114 BOTANY. 



(isogamous), or they may be unlike in size and otherwise 

 quite different also (heterogamous). Thus, all except the 

 highest Protococcoideae, all of the Oonjugatse, all but the 

 higher SiphoneEe and Confervoideffi of the first-mentioned 

 class and nearly all of the second class are isogamous. The 

 families Vaucheriacese, Saprolegniacese, and Peronosporaceae 

 (of the order Siphoneae) and Sphseropleaceae Cylindrocap- 

 sacege and CEdogomaoeae (of the order Oonfervoidese) are 

 heterogamous. Among the Phaeophyceffi, the Pucoideee 

 alone are heterogamous. In all classes above the Chloro- 

 phyceae and Pheeophycese heterogamy is the invariable rule. 

 198. Results of Cell Union. — As vre pass from the lower 

 plants to the higher, there is an increasing complexity in 

 the results of the cell union. In the Chlorophyceae and 

 Phaeophyceae the result is a single egg-like cell (oospore) 

 which sooner or later develops into one or more new plants. 

 In passing to the Coleochaetaceae and Plorideae, we find 

 that in the former the single spore soon becomes invested 

 with a cellular layer of protective tissue, and the spore 

 itself upon germination becomes several-celled. In the 

 Florideee the fertilized cell not only divides early, but each 

 segment emits a branch whose end segment becomes de- 

 tached as a spore, and in the meantime the whole has be- 

 come invested by a layer of protective tissue. In the 

 Charophyceae the growth of the protective tissue precedes 

 fertilization, so that from a protective device which only 

 follows fertilization, we have now the same device develop- 

 ing before fertilization, and serving as a protection to the 

 unfertilized cell. In bryophytes and pteridophytes we 

 recognize in the archegone the homologue of the structure 

 just referred to in the Charophyceae ; in fact it is difficult 

 to separate the latter from the former by any absolute char- 



