FORM OF THE PLANT-CELL. 41 



lated knobs*, and in the stinging hairs of the leaves of Anchusa crassi- 

 folia, where they appear as granular warts. Of the history of the 

 development of these little knobs, especially those of the Malpighiacece, 

 we know nothing. 



17. In some rare cases, as in the spores of some Conferva, there 

 are formed, sometimes upon the external surface of the cell and 

 sometimes only at a particular spot, fibrilliform processes, which 

 exhibit a vibratile motion similar to the cilia found on the mucous 

 membranes of animals. 



For the observation of this highly interesting phenomenon, we are 

 indebted to the labours of Ungerf and Thuret.J I have not yet had an 

 opportunity of confirming these observations. The following is the 

 result of their researches : Thuret distinguishes four forms. In the 

 Conferva rivularis and C. glomerata the spores have at their tapering 

 ends two vibratile cilia. In Chcetopliora elegans var. pisiformis and 

 another species there are four cilia. Prolifera rivularis and P. Candollii 

 of Leon Leclerc ( OEdogonium of Klitzing ?) have in the same spot a 

 crown of vibratile cilia. In the last place, the Vaucheria Ungeri Thu- 

 ret ( V. clavata and ovata DeC.) have the entire surface of their spores 

 covered with vibratile cilia. This last fact was first ascertained by 

 linger. Upon the origin of these cilia nothing is known, and just as 

 little of the mode of their disappearance. Just as suddenly as these cilia 

 disappear when the moving spore has fixed itself, do they appear when the 

 spore leaves its spore-case. The perfectly formed spore-cells are brown 

 in all cases, according to Kiitzing, but those which have the power of 

 moving are green. According to the history of the development of the 

 cells of Algce, given by Nageli, they originally consist of a closed sac of 

 mucus, and subsequently there appears around them a membrane of 

 cellulose. It might be a matter of inquiry whether the imperfect mucus- 

 cells have not the power of forming cilia, which they lose immediately 

 the proper cell-membrane is formed. The membrane bearing the move- 

 able cilia would be then the same layer of mucus as, in the antheridia 

 of the Mosses, Hepaticce, &c., is transformed into spiral fibres, and which, 

 in the cells of the Characece, Naiadece, and many higher plants, form 

 little moving streams. Nageli has also promised shortly an account of 

 the relation between these movements and the nitrogenous contents of 

 the bodies in which they occur. 



18. When the cell has reached a certain degree of development, 

 an essential change takes place in its mode of nourishment: the 

 newly formed cellulose is not taken up by intus-susception, but is 

 deposited upon its inner surface as a concrete layer. This deposi- 

 tion does not, however, take place as a continuous membrane, but 

 is formed in the direction of a spiral, as a spiral fibre or band. 

 Should the cell distend after this deposition, then the spires which 

 laid close together at first are drawn asunder. The less the cell is 

 extended, the firmer is the union of these fibres with its walls. 



* Morren, Obs. sur 1'Epaississement de la Membrane vegetale dans plusieurs Or- 

 ganes de 1'Appareil pileux. (Bullet, de 1'Acad. Itoy. de Bruxelles, torn. vi. No. 9.) 



f Unger, Die Pflanze in Moment der Thienverdung. Vienna, 1843. 



\ Thuret, Recherches sur les Organes locomotcurs des Spores dcs Algees. (Ann 

 des Sc. Nat, Mai, 1843.) 



