THE ARCHEGONIAT^E 205 



tropical forests, the spores contain chlorophyll ; but where the spores 

 are adapted to endure a long period of drought, as in most Cali- 

 fornian species, the ripe spores contain no chlorophyll, but are filled 

 with food materials, largely oil and albuminous granules. Such 

 spores have much heavier walls, also, than those which contain 

 chlorophyll. 



Germination. Where chlorophyll is absent from the ripe spores, 

 the first step in germination is the appearance of chlorophyll in the 

 spore, although the amount is sometimes small. The exospore and 

 perinium are ruptured (Fig. 162), and the spore-contents, included 

 within the endospore, or intine, appear as a papilla, the germ-tube, 

 which often becomes much elongated. Usually from the base of the 

 germ-tube a small papilla is cut off, which rapidly elongates into 

 the primary rhizoid. 



At the end of the germ-tube a mass of cells is developed, which 

 soon becomes a flattened thallus, growing from a definite apical cell. 

 This apical cell, in most cases, is of the two-sided type, found per- 

 manently in Metzgeria and Aneura. Sooner or later, this is replaced 

 by the type found in the mature gametophyte. With few excep- 

 tions, the young plant assumes gradually the characters of the adult. 



Classification of Hepaticae 



The Hepaticse (exclusive of the Anthocerotales) may be divided 

 into two orders, the Marchantiales and the Jungermanniales. The 

 gametophyte in the former is always thallose, and may become very 

 complex ; in the latter, it may be either thallose or foliose, but 

 always is relatively simple in its cellular structure. 



Order I. Marchantiales 



These very characteristic plants possess a prostrate, fleshy thallus 

 (Figs. 1G3, 164), which usually grows upon the earth, to which it is 

 attached by numerous root-hairs of two kinds, large, thin-walled 

 ones, and smaller hairs, with undulate walls, having peculiar spike- 

 like thickenings projecting into the cavity of the cell. 



The branching of the thallus is usually dichotomous, but adventi- 

 tious shoots are common in many forms. With the exception of the 

 tropical genera, Dumortiera and Monoclea, in which the differentia- 

 tion of the tissues is poorly marked, the Marchantiales show two 

 definite regions of the thallus (Fig. 167), a central portion, composed 

 of compact, colorless tissue, sometimes containing special mucilage- 

 cells, or ducts, and cells with oil-bodies. The dorsal tissue, which 

 may merge somewhat gradually into the ventral tissue, is composed 

 of green cells, with large air-chambers, or lacunae. These chambers 



