BRYOPHYTA 



187 



other genera the thallus is several layers of cells thick and without a mid- 

 rib. There is no internal differentiation of tissues. The cells are peculiar 

 in having, as a rule, a single large chloroplast with a conspicuous pyrenoid, 

 a feature of most green algae but not of any other bryophyte. In some 

 members of the group two or more chloroplasts are present. 



The thallus grows by means of a cuneate (wedge-shaped) apical cell, 

 as in the Marchantiales. There are no air 

 chambers or air pores, but some species of 

 Anthoceros and Dendroceros have intercellular 

 mucilage cavities that open by clefts to the 

 ventral surface, and in these cavities colonies of 

 Nostoc may live. Smooth rhizoids are present, 

 but there are no ventral scales. In some species 

 of Anthoceros vegetative propagation is accom- 

 phshed by the isolation of branches, in other 

 species {e.g., Anthoceros hallii) by the formation 

 of small tubers that rest in the soil until the 

 next growing season. 



Sex Organs. In most of the Anthocerotales 

 the antheridia and archegonia are borne on the 

 same plant but in separate groups, the antheridia 

 appearing first. Both kinds of sex organs are 

 embedded in the dorsal surface of the thallus and 

 develop endogenously. 



The antheridium initial is a superficial cell 

 arising close to the growing apex. It does not 

 become papillate, as in the other Hepaticae, 

 but divides transversely, the inner cell giving 

 rise to the antheridium. Between the two 

 cells a mucilage-filled cleft appears that later 

 becomes the antheridial chamber, the roof of 

 which is formed by the derivatives of the outer 

 cell. In the inner cell two vertical walls at right angles to each other 

 now appear, followed by two transverse walls (Fig. 154A). As a result, 

 three tiers are formed with four cells in each tier. The stalk is 

 derived from the lowest tier. Periclinal walls in the two upper tiers sepa- 

 rate the outer sterile jacket from the inner spermatogenous cells (Fig. 

 1MB, C). Further development follows the usual liverwort pattern. 



The mature antheridia are spherical or nearly so, generally long- 

 stalked, and often bright orange-yellow. When the sperms are ripe, the 

 roof of the antheridial chamber bursts. In Notothylas and most species 

 of Anthoceros two or four antheridia develop in the same chamber, all 

 coming from the inner segment of the same initial cell. This divides by a 



Fig. 153. Anthoceros fusi- 

 formis, with three sporo- 

 phytes arising from the 

 gametophyte, X3. 



