CONJUGATE. 103 



are called aplanospores ; when formed without rejuvenes- 

 cence, akinetes. The only known method of sexual repro- 

 duction is by the conjugation of two motile zoogametes, 

 which are not apparently differentiated into male and 

 female elements. The term isogamous is applied to those 

 cases of primitive sexual reproduction where the two ele- 

 ments are similar in size, and without marked differentiation 

 into male and female bodies. In many species, included 

 under the present order, the sexual mode of reproduction is 

 unknown. 



Conjugatse. 



The most pronounced characteristic of the present order, 

 and the one from which the name is derived, is the sexual 

 mode of reproduction, which is effected by the conjugation 

 of two passive or stationary cells. According to some 

 authorities we meet with the first morphological indication 

 of male and female sexual organs in the present order ; this 

 idea will be explained when describing the morphology of 

 Zygnema. Motile zoospores are unknown, asexual repro- 

 duction being carried on by the formation of akinetes or 

 aplanospores, which form resting-spores. The individuals 

 are either unicellular, as in most of the Uesmids, or consist 

 of a single row of cells attached end to end, and almost 

 invariably unbranched. The arrangement of the chlorophyll 

 in spiral bands, symmetrical stars, or discs, is a marked 

 feature of the order. The external mucilaginous sheath is 

 still present, but is not so pronounced as in some of the 

 preceding orders. 



Most species occur in fresh water, a few occur amongst 

 damp mosses, or on moist ground. 



The Diatoms, described under the Protophyta in the 



