112 



STRUCTURAL BOTANY 



midrib, and very thin margins. In damp positions, 

 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, 

 with 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 thallus. 



The anatomical structure of 

 the thallus is excessively 

 simple. It consists entirely of 

 parenchyma, the cells of which 

 are elongated in the midrib, 

 and polyhedral in the rest of the 

 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 are 



FIG. 50. Part of the thallus 

 of Pellia, seen from above. 

 an, the numerous antheridia ; 

 r.h, r.h, the root - hairs. 

 Slightly magnified. (R. S.) 



