HEPA TIC.E. 



303 



growing margin, through which filaments of Nostoc frequently penetrate, forming roundish 

 balls in the tissue of the thallus (Fig. 216 5), which were at one time considered to be 

 endogenous gemmae. The antheridia and archegonia arise apparently without any definite 

 arrangement in the interior of the upper side of the thallus. The formation of the 

 antheridia commences by a circular group of cells of the outer layer separating from 

 the subjacent tissue and thus producing a broad intercellular space, several of the low^er 

 bounding cells of which, after some vertical divisions, rise up in the form of papillae, 

 and form the antheridia, the position of which is represented in Fig. 216, B, an, their 

 mode of formation in Fig. 214. It is only w^hen the grains of chlorophyll in the walls 

 of the antheridia have assumed a yellow colour and the antherozoids are mature, that 

 the roof of the cavity is ruptured, the antheridia opening at their apex and allowing the 

 antherozoids to escape. The archegonia are formed in a manner still more different from 

 that of all other Hepaticae (Fig. 218, C, ar). A row of cells perpendicular to the surface, 

 resulting from the divisions of an upper segment of the apical cell of the shoot, becomes 



/^> 



Fig. 217. — AtUhoceros l^FZ'is ; s£ the young sporogoniuni ; L the involucre (after Hofmeister, Xiso). 



filled with protoplasm ; the lowest cell of this row swells and becomes the central cell of 

 the archegonium. While this cell is growing and rounding off, the other cells of the 

 row become absorbed; the canal of the neck (Fig. 216, D, ar) which conducts to the 

 interior is thus formed, surrounded by six rows of cells. After fertilisation, the 

 oosphere is first divided by an oblique wall ; in the upper ceM, which becomes the apical 

 cell, other walls are formed inclining alternately right and left; but the walls after- 

 wards arise in four alternating directions. While the immature sporogonium is thus 

 becoming transformed into a multicellular body enlarged below (Fig. 216,^), the sur- 

 rounding tissue of the thallus divides repeatedly and grows into an involucre which is 

 arched upwards and through which the elongating sporogonium afterwards pushes its 

 way. The sporogonium, which had hitherto consisted of homogeneous tissue, now be- 

 comes differentiated; the cylindrical Columella, consisting of from 12 to 16 rows of 

 cells, is formed, its cells being elongated in an axial direction, while those of the sur- 

 rounding layer become divided by horizontal walls, and form the mother-cells of the 

 spores and elaters. The outer four or five layers of cells form the wall of the sporo- 



