164 



PLANT MORPHOLOGY 



ings that project into the lumen. In Riccia rhizoids are usually abun- 

 dant, but frequently ventral scales are rudimentary or absent. ^ In nearly 

 all the Marchantiales the ventral scales are arranged in two longitudinal 

 rows; in Marchantia they are in four or more rows. 



Throughout the order decay of the older parts of the thallus results in 

 the isolation of branches, each of which forms a new plant. In some 

 species vegetative propagation occurs by the formation of adventitious 

 branches that become detached. In two genera, Lunularia and Marchan- 

 tia, multicellular gemmae are produced. These are flat, stalked, discoid 

 bodies that arise in groups on the dorsal side of the thallus inside cupules. 



Fig. 131. Male (A) and female (B) plants of Marchantia polymorpha, natural size. 



In Lunularia the cupules are crescentic, while in Marchantia they are 

 cup-shaped. The gemmae arise from the floor of the cupule. Each 

 gemma has two notches, one on either side, and in each notch is an apical 

 cell. Upon separation from the cupule, a single gemma gives rise to two 

 new thalli. 



Sex Organs. The sex organs of the Marchantiales are invariably 

 dorsal in origin, arising either directly on the thallus itself or on a more or 

 less specialized receptacle. Both kinds of sex organs arise in acropetal 

 succession from segments of an apical cell. According to the species, the 

 antheridia and archegonia occur on the same plant or on separate plants. 

 In Riccia each branch of the thallus has a median dorsal groove extending 

 backward from the growing apex; in this groove the sex organs are borne. 

 Although generally scattered irregularly, they sometimes tend to be 

 segregated into separate groups. The sex organs arise singly just behind 



' When Riccia nutans floats on the surface of quiet water, it has numerous large 

 scales and few or no rhizoids. When it grows on muddy banks and flats, it has many 

 rhizoids and few scales. 



