72 REPORT ON BOTANY, MDCCCXLI : 
the structure of a root-stock ; for instance, compare with 
them the root-stock of celery {Apium graveoleus). The 
thick walled cells most probably belong to the vessels or cells 
of the liber. The author, in this instance, also limits his con- 
ception of spiral vessels to such as may be unrolled, and which 
exhibit no membrane between the spires. I will not be posi- 
tive that this membrane always exists, or that it is frequently 
wanting ; but I have mentioned once before, that I would not 
like to consider it as a characteristic, nor even the circum- 
stance of their being unrolled, for the latter may be occasioned 
by the stronger or slighter union of the spires themselves, or 
by the greater or less strength of the membrane between them. 
In my opinion, there are only two kinds of spiral vessels ; one 
where the spiral-formed windings are exhibited in the form of 
a band, and the other where there are porous vessels with 
round holes or transverse bars. We have before noticed the 
treatise of David Don respecting the Cycadem, in which he 
says he had saturated porous vessels with tincture of iodine, 
and found that the openings were coloured ; and thought this a 
proof that they were overspread with a tender membrane. I 
have frequently repeated this experiment since, but the result 
was just the reverse ; the pores were not coloured at all, but 
were always distinctly seen as genuine openings. If Mr. Don 
did not see this, it must have been owing to his not having 
made sufficiently fine incisions, when, consequently, a mem- 
brane was left beneath, which closed up the opening. The cir- 
cumstance of the bars being seen as dark stripes, originates in 
the reflection of the rays of light ; this, however, is frequently 
so deceptive, that these bars may be considered as shortened 
spiral fibres or bands, as has occurred to myself and many 
others. 
B. LEMNACE^. 
Contributions to the more intimate knowledge of Lemna 
arhiza, by J. T. Holfmann, Wiegmann’s Arcliiv, 1840, vol. i. 
p. 138. The author found this hitherto but little known or 
neglected species upon waters in the south of Holland, near 
Gouda. It is distinguished by its form ; it is oblong or 
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