192 Prof. H. Karsten on the Formation, 
The study of the history of development of the porous cells 
in the pith of Hoya carnosa, in the tissue of the stem of Langs- 
dorffia, as also that of the porous and scalariform ligneous cells 
of ferns &c., led me to the knowledge of these conditions of 
structure*. 
At the time it escaped my notice that Unger, in his instrue- 
tive examination of the development of the spiral vessels in the 
root-ends of Monocotyledons, had already arrived at similar 
results. Unger observed that the youngest vessels arising from 
the coalescence of series of cells contained a mucilaginous fluid, 
within which numerous small vesicles soon presented themselves 
and became adherent to the walls of the vessels, which at a later 
period underwent thickening, in part in a spiral manner, in the 
intervals between these vesicles. 
A picture of the spiral thickening of secondary cells is fur- 
nished by certain diseased states of Spiroyyra nitens, which have 
been frequently referred to. When this plant has lain for some 
time in carbonic-acid water, and is afterwards transferred to 
pure water or to a very weak endosmotic solution, the chloro- 
phyll-layers are observed to become, in consequence of diosmosis, 
separated from the swollen secondary walls, as seen in Pl. VII. 
figs. 65, 66. In these now muco-gelatinous membranes they 
leave behind them channel-like depressions, the membrane at the 
parts between them being more strongly thickened, probably 
from the absence here of impediments to diffusion. The pheno- 
menon is very transitory, as the membrane continues to undergo 
change by swelling up, and apparently becoming liquefied in 
the water. 
Another picture, likewise, of a spiral arrangement is at times 
seen in the progress of the changes of the cell-contents of 
Mougeotia when placed in solution of tannin (vol. xi. p. 418). 
In this instance the secretion-cells do not adhere to the wall, but 
occupy the entire cavity of the cell. 
Both these examples are probably types of spiral formation 
as it actually proceeds in nature, though observable with very 
great difficulty. In every case this formation takes place by 
means of a thickening of the cell-membrane in the intervals 
between adherent endogenous vesicles, just as the often observed 
ridge-like prominences on the secondary pollen mother cells 
(vol. xiii. p. 483) originate between the pollen-cells, the proper 
* De Cella vitali, p. 33, tab. 1. figs. a-d; Vegetationsorgane der Palmen, 
tab. 8. fig. 16; Bau der Cecropia, Nova Acta, vol. xxiv. tom. I. p. 88, 
tab. 13. fig. 4; Langsdorffia, Nova Acta, vol. xxvi. tom. 11. tab. 63, 
fig. 5. 
+ Linnea, 1841, p. 385, taf. 5. See also Grundz, d. Anat. u. Phys. 
1846, pp. 11 & 46. 
