334 Conjugate 



In some instances the rhizoids may be formed as a result of the stimulus of contact, 

 as in the profusely branched haptera described by Delf ('13) in an attached Spirogym, 

 but in other cases, such as the replacement of a conjugation-tube by a branched rhizoid, the 

 stimulus is obviously of another kind. 



Fragmentation of the filaments is of frequent occurrence in the Zygne- 

 macese and is quite a normal phenomenon in various species of Moitgeotia 

 and in certain of the narrower species of Spirogyra (consult p. 349). 



The mature zygospore has typically three membranes. 



On the germination of the zygospore only a single young plant (sporeling) 

 is formed (fig. 210 G-E; fig. 218 G). 



The classification of the Zygnemaceas presents certain difficulties. The 

 outstanding features upon which a classification could be based are obviously 

 the method of formation of the gametes, the manner of conjugation, and the 

 nature and disposition of the chloroplasts. In all except a very few of the 

 more recent works treating of the Conjugatse the methods of gamete-formation 

 and of conjugation have been exclusively used as a basis of classification. It 

 was De Bary ('58) who first sharply separated the Mesocarpese (type genus 

 Mougeotia) from the Zygnemere (typical genera Zygnema and Spirogyra) on 

 differences in their mode of conjugation, and this separation was further 

 supported by the careful studies of Wittrock ('72 ; 78) on spore-formation in 

 the Mesocarpeae. The taxonomic value of the mode of formation of the 

 gametes and of the nature of the conjugation has, however, been seriously 

 discounted by recent discoveries, amongst which the genus Pyxispora (W. & 

 G. S. W., '97 ; fig. 216 A (7) must take first place. In this Zygnema-like 

 genus the gametes are formed from only part of the protoplast of the game- 

 tangium, as in Mougeotia, and therefore there are now two known genera 

 with widely dissimilar chloroplasts each having the M ' ougeotia-type of conju- 

 gation. This fact is alone sufficient to raise the question as to whether or not 

 too much importance has been attached in the past to the differences in 

 conjugation exhibited by the various members of the Zygnemacea?. When 

 it is also found that in the Spirogyra-like genus Sirogoniitm and in the 

 Mougeotia-]ike genus Temnogametum (fig. 212) conjugation only takes place 

 between the gametes of gametangia which are specially cut off, then it must 

 be confessed that the advisability of utilizing the mode of conjugation as a 

 basis of classification is still more seriously questioned. 



A consideration of all the known facts concerning the modes of conjugation 

 in the various genera of the Zygriemaceae causes one to enquire if there are 

 not other characters which can be utilized as a basis of classification, and it is 

 here that the suggestions of Palla ('94) are most helpful. Palla, who was 

 afterwards supported by Blackmail & Tansley ('02), suggested that the 

 chloroplast-characters rather than the modes of conj ugation should be utilized 

 as a basis of distinction between certain definite groups of filamentous 



