DEVELOPMENT OF PLANTS 265 



The thallus is of a primitive type, often with simple lobings 

 and therefore, suggestive of relationship with the simpler 

 Jungermaniales (Fig. 198). A peculiar feature of the order is 

 the occurrence of mucilage cavities which communicate with the 

 air through small openings on the under side of the thallus. 

 These cavities are always occupied by one of the blue-green algae. 

 Xostoc, which possibly assists the plants in the retention of water 

 owing to their mucilaginous character. Another interesting fea- 

 ture of the order is the occurrence of a single chloroplast in each 

 cell, as in Colcocliactc and several other genera of the algae. The 

 archegonia and antheridia, while resembling those of the pre- 

 ceding orders, are sunken in the tissues and also originate some- 

 what after the manner to be noted in the ferns. 



The most suggestive variation of the Anthocerotales appears 

 in the development of the sporophyte. We have noted that the 

 asexual generation in forms like Ricciocarpus was a microscopic 

 plant consisting of a delicate spore-bearing capsule. In Mar- 

 chantia the sporophyte is still minute but differentiated into a 

 foot, stalk and capsule, and in the Jungermaniales this becomes 

 much more pronounced. In the Anthocerotales the sporophyte 

 assumes a growth and differentiation that is much more extensive 

 and complex and it also presents features that indicate a rela- 

 tionship with the mosses and possibly with the ferns. The germi- 

 nation of the gemetospore gives rise to a rather cylindrical or 

 pod-shaped body (Fig. 199). The basal portion of the sporo- 

 phyte develops into a massive foot, often provided with rhizoidal- 

 like outgrowths, which serve as a very efficient absorbing organ. 

 The upper portions of the sporophyte present a remarkable 

 series of differentiations. The outer part of it consists of chloro- 

 phyll-bearing cells in which, for the first time, genuine stomata 

 appear (Fig. 199, ch). Within this zone of chlorenchyma is a 

 dome-shaped layer of spore mother cells alternating with sterile 

 cells which in some genera develop as elaters. In the center of 

 the sporophyte is a mass of sterile cells, the columella, which 

 assist in conducting food from the foot to the forming spores. 

 Directly above the foot is a region of rapidly dividing cells which 

 causes the sporophyte to elongate by basal growth and push out 



