40 



MORPHOLOGY 



like pyrenoids, which are often surrounded by a starch jacket. The 

 nucleus is swung in the center of the vacuolate cell by strands of cyto- 

 plasm that connect the sheath of cytoplasm about the nucleus with the 

 peripheral layer of cytoplasm. The cell division is as described for 

 the Mesocarpaceae. 



Sexual reproduction. — Sexual reproduction is most characteristic 

 (figs. 107-110, and fig. 112). Conjugating tubes put out from the cells 

 of adjacent filaments and fuse, until the 

 two filaments connected by conjugating 

 tubes resemble a ladder. The protoplast 

 of one cell passes through the conjugating 

 tube into the connected cell, and the two 

 protoplasts fuse, forming a large, heavy- 

 walled zygospore. The conjugating 

 protoplasts in this case differ in behavior, 

 one being passive and the other relatively 

 active, so that there is apparent a distinc- 

 tion of sex, although the two protoplasts 

 are similar in appearance. This distinc- 

 tion often extends to the filaments, one 

 filament emptying all of its protoplasts 

 into the cells of the connected filament; 

 in which case the former filament can be 

 regarded as male and the latter one as 

 female. It is very common to see a 

 filament, all of whose cells are empty, 

 connected with another filament, each of 

 whose cells contains a zygospore. On 

 the other hand, the same filament may give and receive protoplasts ; 

 and in some species conjugating tubes connect adjacent cells of the 

 same filament. Occasionally, also, bodies which resemble zygospores 

 are found within cells that have established no connections, and so they 

 have been formed without fusion, a phenomenon called parthenogenesis 

 (fig. 112). Great variations in the establishment of connections for 

 conjugation may be found in almost every collection of zygospore- 

 forming material. The zygospore is the winter condition of the plant, 

 and upon germination gives rise directly to a new filament. 



Conclusions. — The body of the Conjugates consists of a single cell or 

 a simple filament, and in its vegetative phase is distinguished by large 



Figs, hi, 112. — Zygnema: 

 in, cells showing the two radiating 

 chloroplasts, between which is the 

 nucleus; 112, conjugation, showing 

 two zygospores (each with two 

 nuclei and four chloroplasts) formed 

 by the fusion of two protoplasts, 

 and a third zygospore-like cell not 

 formed by fusion (illustration of 

 parthenogenesis). 



