CONJUGAT.^ 259 



thin transparent mucilaginous sheath. According to Klebs (Untersuch. 

 Bot. Inst Tubingen, 1886, p. 333) this sheath is composed of two dis- 

 tinct portions, a homogeneous substance which is but sHghth- refringent, 

 and a portion which consists of minute rods placed at right angles to 

 the cell-wall. He regards this mucilaginous sheath as entirely independ- 

 ent of the substance of the cell-\Yall, and derived from the protoplasmic 

 contents of the cell by diffusion through the cell-wall. The same struc- 

 ture probably prevails also in the Confervaceas and other filiform algse 

 growing in fresh water. The Desmidiacese possess a remarkable power 

 of apparently spontaneous motion, which will be spoken of in detail 

 under that order. 



The only sexual mode of reproduction in the Conjugatce is the 

 €07ijugatio}i of stationary cells, found nowhere else except in some of the 

 Zygomycetes. This consists, in the unicellular genera, of the complete 

 •union or fusion of the protoplasmic contents of two individuals ; in the 

 multicellular genera, of the isogamous union of the whole or a part of 

 the contents of gametes or non-motile unciliated cells into a zygosperju ; 

 conjugation may take place between cells belonging to the same or to 

 different filaments. Whether the two conjugating cells are physiologi- 

 cally equivalent or not will be discussed under the separate orders, and 

 the process described more in detail. Klebahn finds the union of the 

 two nuclei in the zygosperm to take place only slowly in Zygnema (Ktz.) 

 -(Zygnemaceae) ; while in Closterium (Nitzsch) (Desmidiaccce) they 

 remain distinct even in the mature zygosperm. 



The Zygnemaceae must be regarded as the typical family of Conju- 

 gatse, from which the Desmidiacese have probably been derived by 

 retrogression, exhibited, in most cases, by the reduction of the filament 

 to a single cell. The Mesocarpaceae display an approach to a higher 

 type of sexual reproduction in the more complicated processes connected 

 with the formation of the zygosperm. The reasons for excluding the 

 Diatomaceae from the Conjugatae, contrary to the opinion of some 

 -writers, and placing them in a totally different group, will be given 

 hereafter. 



Many of the Conjugatse are extremely abundant in fresh water, 



whether running or stagnant, to which they are almost entirely confined ; 



some of the filiform species grow also on moist ground and among 



moss. 



Literature. 



De Bary — Untersuchungen liber die Conjugaten, 1S58. 



Gay — Essai d'un ]Monographe des Conjuguees, 1884. 



Bennett — Journ. Linn. Soc. , xx., 1884, p. 430. 



Klebahn — (Zygosperm) Ber. Deutsch. Bot. Gesell., 1SS8, p. 160. 



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