628 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



tive cells, as to the physiological conditions which determine their formation, and 

 finally, questions as to the division of labour in the protoplast itself, the function 

 of the nucleus and its relation to the protoplasm, and how far the latter can be 

 resolved into separate, but mutually co-operative parts. 



The framework of our knowledge on the former group of questions has been, and 

 is being, built up largely from a study of the Green Alga3; of the latter — and even 

 more fundamental — we know, as yet, very little, but the same group of plants is 

 already beginning to yield important results, and we have every reason to expect 

 an even richer harvest in the immediate future. 



We shall now proceed to a brief review of the groups into which the Chloro- 

 phyceaB may be divided, mention being made of the more interesting forms in each 

 group. 



The sub-class Chlorophyceas may be defined as follows : — Thallus of very various 

 form, one- or many -celled, coloured green by chlorophyll which is contained in 

 chromatophores of very -various shape: the green colour rarely masked by other 

 pigments. Reproduction by motile zoospores, and by gametes, which either resemble 

 small zoospores, and are equal in size {isoplanogainetes), or are differentiated into 

 two categories; first, relatively small, active male gametes — called spermatozoids 

 when they are very highly differentiated — and secondly, relatively large, passive 

 female gametes — known as eggs or oospheres when they are quite passive. The cell 

 produced by the fusion of the bodies of two gametes is called the zygote, and gives 

 rise to a new plant either mediately or immediately. When the gametes are 

 sexually differentiated, the zygote (now called a fertilized egg or oospore) is pro- 

 duced only by the fusion of a male with a female gamete. 



Other reproductive cells are known as aplanospores and akinetes. Aplanospores 

 are formed by the protoplasm of a cell of the thallus rounding itself off and putting 

 on a new cell-wall, or dividing into several parts, each of which acquires a separate 

 cell-wall. Akinetes are simply single cells of the thallus, whose original walls 

 thicken directly, the cells becoming separated from the rest of the thallus. These 

 two categories of reproductive cells germinate at once to form new plants. The 

 Chlorophycege comprehend the following alliances: — Protococcoidege, Siphonese, 

 Confervoidese, Conjugatae, and Charales. 



Alliance VI. — Protococcoideae. 



Families: Chlamydomonadece, Volvocece, Pleiirococcacece, Endosphceracem, 



Hydrodictyacece. 



Unicellular forms, actively swimming, floating, or fixed in habit, living either 

 singly or united into colonies. 



Family GldarixydovionadecB. This family consists of minute green (sometimes 

 red) organisms which spend the greater part of their life actively swimming about 

 in water. Several species of the genus Chla7)iydomonas are very common in 

 stagnant water. Each CJdaniydomonas-ceW consists of a roundish mass of proto- 



