LAMINARIALES (HETEROGENERATAE) 185 



retention of the ovum on the parent plant. This again may prove to 

 be a significant feature in a consideration of the origin of a land 

 flora. 



Alariaceae: Alaria (ala, wing). Fig. 125. 



This genus is widely distributed throughout the northern 

 hemisphere, the common species being A. esculenta. There is a 



Fig. 125. Alaria esculenta. A, plant oi A. oblonga with sporophylls. B, sporangia 

 and paraphyses ( x 200). C, germling sporophyte ( x 100). D, female gametophyte 

 ( X 80). (A, after Oltmanns; B-D, after Newton.) 



short, soHd, unbranched stipe which is attached to the substrate by 

 means of small branched rhizoids. It is naked below with an inter- 

 calary growing zone that allows for continual renewal, whilst above 

 the growing region the stipe expands into a flattened rachis which 

 bears each year a fresh crop of marginal rows of sporophylls. The 

 frond finally terminates in an expanded sterile lamina with a well- 

 marked mid-rib, which is also an annual production. In addition to 

 the intercalary growth there is also a marginal growth that imparts 



