Mesocarpede 341 



not primitive. In all its characters it suggests a set back from Mougeotia and 

 in view of the conditions prevailing in M. producta (fig. 211 //"and /), it is 

 possibly unwise to separate Gonatonema as a genus distinct from Mougeotia. 



The genus T&mnogametum (W. & G. S. W., '97), which is exclusively 

 tropical, one species (T. heterosporum ; fig. 212) being known from West 

 Central Africa and one (T. Ulmeami) from Brazil, is unique among the 

 Mesocarpese in the fact that the sexual cells are specially cut off. The 

 vegetative filaments are precisely similar to those of Mougeotia, but the 

 gametangia are short cells cut off from the more elongated vegetative cells. 

 They are cut off either singly, in which case conjugation is scalariform 

 (fig. 212 A E), or in pairs, when conjugation is lateral (fig. 212 A, F 

 and G). 



Of considerable interest is Debarya (Wittrock, '72), another genus in 

 which the vegetative cells are precisely like those of Mougeotia, but in which 

 each gamete is formed from the entire protoplast of the gametangium. 

 Conjugation is scalariform and the zygospore is lodged in all cases in the 

 conjugation-canal between the gametangia. In D. calospora the chloroplast 

 may sometimes be destitute of pyrenoids. 



In most species of Debarya the walls of the gametangia undergo a pecular thickening 

 during the fusion of the gametes and the formation of the zygospore (G. S. W., '04 ; '07 ; 

 '09). In extreme cases the relics of the gametangia become four solid processes which 

 remain permanently attached to the zygospore as in D. Hardy i (fig. 213 A F) and 

 D. desmidioides (fig. 213 G A'). In D. Hardyi the thickening begins as soon as the 

 gametes commence to pass into the completed conjugation-tube. ' The terminal transverse 

 walls increase greatly in thickness by the deposition of layer after layer of cellulose, and a 

 slight thickening of the side walls also occurs. As this thickening goes on the cavity of 

 the gametangium is gradually reduced and a hemispherical, or sometimes a bluntly conical 

 mound of cellulose projects into the emptying gametangium. The metamorphosis of the 

 gametangium, which might almost be described as a "solidification," keeps pace with the 

 receding of the gametes, and when the latter have completely coalesced in the wide 

 conjugating-tube, the proximal ends of the four solid processes project as four rounded 

 buttons into the cavity of the zygospore. The mature zygospore possesses four cylindrical 

 truncate horns, each of which has arisen without external change of form from one half of 

 a gametangium, the latter having become solid by the deposition of an internal thickening 

 of cellulose' (G. S. W., '09); consult fig. 213, BF. In D. africana G. S. West there is 

 no solidification, but the walls of the gametangia increase considerably in thickness, the 

 thickening being evenly laid down except for terminal pits. In D. ylyptosperma (De Bary) 

 Wittr. the relics of the gametangia become very clear and refractive, exhibiting the delicate 

 striations parallel to the transverse walls which are very noticeable in D. Hardyi. They 

 have all the appearance of solidity, although the actual details of the change have not been 

 followed out. 



The genera of the Mesocarpesc are : Mougeotia Agardh, 1824 ; Debarya Wittrock, 1872 ; 

 Gonatonema Wittrock, 1878; T&mnogametum W. & G. S. West, 1897. 



Species of Mougeotia occur abundantly in almost all parts of the world and are especially 

 frequent in the more elevated regions of temperate countries, in which areas they only 

 rarely conjugate, passing the winter in a vegetative state. The smallest are M. elegantula 



