116 Chlorophycece 



changes which take place in a filament on the conversion of its vege- 

 tative cells into gametangia changes which are rendered manifest 

 by a general increased activity should not be so far modified at 

 different parts of the same filament as to produce a differentiation 

 of sex. This affords an explanation of the rare cases of " cross- 

 conjugation" to be described later on, but at the same time there 

 is no doubt that it is a general rule in the Zygnemeae that the cells 

 of any one filament all become of the same sex. 



t/ 



Little is known concerning the effect of temperature and 

 climatic conditions on the methods of reproduction of the Coiijugatse. 

 A high altitude, which is usually accompanied by a relatively low 

 temperature, certainly favours the formation of 'cysts,' and an 

 increase of temperature is in many cases an aid to conjugation. 

 With regard to the British Islands, conjugating examples of the 

 Zygnemacese are more frequently met with in low-lying areas than 

 in upland districts, and most frequently in the western or south- 

 western counties. Observations tend to prove that the Conjugate 

 in their natural habitats withstand extremes of temperature very 

 well, and that they are capable of adapting themselves to very- 

 different conditions from those under which they normally exist. 



All plants of this order, whether unicellular or filamentous, are 

 very slimy to the touch. This is due to the gelatinous nature of 

 the outer layers of the cell-wall, and this character alone frequently 

 enables the collector to distinguish Conjugates from most other 

 filamentous green Alga?. 



The order is subdivided into three families, of which one the 

 Temnogametacea? is only known from Equatorial Africa. The 

 two British families are : 



Family 1. Zygnemacece. Thallus filamentous, consisting of a single series 

 of cylindrical cells, each cell possessing one or several chloroplasts of definite 

 form. Any cell may become a gametangium. 



Family 2. Desmidiacece. Thallus unicellular ; cells sometimes loosely, 

 sometimes closely joined into simple filaments, in most genera constricted into 

 two equal and symmetrical halves. Chloroplasts one or several in each cell, 

 usually of definite form. Many of the plants of this family are remarkable for 

 their beauty of form. Any cell may become a gametangium. 



Family 1. ZYGNEMACE.E. 



This family of Conjugates has a world-wide distribution and 

 includes some of the commonest and most striking of freshwater 

 Alga?. The thallus is in every instance filamentous and consists of 



