180 NOTES AND MEMORANDA. 
work. His energies were always ready to turn themselves 
to any part of his subject where facts seemed to need inves- 
tigation, or the results of others to challenge re-examination 
or criticism. His papers are, in their way, models of “ con- 
tributions to knowledge.” 
Von Mohl’s first publication in 1827 was a prize thesis on 
the structure of climbing plants, in which he endeavoured to 
show that the stems have a dull kind of irritability, so that 
they bend towards any object which they touch. This expla- 
nation has given place to a better knowledge of the pheno- 
mena; but Mr. Darwin, to whom that service to science is 
largely due, bears witness to the prima facie probability of 
Von Mohl’s view (‘ Journ. Linn. Soc.,’ Bot. vol. ix, p. 10). His 
inaugural dissertation in 1828 (already alluded to) gave the 
first account of the true structure of the dots or “ pores ” fre- 
quently met with in the walls of cells (‘ Ueber die Poren des 
Pflanzenzellgewebes’). He showed that they were thinner 
portions of the cell-membrane, 
In 1831 Von Mohl, as already mentioned, contributed to 
the ‘ Historia Naturalis Palmarum’ of Von Martius an 
elaborate account in Latin of the structure of the stems and 
roots of palms, under the title “De Structura Palmarum.” 
This was republished in German in his ‘ Vermischte Schriften’ 
in 1845, and was translated for the Ray Society in 1849 by 
Prof. Henfrey. Von Mohl gave the final blow to the theory 
of the internal growth of monocotyledonous stems first pro- 
pounded by Desfontaines, and upon which De Candolle had 
founded the division of vascular plants into Exogens and 
Endogens. In this memoir he appears to have first described 
the origin of ducts from rows of closed cells, a point which he 
further developed in the following year in a paper, “ Ueber 
den Bau der pordsen Gefisse. 
The publication by Von Mohl in 1835 of his discovery of 
the multiplication of cells by division (‘Ueber die Vermeh- 
rung der Pflanzenzellen durch Theilung’) in Cladophora 
glomerata has been the starting-point of all subsequent in- 
vestigations into the development of the tissues and organs 
of plants. It revealed, in fact, the precise mode by which 
vegetative growth is accomplished. Mirbel, in his memoir 
on the development of Marchantia, communicated to the 
Académie des Sciences in 1831 and 1832, but not published 
till 1836, had described the formation of pollen-grains by the 
quadripartite division of a mother-cell. This, however, 
though an extremely important observation, is not a case of 
growth, properly speaking, and does not affect Mohl’s histo- 
rical position in the matter. In 1838 Schleiden announced 
