114 Chloro})hycecK 



localities ; thickness of filaments 78 90 p. (fig. 42 A, F H). V. terrestris 

 Lyngb., and V. hamata (Vauch.) Lyngb. (fig. 43 C and D), occur in profusion 

 on damp ground, often forming thick mats on gravel paths and on the surface 

 soil of damp flower-pots. V. sericea Lyngb., which is the smallest British 

 species (thickness of fil. 48 55 /u, ; fig. 42 B and D, 43 E), and V. aversa Hass. 

 usually occur entirely submerged in the waters of ditches and ponds. V. dicho- 

 toma (Lyngb.) Ag. is the largest British species (thickness of fil. 180 220 ^) 

 and is dioecious. Some forms of it are truly marine, but others occur in 

 brackish water. 



Order VIII. CONJUGATE. 



The order Conjugatse is one of the best defined and most 

 natural of the groups of the Chlorophycese. The thallus is 

 unicellular in the Desmidiacese and the individuals exhibit great 

 specialization of form. In the Zygnemacese, which is the only 

 other family of the order, the plants are multicellular, consisting 

 of unbranched filaments of cylindrical cells. These filaments are, 

 however, fragile and often become dissociated into their individual 

 cells. All the plants of this order, whether unicellular or multi- 

 cellular, are remarkable for the great development of the gelatinous 

 pectose constituents of the cell-wall. There appears to be a 

 continual exudation of this gelatinous material, until, in some 

 instances, it is of much greater bulk than the individual plant, 

 and it frequently happens that the unicellular forms occur 

 embedded in a mass of transparent jelly formed by the coalescence 

 of their outer gelatinous coverings. 



One of the most conspicuous features of the order is the 

 presence of chloroplasts of large size and definite form. They vary 

 in number from one to about eight or twelve in each cell and they 

 exhibit great variety in form and symmetry. Each chloroplast 

 contains one or more conspicuous pyrenoids. Boubier 1 has 

 observed in species of Spirogyra and in Mouyeotia scalaris Hass. 

 what he terms ' compound pyrenoids.' These consist of an 

 agglomeration of pyrenoid structures enclosed in a membrane and 

 containing in the centre a pyrenocrystal. 



Multiplication of the filamentous forms sometimes takes place 

 by the fragmentation of the filaments, each cell undergoing rapid 

 division and forming a new plant. 



Asexual reproduction may be brought about in the Zygnemacese 



1 Boubier in Bull. Herb. Boissier, vii, 1899. 



