GttOUP I. THALLOPHYTA: ALG^E : CHLOROPHYCEyE. 243 



rounded by a delicate membrane within which they come to rest and cohere 

 to form a Pediastrum-plant. 



In Coelastrum and Sorastrum the gonidia formed in the mother-cell either at 

 once unite to form a new coenobium, or they may be set free, and then each 

 undergoes repeated division to form a new coeuobium. In view of the state- 

 ment that these two genera are not coenocytic, but cellular, and of the fact that 

 in them the coenobium may be formed by cell -division, it appears that their 

 affinities are rather with the Protococcoideae than with the Siphonoideos. 



Series IV. CONFERVOIDEAE. The forms included in this series may be arranged 



in the following orders : 

 Sub-series A. Azoosporees : no zoogonidia or zoospores. 



Order 1. Conjugate : sexual reproduction by aplanogametes. 

 Sub-series J3. Zoosporees ; asexual reproduction by zoogonidia and zoospores. 

 Isof/amous Orders : 



Order 2. Ulothrichacees : body filamentous, unbranched. 



Order 3. Chestophoracees : body filamentous, branched. 



Order 4. Ulvacees : body a flat or tubular cellular expansion. 

 Oo (famous Orders : 



Order 5. (Edogoniaceae: body filamentous, unbranched (except Bulbochaete) . 



Order 6. Coleocbestaceae : body filamentous, branched ; oogonium with a 

 trichogyne ; sporophyte multicellular. 



An asexual formation of spores takes place in all the Confervoidees, except the 

 Conjugates. The sporophyte, in all cases in which it is fully known, gives rise 

 to zoospores. The gametophyte multiplies itself by means of zoogonidia, and 

 in some cases (Chaetophoracees) by means of resting gonidia. The gonidangia 

 are not differentiated, as any cell of the body may act as one. Several zoospores 

 are usually developed in a single sporangial cell, but in some cases only one 

 (e.g. zoogonidia of CEdogonium and Coleochoste, zoospores of Coleocheste). 



A sexual formation of spores takes place in all the Confervoideae ; in the 

 isogamous forms the product of conjugation is a zygospore ; in the oogamous 

 forms it is an oospore. In the isogamous forms the sexual organs, or game- 

 tangia, are not differentiated; any or all of the cells of the body may act as 

 gametangia ; the sexual cells are (except in the Conjugates) free-swimming simi- 

 lar planogametes ; in the Conjugates the gametes are not set free into the water 

 and they are not ciliated. In some cases (e.g. Ulothrix, Chroolepus) the piano- 

 gametes, if they fail to conjugate, are capable of independent germination, thus 

 showing their imperfect sexual differentiation. The plauogametes usually 

 differ from the zoogonidia in being smaller. In tbe oogamous forms the s'exual 

 organs are antheiidia and oogonia ; they are more or less markedly differen- 

 tiated. The sexual cells are oospheres and spermatozoids. The oospheres are 

 formed singly in the oogonium. The spermatozoids are formed singly in the 

 mother-cell; they resemble the zoogonidia of the respective plants, but are 

 smaller and are not green. 



The life-history of the Confervoideae has been investigated with success in 

 only a few forms (some Conjugates, Ulothrix, CEdogonium, Coleochaete). In 

 the Conjugates, as there is no asexual production of spores, there is no alterna- 

 tion of generations ; in the others such an alternation can be traced. In these 



