390 A TEXTBOOK OF THEORETICAL BOTANY 



They are divided into four orders of which we shall consider examples 

 from the following three : — 



1. Jungermanniales {e.g., PeUia). 



2. Marchantiales {e.g., Marchantia). 



3. Anthocerotales {e.g., Anthoceros). 



Jungermanniales 



The Jungermanniales are Hepaticae in which the thallus is never differ- 

 entiated into distinct layers of tissue, and pores are never present. The 

 rhizoids are of one kind, unicellular, and always smooth-walled. The sex 

 organs are arranged in groups on the thallus or its branches, though in the 

 latter case these branches are not morphologically specialized as in the 

 Marchantiales. The sporogonium is often provided with a long stalk bearing 

 a capsule which dehisces by four valves. 



This order is the largest of the Hepaticae and is subdivided into two 

 sub-orders : Anacrogynae, mostly thallose, in which the archegonia arise 

 from behind the apical cell, and Acrogynae, mostly foliose, in which the 

 archegonia arise from the apical cell itself. 



We shall consider one example of the order, Pellia epiphylla, which is a 

 member of the Anacrogynae. 



Pellia epiphylla 



Pellia epiphylla occurs commonly throughout the country by the sides 

 of streams, in damp woods and under hedges. Occasionally it appears on 

 old humus-laden sand dunes. It consists of a flat, lobed thallus which 

 branches repeatedly, so that the lobes often overlap one another (Fig. 372). 

 Moreover, since many plants generally grow together, the Liverwort may 

 cover quite large areas of ground. A detailed examination of a piece of the 

 thallus shows that the upper surface is considerably darker green in colour 

 than the lower, and is traversed by a purplish-coloured midrib. On the 

 under surface numerous rhizoids are borne in the region of the midrib, 

 and by these the plant is attached to the ground. 



The internal structure of the thallus is simple, and is composed of 

 polyhedral cells forming a uniform tissue called parenchyma. Lhilike the 

 plants which we have considered so far, which were predominantly made 

 up of filaments, the thallus of the Liverwort is composed of layers of cells 

 joined together in a honeycomb-like manner. We shall find that this form 

 of tissue forms the basis of the tissue-organization of all land plants. The 

 chloroplasts are restricted chiefly to the upper layers of cells, but starch 

 grains occur in the cells throughout. These starch grains constitute the 

 reser\'e food material resulting from Carbon assimilation. The layer of cells 

 which covers the upper and lower surface of the thallus is called the 

 epidermis. The rhizoids grow from the cells of the lower epidermis and are 

 unicellular. 



