chloroplasts of ordinary 

 size; note that the palisades are not 

 symmetrically placed but appear to be 

 oriented with respect to the incident 

 light; the epidermis (e) is two cells 

 thick and represents a peripheral water 

 tissue; highly magnified. 



filaments, which contrast with the compact tissues elsewhere; in the leaf of 

 Polytrichum similar filaments form vertical plates hanging loosely from the leaf 

 body. Near the base of some moss capsules the chlorenchyma is differentiated 

 into palisade and sponge tissue, much as in seed plants (fig. 263). In lichens 

 the chlorenchyma consists of the algal layer, which lies close to the surface in shade 

 forms and deeper in sun forms (fig. 1112). 



The influence of external factors upon the form and orientation of 

 chlorench3mia cells. — The plasticity of chlorenchyma. — Chlorenchyma 

 is one of the most plastic of plant tissues, its thickness, its compactness, 

 and even its differentiation into pahsade and sponge often being subject 



