VOL. III.] Some Notes on Azolla. 343 



made, as the first stages take place within the completely closed 

 spore. From the macrospore a small triangular prothallium is pro- 

 duced, which breaks open the apex of the spore, and pushes up be- 

 tween the three appendages on the top. A single archegonium is 

 formed at a very early stage, in the center. This resembles in its 

 essential features the archegonium of the ordinary ferns. In case 

 the first archegonium is not fecundated, several others may be formed, 

 but the growth of the prothallium is limited, and appears to ceaSe 

 after the reserve fund in the spore is used up. If the first archego- 

 nium is fertilized, the egg-cell after secreting a cellulose wall about 

 itself divides by a transverse wall. From the upper of the two 

 primary cells the stem and fine leaf of the young plant arise; from 

 the lower, the primary root and the foot (the organ by which the em- 

 bryo absorbs its nourishment from the spore). 



The microspore produces an extremely simple prothallium bearing 

 a single antheridium. 



The ripe spores sink promptly when placed in clear water, but as 

 the embryo develops, large intercellular spaces are formed, which, 

 Tilling with gases, cause the young plant to rise to the surface. 



The development of the prothallium, so far as could be determined, 

 is completed in about one week from the beginning of germination; 

 and it is almost as long before the young plant rises to the surface of 

 the water. These figures are necessarily only approximate, as there 

 is no means of telling how far germination has advanced without 

 kilUng the plant, and there is a great deal of difference in the time 

 when germination begins. 



All species of Azolla have always associated with them a nostoc- 

 like plant of the genus Anabaena. The necklace-like chains of cells 

 of this plant are always found tangled about the growing point of 

 the Azolla stem, and as the leaves develop, a cavity is formed in each 

 one, into which the Anabaena filaments creep and form a colony. 

 They do not seem to affect the growth of the Azolla, but are simply 

 sheltered by it. As the sporocarps are forming, the Anabaena makes 

 its way into the open top where the cells enter a resting condition to 

 assume growth again when the spores germinate. When this takes 

 place, the Anabaena filaments surround the growing point of the 

 embryo, which is thus brought into contact with the parasite from 

 the very first. 



