THE GREEN ALG.E 641 



gelatinisation of the walls to take place to give rise to a 

 palmelloid form like Glceocystis, etc. 



The first beginning of this Chlorella-tendency, as we may 

 perhaps call it, was probably the drawing-in of the cilia of 

 the Chlamydomonas-mdividual prior to division. There is some 

 latitude in this respect in the different species, and we need 

 only imagine an increasing prolongation of the interval between 

 the drawing-in of the cilia and the process of division to arrive 

 ultimately at the state of affairs obtaining in Chlorella. Looking 

 upon Chlorella in this way, we can derive from it a considerable 

 number of genera in which the daughter-cells cohere to form 

 colonies in diverse ways (viz. Scenedesmus, Coelastrum, Dictyo- 

 sphceriutri). At first sight these forms do not appear to have 

 very much in common with Chlorella, but we owe to Senn's (62) 

 researches the discovery of the fact that the Chlorel la-type may 

 appear in them under certain special conditions. These colonial 

 forms differ from the loose colonies of Chlorella in having their 

 cells joined together by local development of their outer 

 mucilage-envelopes. When there is an abundance of oxygen 

 in the surrounding medium, however, the development of the 

 connecting processes between the individual cells does not take 

 place. After a number of generations the cells in many cases 

 become rounded off, and in this condition are practically indis- 

 tinguishable from individuals of Chlorella (fig. 2, g, l) ; as long 

 as the influencing conditions remain the same, these cells, more- 

 over, continue to divide just as in Chlorella, and the products 

 of division are of the same type. This reversion to the Chlorella 

 condition is certainly of phylogenetic significance, and warrants 

 our placing this genus at the commencement of the present 

 series. 



In the genus Scenedesmus, which forms short chains of four 

 or eight cells cohering by their lateral surfaces, two species 

 have been studied from this point of view. In S. acutus the 

 mucilage-envelope is particularly prominent between the flat 

 surfaces of the cells, thus cementing them together, but it also 

 forms the hyaline knob-like thickenings at the two ends of each 

 cell (fig. 2, k). In S. candatus (S. quadricaudd) the cells cohere 

 by little bridges of mucilage extending between the free tips of 

 adjoining cells, the mucilage also forming the four characteristic 

 processes to which the species owes its name (fig. 1, r). In 

 S. acutus the Chlorella-stage is fairly easily obtained (fig. 2, l), 



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