CELL. 



[ 122 ] 



CELL. 



vesicles in the embryo-sac of the Flowering 

 plants(P1.38.figs.l-4). Other cells sometimes 

 occm", formed in the same way, at the oppo- 

 site end of the embryo-sac. The embryo- 

 sac also frequently becomes filled, after fer- 

 tilization, by a large increase of free cells 

 developed out of the layer of protoplasm or 

 primordial utricle lining the walls; these 

 accumulate in the sac, and sometimes be- 

 come consolidated into a tissue (albumen) 

 in which the embiyo lies imbedded (NujjJiar); 

 more frequently they are re-absorbed during 

 the growth of the embryo. The embryo 

 itself is developed from the germinal vesicle 

 by cell-division such as is described under § 1 

 (PL 38. figs. 5, 6). 



The notion of the independent origin of 

 cells from organic substances by generatio 

 equivoca seems to require no notice; but 

 allusion may be made to certain curious 

 pheenomena which have been called ' abnor- 

 mal cell-formations,' occurring in some of the 

 Confervoids, The protoplasm of the Sipho- 

 neae is verj- apt to collect into globular masses 

 in injm-ed filaments, and these globular 

 masses apparently acquire a cellulose coat in 

 some cases; they have been observed in 

 Vaucheria and Bryopsis ; a somewhat similar 

 phaenomenon often occurs in the contents of 

 the cells of Sjnrogyra. It appears to be a 

 kind of gonidial reproduction, in which a 

 portion of the hving contents are enabled to 

 save themselves from the general decompo- 

 sition. 



Membrane. — In all young organs, in suc- 

 culent structures, and all the delicate tissues 

 of the higher plants, and in the majority of 

 the Cellular plants in almost the entire or- 

 ganization, the cellular membranes consist 

 of a thin structureless pellicle, possessing a 

 considerable degree of toughness and a cer- 

 tain amount of elasticity. (C. J. Agardh 

 has indeed lately asserted that cell-membrane 

 is composed of spiral fibrous structure, but 

 this doubtless is an error as regards the pri- 

 mary membrane.) It is readily permeable 

 b\ water, while no orifices of any kind can 

 be detected in it; but young and indeed soft 

 cell-membranes generally imbibe more or 

 less water, and swell to some extent, often be- 

 coming more or less gelatinous. It is statedby 

 Schleiden that the membranes of nascent cells 

 are soluble in water, but general experience 

 does not confirm this statement; the only ap- 

 proach to a corroboration of it that we have 

 met with, is the fact of the external membranes 

 of many of the filamentous and unicellular 

 Algae becoming gelatinous and gradually 



dissolving away as the inner membranes are 

 successively deposited, forming a gelatinous 

 coat {ex. gr. Protococcus, Nostoc, Desmi- 

 diaceae, Diatomaceae, Zygnema, Oscillatorieae. 

 &c.); the same also taking place in the de- 

 velojjment of spores and pollen grains, which 

 are set free by the parent-cell membranes 

 becoming dissolved. This, however, is 

 scarcely a direct solution in water, and comes 

 rather under the head of a decomposition. 



Young and delicate cell-membranes are per- 

 fectl}'^ transparent and colourless, as is seen 

 in the Yeast plant, in the mycelium of Mil- 

 dews, in the cellular tissue of tuberous struc- 

 tures like the Potato, in piths (after the 

 mucilaginous cell-contents have been re- 

 moved). As they grow older, they often 

 become colom*ed, sometimes very deeply, 

 which is supposed, however, to depend on 

 the infiltration of foreign matters. In the 

 state of simple cell-membranes, when no in- 

 filtration of foreign matter has occurred, the 

 application of sulphuric acid of moderate 

 strength, with solution of iodine in solution 

 of iodide of potassium, brings out a bright 

 blue colour, and this is regarded as a test for 

 cellulose, the universal basis of vegetable 

 cell-membrane. 



When the cell has attained a certain age, 

 new deposits of membranous substance take 

 place inside, and the membrane thus acquires 

 more or less thickness, together with a very 

 varied appearance, according to the character 

 of the deposits. The new layers are known 

 as Secondary Layers, and the term 

 Cell-wall is perhaps the most convenient 

 collective term which can be applied to the 

 various structures produced by the deposition 

 of new layers of cellulose upon the inside of 

 the primary cell-membrane. Although these 

 new deposits are thin layers of cellulose like 

 the primary membrane, they are rarely so 

 totally devoid of detail structure, and in 

 the majority of cases exhibit orifices and 

 irregularities of the most striking character. 

 Moreover, in one class of cases, they are not 

 deposited as a continuous coat, but as a 

 fibrous structure apjdied upon the primary 

 membrane, as in spiral-fibrous-cells ; and in 

 wood-cells they are formed one above an- 

 other to such a thickness that the cell-wall 

 loses its original membranous character, and 

 becomes a solid case, with the internal cavity 

 reduced to a comparatively small chamber in 

 the centre. 



The simplest condition of a thickened cell- 

 wall is that met with in the unicellular and 

 filamentous Algae, where the primary mem- 



