18 
REPORT ON BOTANY, MDCCCXLl : 
form a mucus that is likewise turned blue by the tincture of 
iodine. See Icon, Anat. Bot., table 16, fig. 14, a. h. 
It is a matter of surprise to me, that Payen has paid no 
regard to the cellular substance of potatoes, which seems to 
approximate to the membrane of the Lichenes. As to Ce- 
traria islandica, it hehayes itself as I hare represented in 
the third volume of the selected Anatom, Bot, Figures, table 
5, figs. 1 and 3 (1841). The most external layer, which 
generally consists of an intercellular substance, does not 
become coloured by iodine ; the second layer, consisting of 
very tender close tubes, exhibits a strong colouring by iodine ; 
and again, the most internal, very loose layer, which consists 
of larger tubes, is not changed by iodine. It is different with 
Loharia pulmonaria, and Roccella tinctoria ; the external 
layer does not become coloured, hut the internal, very loose 
layer, consisting of rather thick tubes, does ; the second layer, 
which is coloured blue in Cetraria islandica, is also entirely 
wanting here. See table 5, fig. 13, and table 6, fig. 5. 
The production of new cells, in relation to those already 
existing, may be conceived to take place in three different 
ways. The new cells are produced at the ends, edges, or 
within the circumference of the older cells ; or they are pro- 
duced between the older cells ; or they are produced within 
the old cells, by a mother cell being torn, or otherwise de- 
stroyed, which causes the new cell to come forward. The 
last mode of production is assumed by Schleiden, who describes 
the new cells as produced from a dark nucleus “ cytoblast” 
(which E. Brown has particularly observed in the young cells 
of the Orchideoe) within a mother cell. I have made some 
researches on this subject, and the result of them I have given 
in the second volume of the leones Selectee Anat, Bot, (1840), 
tab. 6, figs. 1-8. The young leaves are most easily investi- 
gated in juicy plants ; the bud is only a round projection of 
the trunk, on which the leaves are situated like small granules. 
Fig. 1 represents such a leaf of Sempervivum arhoreum, 
magnified 315 times in diameter ; and fig. 2 exhibits the point 
of such a leaf, magnified 600 times. A dark material is per- 
ceived in the cells of fig. 1, which is less distinct in the cells of 
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