LEAVES 



533 



In most of the lower plants the chloroohvll-hparing 



makp up the, body 



th^ plant and are not delimited into spprial fissnpg I n 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 centralfy\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) \^bose chloroplasts are grouped 

 chiefly at the basal end^ and loosely 

 arranged sponge cells (/); note the 

 heavy epidermis (e), 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 crpss section of the 

 upper part of a succulent xerophytic 

 leaf, that of the purslane (Portulaca 

 oleracea), showing a festoon (/) of 

 funnel-shaped cells with large basal 

 chloroplasts grouped about a small vein 

 (v), and an outer ring of palisade cells 

 (/>), 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-eaosules- the chlorenchyma ig Hiffprpntla^fl. 



Jnto palisadp and jnric ^'-JW , TTII^h 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 

 chlorenchyma cells. The plasticity of chlorenchyma. Chjflrpricjhyma<l 

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

 and even its differentiation into palisade and rponge often being subject 



