C/ilorococcineas 209 



Algse since they occur in the Chsetophoracese, in Dicranocheete, and in Glceocheete. It is 

 probable that the Cheetopeltidacese as here denned are a mixed assemblage, but our present 

 knowledge of the included forms is insufficient to deal with them in a satisfactory manner. 



Sub-order CHLOROCOCCINE/E. 



In this division of the Protococcales vegetative cell-division does not occur. 

 Reproduction takes place by zoogonidia and in some forms by isoplanogametes. 

 The zoogonidia on coming to rest immediately begin to grow into new indi- 

 viduals. The cells often grow to a considerable size and become simple 

 ccenocytes by the division of the original nucleus. This non-motile vegetative 

 phase is the dominant one, the coenocytes assuming various forms. Most of 

 the Chlorococcineas are gregarious, but those belonging to the Hydrodictyace;e 

 consist of free-floating ccenobia of coenocytes which arise by the apposition of 

 the zoogonidia. 



It has been suggested that the more complex coenocytic types of the 

 Siphonales and Siphonocladiales have been evolved from this group (Blackman 

 & Tansley, '02), and although there is little direct evidence in support of 

 this suggestion it is not improbable that the siphonaceous Algae may have 

 had such an origin. In the present volume the Chlorococcinese is sub- 

 divided into two families as follows : 



Fam. Planosporacess. Cells or coenocytes solitary or gregarious, for the 

 most part fixed. 



Fain. Hydrodictyacese. Coanocytes united to form definite free-floating 

 coenobia. 



Family Planosporacese. 



This family is established to include all those non-coenobic members of 

 the Protococcales which are reproduced solely by zoogonidia or by isoplano- 

 gametes. It is quite a natural family, embracing a number of different genera 

 which by virtue of their method of reproduction have in many cases estab- 

 lished themselves as epiphytes and endophytes, the zoogonidia being able to 

 swim direct to their destination and there grow. 



Each individual is a single cell or coenocyte, free-floating, epiphytic or 

 endophytic. In Chlorococcum the aggregation of individuals may be so dense as 

 to form a stratum. The outward form of the individual varies much, ranging 

 from the globose cells of Halosphwra to the elongated stalked cells of 

 Characium. In some of the endophytic forms, as in Phyllobium, long tubular 

 outgrowths permeate the tissues of the host. Dicranochfete is unique in the 

 possession of dichotomously branched bristles. 



From 4 to 128 zoogonidia arise in a mother-cell, and are liberated either 

 by the dissolution of the wall at some unlocalised point or by the separation 

 of a distinct lid. In some of the Chlorochytrieae isogametes occur, and the 

 zygote germinates without any period of rest. 



W. A. 14 



