Development, and Structure of the Vegetable Cell, 198 



membranes of which, being subsequently thickened, then appear 

 to be continuations of these ridges. 



It is probable that the porous walls are produced sometimes, 

 although but seldom, by mere folds, at other times by thickenings 

 similar to that described in Spirogyra, but sometimes also by 

 not only the membrane of the mother cell, but partially those 

 of the vesicles adherent to it, becoming lignified, in the same 

 way as the reticulated outer membrane of spores and pollen-cells 

 and also the simple cellular layer formed by the seed-coverings 

 of the Orchidete, Burmanniaceae, Gentianea, &c. 



That the production of the vessels composed of spiral cells 

 (which are to be regarded as the first vessels in the cambial 

 tissue of the apices of the roots, and therefore, no doubt, also of 

 the buds of the stem and branches) is assisted by the richness 

 of this tissue in organic nitrogenous compounds, was evidenced 

 to me by experiments with roots of Iriartea ; and that these 

 compounds, by increasing the quantity of the endogenous cel- 

 lular structures, also appear to induce the general spiral dispo- 

 sition of the organized cell-contents may be assumed from the 

 observations upon the position of the chlorophyll-sac of Spiro- 

 gyra cited at p. 25. Direct special researches will elucidate this 

 point. 



Certain retrograde metamorphoses of porous vessels which I 

 have observed appear to me tu be capable of furnishing con- 

 firmation to the investigations of their anatomical structure 

 made by Unger and myself. 



The walls of the thickened porous cells and vessels filled with 

 cork-cells undergo absorption (as described at p. 272, vol. xiii.) 

 in such a manner that the external membranes are the first to 

 disappear. This can be particularly well seen in the much- 

 thickened cells of the medullary sheath, the innermost coats of 

 which, shortiv before their complete deliquescence, exhibit pores 

 of considerable size (PI. V, tig. 15). 



Under these circumstances we may not unfrequently detect 

 in the walls of porous vessels in course of absorption a structure 

 which is in accordance with the production of these pores in 

 consequence of the adhesion of vesicles to the inner surface of 

 the cell-membrane which is afterwards porously thickened. 



A portion of such a cell-wall, more strongly magnified, is 

 shown in fig. 6. It is composed of almost horizontally disposed 

 annular bodies, imbedded in an intercellular substance, and 

 having interposed between them a homogeneous continuous 

 band cemented to them by the intercellular substance. 



That these annular bodies are to be regarded as small cells, 

 thickened strongly all round, and but slightly above and below, 

 is evidenced (leaving out of consideration the already recognized 



Ann. 6c Mag. N. Hist. Ser. 3. Vol. xiv. 13 



