44 



MANUAL OF BOTANY 



antherozoids, which care sometmies discharged into the con- 

 ceptacle and make their way out througli the ostiole. In 

 other cases the antheridia are detached unruptured and escape 

 from the conceptacle, the antherozoids being subsequently 

 liberated. 



The oogonia arise from two-celled hairs, of which the upper 

 cell only gives rise to the sexual organ, the lower one forming a 

 stalk to it {fig. 794). Each oogonium develops usually eight 

 oospheres, of generally spherical shape and having no cilia. The 

 wall of the oogonium is composed of two laj^ers, and when the 

 oospheres are mature the outer layer bursts {fig. 796), and the 



Fig. 792. Fig. 793. 



Fig. 792. G-rowing point of Pdce/ia. a. Apical cell. After Kny. Fig. 



793. Thallusor thalloiue of the common BladJer Seaweed {Fuciis ve.siculo- 

 siis). f, t. Groups of conceptacles. v, v. Bladders of air. 



inner one, covering the oospheres, is extruded. When this bursts 

 later the latter are set free into the water. Fertilisation always 

 takes place outside the conceptacle, each oosphere becoming 

 surrounded by a number of the ciliated antherozoids {fig. 797), 

 one of which ultimately fuses with it, forming the oospore or 

 zygote, which then secretes for itself a cellulose covering. The 

 zygote generally germinates at once. 



Besides the sexual organs the conceptacles contain a number 

 of barren hairs or paraphyses {fig. 794), which frequently are so 

 numerous towards the top of the ca^•ity as to protrude through 

 the ostiole. 



