112 



STRUCTUKAL BOTANY 



midrib, and very thin margins. On damp ground, 

 where Pellia attains its greatest luxuriance, the thallus 

 is much broader than in the aquatic form, but still 

 elongated, with the branches spread out nearly flat, 

 and the midrib very strongly marked. On dry sandy 

 soil the plant assumes a very different form ; the 

 thallus remains short and stunted, with densely crowded 

 branches overlapping each other. The whole plant is 

 much thicker and tougher, and consequently the midrib 



becomes indistinct. In spring, 

 when the plants begin their 

 new growth, they send out a 

 great number of small crowded 

 branches, giving a parsley-like 

 appearance to the growing edge 

 of the thalius. 



The anatomical structure of 

 the thallus is excessively 



r.'h. 



simple. It consists entirely of 

 FIG. 50. Part of the thallus parenchyma, the cells of which 



of Pellia. seen from above. n , -, i M 



an, the numerous antheridia ; are elongated in the midrib, 

 r.h, r.k, the root - hairs. an d polyhedral in the rest of the 



Slightly magnified. (E. S.) , ri , 1 , ., , 



thallus. Chlorophyll-granules 



occur chiefly in the more superficial cells. They are most 

 abundant in the cells on the upper surface and in all 

 cells of the thinner marginal portions. The whole tissue 

 is rich in starch-grains. The epidermis scarcely differs 

 from, the rest of the tissue, but has a thin cuticle, at 

 least on the under surface of the thallus. In the interior 

 of the middle part of the thallus there are sometimes groups 

 of cells with much thicker walls than their neighbours. 

 The walls, however, are of cellulose, and there is no 

 further differentiation. The root-hairs, or rliizoids, are 



