346 Conjugate 



and Z. didymum 1 , the generic character being the formation of ' progametangia' which 

 ultimately fuse to form the zygospore. He illustrated this by two figures of conjugating 

 filaments of Z. didymum. Various authors, notably Wille ('09) have accepted De Bary's 

 conception of the genus Zygogonium. 



In the first place it should be mentioned that the sole character upon which Kiitzing 

 ('43) separated Zygogonium from Zygnema Agardh ('24) was the lodgment of the zygospore 

 in the conjugation-canal, and since recent evidence proves this to be a character of no 

 taxonomic value (vide Q. S. W., '09) De Bary was in error in trying to establish a genus 

 under the name of ' Zygogonium ' based upon other characters .such as were never mentioned 

 by Kiitzing. With the one exception of Z. ericetorum no species included in Klitzing's 

 Zygogonium could have any place in De Bary's genus of that name. Secondly, it is 

 essential that the evidence for the existence of ' progametangia ' should be carefully 

 examined, since the two figures given by De Bary in 1858 illustrating these structures 

 constitute the sole record of their existence. The conjugation of Zygnema ericetorum was 

 described and well figured by W. & G. S. West ('94) under the name of ' Z. pachydermum.' 

 West & Starkey ('14; '15) have recently re-examined this material, which was collected in 

 the West Indies, and find that it cannot in any way be distinguished from Z. ericetorum, 

 the cytological characters being identical in all respects. The conjugation was in every 

 instance like that of any other Zygnema in which the zygospore is lodged in the conjugation- 

 canal and no 'progametangia' were observed (consult fig. 215.4 C). There is, therefore, 

 as yet no confirmation of the formation of progametangia in Z. ericetorum, and it is quite 

 possible that the two figures, first published more than half a century ago by De Bary and 

 since copied into many text-books, are abnormalities and do not represent healthy 

 conjugation. 



West & Starkey ('14 ; '15) made numerous cultures of Z. ericetorum, extending over a 

 period of two years, and many of these cultures were subaerial, but all attempts to induce 

 conjugation failed, although aplanospores were obtained in abundance. On the whole, it 

 may be said that Z. ericetorum is about the most inert and unresponsive Alga at present 

 known. 



The cytological structure of Z. ericetorum differs from that of all other species of the 

 genus. Wille ('97) stated that there were two chloroplasts, each with one pyrenoid, but twelve 

 years later (Wille, '09) he states that there is one axile chloroplast with one pyrenoid. 

 Both these statements are erroneous. There is but a single axile chloroplast in each cell, 

 deeply constricted and sometimes twisted in the middle, with one pyrenoid in each lobe. 

 The lobes of the chloroplas.t have an irregular outline, but are not produced into the 

 branched processes so characteristic of most species of Zygnema (compare fig. 215 D and E 

 with fig. 216 E). 



On some of the heaths and moors of the British Islands Z. ericetorum sometimes fulfils 

 an important function. In the drier and hotter periods of the year thickly-matted sheets 

 of this Alga, often many square feet in extent, are found covering extensive patches of 

 almost bare sand or peat, round such plants as Drosera, Carices, etc. These mats of 

 Zygnema have great absorptive capacity, greedily taking up water, and in this way they 

 regulate the moisture of the surface soil, the thriving of some of the smaller phanerogams 

 depending to a great extent on the presence of the Zygnema^. 



1 The present author has examined Rabenhorst's original specimens of Zygogonium didymum 

 (issued in Eabenhorst's Alg. Exsic. no. 182) aud finds them identical in every respect with 

 Zygnema ericetorum. 



- This phenomenon is much more evident in some parts of the tropics and attention was first 

 called to it by Welwitsch in the Journal of Travel and Natural History, vol. i, 1868 (consult p. 31). 



