150 MORPHOLOGY. 



justified in expecting important results to science from them on account 

 of the simplicity of their structure and life. To effect this, however, it 

 will be necessary, if we would avoid thorough confusion, entirely to 

 exclude the Diatomece and the true Oscillatorice, which, according to my 

 view, are equally dubious. 



82. The Alga, consist generally of cells in a low condition of 

 development, having for the most part gelatinous walls; in the 

 FucoidecR and Floridecs we find, in the interior, more elongated or 

 broader cells, which, by their distinct porous canals, indicate the 

 presence of thickening layers, so that the cavity of the cell has fre- 

 quently a beautifully ramified appearance. These cells are very 

 often found to be arranged with the greatest regularity. In 

 most of the Algce, the tender mucous integument of the inner 

 surface of the cell-wall is especially developed, and here motions of 

 currents of fluid may frequently be traced, as, for instance, in the 

 species of Spirogyra. The chlorophyll often appears as an in- 

 vestment of the cell-wall, in the form of spiral bands with jagged 

 edges ; the granular contents of the cells (the starch) are generally 

 very coarse-grained. In the more complex species, we may distin- 

 guish a smaller, more closely packed cellular tissue, as rind (cor- 

 tex) from the larger-celled porous pith (medulla). The vesicles 

 contain very porous, spongy cellular tissue. All the Algae exhibit 

 a more or less distinctly apparent secreted layer of gelatinous, 

 amorphous substance, covering the whole external surface. 



The structure of the Algce. is, on the whole, very simple, if we do not 

 include amongst them the dubious Diatomece, &c. with their siliceous 

 shields ; and which, as has been already stated, are quite out of place 

 here. I give an exact representation (Plate II. figs. 1 to 6.) of the sili- 

 ceous shield of Navicula viridis, one of the commonest of the Diatomece, 

 of which I have not hitherto met with so minute, nor indeed any accu- 

 rate, representation. It will show that this curious structure is wholly 

 without analogy in the vegetable kingdom*, and cannot be derived from 

 laws of vegetation with which we are at present acquainted. One of the 

 most striking phenomena is the deposition of chlorophyll in the spiral 

 jagged bands observed in the species of Spirogyra. The formation of a 

 secreted layer upon the surface of the Alga appears to throw consider- 

 able light upon the nature of the cuticula in the higher plants. Ca- 

 bomba aquatica shows great affinity to these formations, by its very 

 gelatinous cuticula. It has, however, been shown by the observations of 

 Kutzingf, that this layer in the Algce must certainly be a secreted sub- 

 stance, as, when removed or injured, it is replaced by a liquid mucus, 

 that gradually hardens. On the other hand, the clothing of the inte- 

 rior of the actively vegetating cell, with a semi fluid layer of a nitrogenous 

 substance, frequently circulating in currents (inappropriately termed 

 an amylid cell by Kiitzing), is most remarkably conspicuous in the 

 Algce. Compare, with reference to the above, the Plate II. fig. 7. with 

 the accompanying explanation. 



* Meyen passes this by as a matter of common occurrence because silica is found in 

 other plants, and thus entirely overlooks the essential difference. 

 ) Kiitzing, Phycologia generalis, p. 87. 



