78 THE QUANTITATIVE METHOD IN BIOLOGY 



§63.— SPECIAL SYSTEMS OF SEGMENTATION (con- 

 tinued). 11. THE TETRAHEDRAL SYSTEM.— This well- 

 known system of segmentation is observed in the pollen of a 

 number of Phanerogams (Erica, Rhododendron, etc.), in the 

 spores of many Cryptogams (Selaginella, Saccharomyces, etc.), 

 etc. This system is a primary system from which more com- 

 plicated systems are derived. Interesting examples of such 

 complications are observed in the pollen of Mimosa and certain 

 Orchids. 



§64.— SPECIAL SYSTEMS OF SEGMENTATION (con- 

 tinued). HI. PEDIASTRUM, EUASTROPSIS.— Among the 

 algae we find several peculiar dispositions of the segments, some 

 of which are remarkable by their singularity and elegance.^ 



A specimen (coenohium, individual x) of Pediasirum^ con- 

 sists of a certain number of cells (individuals x-\-i) which are 

 united into a regular discus. The constituent cells of a given 

 discus are produced by successive divisions of one cell, just as 

 the cells of a Spirogyra; but in Pediastrum the cells do not 

 remain attached to each other after each division ; they separ- 

 ate and become for a certain time independent, swimming 

 within a vesicle which proceeds from the mother-cell. At a 

 given moment the hitherto isolated cells are united by juxta- 

 position ; they become adherent and constitute a disciform 

 society called ccenobium (specimen, individual x). 



This discus is, in the full sense of the word, a society of cells 

 which find themselves in a state of equilibrium. We discern 

 here (as in Spirogyra) a differentiation into two sorts of ceUs ; 

 the marginal cells are of different form from those in the centre 

 and they are furnished with a pair of diverging processes.^ 

 This differentiation is brought about by social causes (§55), for 

 all the cells are ahke (without processes) as long as they are 

 independent. The number of cells is 4, 8, 16, 32 ... as in a 

 typical Spirogyra. 



In the genus Euastropsis (allied to Pediastrum) the develop- 

 ment proceeds on the whole as in Pediastrum, but the individual 

 X (ccenohium) consists of two cells. 



Our information about the relative disposition of the cells 

 in Pediastrum and Euastropsis is somewhat incomplete. A 



1 See the classic illustrated works on fresh-water algae ; for instance, G. S. 

 WEST, A Treatise on the British Fresh-water AlgcB (Cambridge University 

 Press, 1904). 



^ Ibid., p. 209. 



^ These processes are an example of differentiation within the limits of one 

 cell. (Compare the sexual outgrowths in Spirogyra. § 60.) Certain cells in 

 certain mosses (marginal cells of the leaves of Tortula ruralis) are furnished 

 with processes which recall those of the marginal cells of Pediastrum. Com- 

 pare also several Desmids. 



