112 PLANT LIFE. 



bladders, or specialized hollow portions of the frond contain- 

 ing air. In the genus Nereocystis, these floats are barrel- 

 • shaped, and often attain a length of two yards. 



Asexual reproduction by means of motile zoospores is 

 general throughout the group. The zoospores are more or 

 less pyriform, furnished with two cilia of unequal length, 

 a longer one pointed forwards, that is, in the direction of 

 the pointed colourless end of the zoospore, and a shorter 

 one pointing backwards. The cilia originate laterally and 

 nearest the pointed end ; whereas, in the groups previously 

 described, they originate from the narrow end. The zoo- 

 spores are produced in zoosporangia of two distinct types : 

 the unilocular sporangium, containing a large number 

 of zoospores in one cavity ; the multilocular, consisting of 

 numerous small cells, each containing a single zoospore. 



Unilocular sporangia vary in form from subglobose, 

 through elliptical to pyriform, and are comparatively large ; 

 whereas the multilocular form is usually elongated, hair-like, 

 and transversely septate. The zoosporangia are either 

 external, terminating short branches, or imbedded in the 

 substance of the thallus, usually in considerable numbers, 

 forming sori or definite groups ; in the latter arrangeiiient 

 clavate cells termed paraphyses are usually mixed with the 

 zoosporangia. In some species both kinds of zoosporangia 

 are produced on the same plant at the same time, but more 

 frequently the two kinds are produced at different seasons 

 of the year ; in others the two forms of zoosporangia are 

 produced on different plants of the same species, whereas 

 in others again only one or other of the two forms of 

 zoosporangia are known to exist. For example, unilocular 

 zoosporangia are alone met with in the genus Laminaria and 

 its allies ; both kinds in the genera Ralfsia, Chordaria, etc. 



