no PLANT LIFE. 



be either marginal, or forming clusters on the surface caljed 

 sori. The supposed sexual organs are produced in similar 

 positions to the tetrasporangia, but on distinct plants, and 

 never on those producing the asexual reproductive bodies. 

 The oogonia consist of special cells, each containing a single 

 large, motionless oosphere. The antheridia consist of a 

 number of cells, which are divided into numerous small 

 cells, each of which contains a single motionless pollinoid. 

 The presence of a trichogyne has not been demonstrated, 

 and whether fertilization of the oosphere takes place is not 

 at present known. 



The Dictyotacea resemble the Floridece in having tetra- 

 spores, and also in the motionless pollinoids, and may 

 perhaps be considered as the progenitors of the character- 

 istic sexual and asexual modes of reproduction met with in 

 the Floridea, the sexual organs not being completely evolved 

 in the present group. 



Pliseosporese. 



The present order may be considered as temporary and 

 purely artificial, owing to absence of information respecting 

 the life-history of numerous included forms ; the common 

 points of agreement being the asexual mode of reproduction 

 by motile zoospores, and the olive-brown colour. With 

 very few exceptions the species are marine, and vary in 

 size from microscopic forms to species- that attain a length of 

 hundreds of feet, as in Macrocystis pyrifera, which occurs on 

 the coast of California and in the Antarctic Ocean. Some of 

 the smaller forms are epiphytic, and others parasitic, on 

 other algse. The structure of the thallus also varies con- 

 siderably in different genera, being crust-like and radiating 

 in Ralfsia ; composed of slender filaments consisting of a 



