190 THE PHENOMENON OF 



the families of the Palraellacege,* Chroococcaceae,! Nos- 

 tochinese, &c., only unfortunately we are very deficient in 

 satisfactory investigations into these gelatinous struc- 

 tures, both in morphological and chemical respects. While 

 they were formerly mostly regarded as excretions through 

 the cell-walls (extra-cellular substance), Nageli,| if I do 

 not misunderstand the passage, explains them as outer 

 layers of the cell-wall itself, under the name of enveloping- 

 raembrane {HilU-membran). 



In Glceocapsa, Glaocystis, and the other examples 

 mentioned above, this explanation seems to me undoubt- 

 edly correct, while in other cases the gelatinous mass 

 appears to be a real secretion on the surface of the cell- 

 membrane ; as, for instance, in the gelatinous envelope, 

 which, according to Thwaites, surrounds the conjugated 

 individuals of the Diatomacese, or the amorphous, very 

 fluid jelly in which many of the Desmidiacese live, as, for 

 instance, Penium curtum, the individuals of which undergo 

 the " skinning" above described, in their division,^ inside 

 this jelly. In many cases these gelatinous envelopes 

 appear not so much like altered and swollen layers of 

 cell-membrane, as coats originally secreted in a fluid- 

 gelatinous form. At the same time undoubted cases 

 show that a gelatinous softening, sweUing-up, and final 

 solution of cell-membrane, formed of cellulose of normal 

 character, does really occur. My observations on the 

 frequently mentioned Water-net {liydrodictyoTi), afford 

 an opportunity for a minute description of such a process. 

 The cells of this plant exhibit, in their full-grown condition, 

 a tough and firm cell-membrane, about ^igth millim. in 

 thickness. By close examination, we may distinguish 

 three layers in this, the outermost of which is the thin- 



* Vide, for instance, Palmella, (Niigeli, ' Einz. Algen.,' t. iv, d.) 

 f_ Ex. ffr., Gleeocapsa, (ibid., t. i, r,) Aphanocapsa, (t. i, B,) Olaothece, 

 (t. i, g), Aphanothece, (t. i, H.) 

 % 'BinzeU. Alg.,' p. 13. 

 § Vide supra, p. 181. 



