HEPAT1CAE. 



145 



Metzgeria, Amur a, Pellia, and others. Rhizoids appear on the shaded side, and 

 near the apex in most species club-shaped papillae which secrete mucilage ; a layer 

 of the cellulose beneath the thin cuticle in these papillose hairs swells considerably 

 and bursts the cuticle, and the mucilage is spread over the growing point, which is 

 thus covered by a layer of this substance and protected from the danger of desic- 

 cation \ In place of these simple hairs some species have lamelliform outgrowths 

 which may be called scales ; such structures are found in the 

 Marchantieae which are distinguished for their high anatomi- 

 cal differentiation. Blasia too has a riband-like stem with 

 two rows of leaves on its under surface (amphigaslria) in 

 the form of small denticulate scales ; but it also has leaves 

 inserted parallel to the longitudinal axis of the stem, which 

 at first sight look like projections from the flat stem and 

 were formerly so described 2 . Next to Blasia comes the 

 genus Fossombronia with its stem not broadly expanded but 

 much flattened on the upper face, and on each side a row 

 of obliquely inserted leaves. These forms together make 

 up the thalloid, or as it was once called thefrondose, division 

 of the Hepaticae, as opposed to the foliose Hepaticae of the 

 family of the Jungermanniaceae ; in this latter division, which 

 includes also the thalloid forms Aneura, Blasia, Fossombronia 

 and some other genera, the vegetative body is a slender 

 filiform stem bearing sharply differentiated leaves (Junger- 

 mannia, Radula and Frullania). There are three rows of 

 these leaves, two on the sides, and one on the face turned 

 towards the substratum ; the last, the amphigastria, are smaller 

 than the others and are sometimes reduced to hair-like struc- 

 tures, or they are wanting, as in Jungermannia bicuspidata 

 and others. It has been already stated that the two divisions 

 of the Hepaticae are connected by intermediate forms afford- 

 ing easy transitions. The leaves of all the Hepaticae are 

 simple cell-surfaces, and have not even the mid-rib which is 

 common in the leaves of the Mosses. The anatomical struc- 

 ture of the stem and thallus is extremely simple. An epi- 

 dermis differentiated from the tissue beneath it is found only 

 in the Marchantieae, and the epidermis of this family has 

 stomata of a peculiar kind. Peculiar slits for the secretion 

 of mucilage occur on the under side of Anthoceros; other- 

 wise the differentiation of tissue is confined to the appear- 

 ance of elongated cells in the midrib of the thalloid forms, as in Blasia. Preissia 

 has thickened fibres, the extremities of which overlap like the sclerenchyma-fibres in 



FIG. 94. Riella helicophylla. 

 Plant in its natural aspect, grow- 

 ing erect and secured by rhizoids 

 at its base. It consists of an 

 axis'and the wing which is wound 

 round it in a spiral. From the 

 Exploration scientifique de FA1- 

 gerie. 



1 In Antkoceros the mucilage is formed not from the hairs, but in the intercellular spaces of the 

 under side of the thallus, which open to the outside by slits (mucilage-slits) ; see also below. 



2 [These appear to be the 'pistilla ' of Sir W. J. Hooker in his British Jungermannieae, the 

 ' pistilla sterilia ' of the Synopsis Hepaticarum of Gottsche, Lindenberg, and Nees ab Esenbeck.] 



0] i 



