244 PART IV. CLASSIFICATION. 



Ulva) without any special morphological differentiation ; this is 

 true likewise of the coanocytic Algse Siphonoidese, such as Botry- 

 dium, Vaucheria, Sphseroplea, Cladophora, where the whole or 

 part of the body may act as a sporangium. Specially differen- 

 tiated sporangia occur only in some Phseosporese, and in the 

 Rhodophycese where they usually produce each four spores (tetra- 

 .spores) and are hence termed tetrasporangia : specially differenti- 

 ated sporangia are also developed in the cystocarp of the Rhodo- 

 phycese where they are termed carposporangia : these organs are 

 in all cases unicellular. 



In the Cyanophycese the formation of spores is effected without 

 any sporangium, for in these plants a cell of the body is converted 

 into a spore by simple encystment. 



As a rule a sporangium gives rise to a number of spores ; 

 but only one is formed in the sporangium of Vaucheria and 

 of (Edogonium (see p. 85), and in the carposporangium of the 

 Rhcdophycese. 



Sub-Class I. CYANOPHYCESE (also called Phycochromacese), or 

 blue-green Algse. The body consists of a single, more or less nearly 

 spherical cell, as in most of the Chroococcacese (e.g. Gloeocapsa, 

 Fig. 134) ; or it is a multicellular layer one cell thick (e.g. Meris- 

 mopedia); or it is filamentous, consisting of a row of cells (e.g. 

 Oscillaria, Nostoc, Rivularia, Scytonema). When the body is 

 filamentous, it sometimes presents a distinction of base and apex 

 (e.g. Rivularia) ; and it is frequently branched. In most cases 

 growth and cell-division go on in all the cells of the body, but 

 in the Scytonemacese only at the apex. The plant is usually 

 free, but it grows attached in some species of Rivulariacese and 

 Sytonemaceae. A characteristic feature of the sub-class is the 

 more or less bulky mucilaginous cell-wall which invests the cells 

 and filaments. The filaments of the Oscillariacese exhibit a glid- 

 ing, oscillating movement, but the mechanism of it is not fully 

 understood. 



Reproduction is mainly effected in a purely vegetative manner. 

 In the unicellular forms (Fig. 134) each cell-division necessarily 

 leads to the formation of new individuals. In the flattened forms 

 (e.g. Merismopedia), when the body reaches a certain limit of size, 

 it simply breaks up into a number of portions each of which 

 becomes a new individual. In the filamentous forms, vegetative 

 propagation is effected by the breaking up of the filament into 

 lengths, each such portion being termed a hormogonium ; in most 



