Development, and Structure of the Vegetable Cell. 193 
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 Orchidexe, Burmanniaceze, Gentianee, &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 Spzro- 
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 to be capable of furnishing con~ 
firmation to the imvestigations 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. xii.) 
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, shortly before their complete deliquescence, exhibit pores 
of considerable size (Pl. V. fig. 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 
Amn. & Mag. N. Hist. Ser. 3. Vol. xiv. 
