340 PART III. THE CLASSIFICATION OF PLANTS, 



MarchantieaB the walls of these cells are generally thickened and pitted ; some 

 of the cells contain mucilage, and in Fegatella the mucilage-cells form con- 

 tinuous rows ; other cells contain a dai'k-coloured oil-drop, though such cells 

 also occur in the air-chamber-layer ; in Preissia brown-coloured sclerotic fibrous 

 cells occur, arranged in longitudinal strands. 



The ventral surface is formed by a layer of cells which, in the simpler forms, 

 is not specially differentiated, but in some the cells of this layer are remarkable 

 for their small size ; in Marchautia and Preissia there are several layers of these 

 small cells, forming a sort of ventral cortex. 



The ventral scales consist of a single layer of cells, the walls of which 

 generally assume a violet colour ; each scale is developed from a single super- 

 ficial cell, or, as generally in the Biccieae, from a transverse row of cells. In 

 Marchantia polymorplia, in addition to the scales which arise from the midrib, 

 there are others which spring from the surface of the lamina. 



Unicellular root-hairs are produced in all Marchantiaceae ; the commonest 

 form has thin walls ; in the MarchantieaB a second form occurs, in which peg- 

 like thickenings of the wall project into the cavity of the cell : the simple root- 

 hairs are developed mainly on the midrib, the thickened hairs mainly on the 

 lamina. 



Gemmae are produced in Lunularia and Marchantia in special receptacles, 

 termed cupules, borne on the dorsal surface of the shoot ; in Lunularia the 

 cupule is crescent-shaped, in Marchantia it is circular (Fig. 240 B). The cupule 

 is formed by an outgrowth of the air-chamber layer, and in Marchantia its 

 margin is prolonged into laciniae. The gemmae spring from single cells of the 

 floor of the cupule, which elongate upwards and divide transversely into a 

 etalk cell and a terminal cell, which, by repeated growth and division, forms a 

 flattened plate of tissue, several layers of cells thick at the middle, thinning 

 out to a single layer at the margin, with a growing-point in a depression on 

 each lateral margin. The symmetry of the gemmae is isobilateral ; but when 

 they fall on to the soil and begin to grow, the undermost surface becomes the 

 ventral, and the uppermost the dorsal. Some of the superficial cells have no 

 chloroplastids ; those of the surface next the soil grow out into root-hairs. 



B. The SPOKOPHYTE. The degree of morphological and histological differ- 

 entiation of the sporophyte presents wide divergences in 1 the different groups. 

 The oospore undergoes division by a wall, generally inclined at an acute angle to 

 the long axis of the archegoniurn, the basn.1 wall, into two halves, the epibasul 

 and the Jiypobasal ; each of these is then divided into two by a wall at right 

 angles to the basal wall, and each of these again by a wall at right angles to 

 the two preceding walls ; thus the embryo comes to consist of eight cells 

 (octants). 



In the Riccieae, the whole embryo simply forms a spherical capsule : in the 

 Marchantiese, the capsule is developed entirely from the epibasal cells, whilst 

 the hypobasal cells give rise to a bulbous foot, which attaches the embryo to the 

 parent, and to a short stalk which bears the capsule, and is formed at a rela- 

 tively late stage by intercalary growth. In the Corsinieae the embryo is 

 differentiated into capsule and foot, but it is uncertain whether or not the limit 

 between these two organs is marked, as it is in the Marchautieae, by the br.sal 

 wall {see Fig. 239). 



