252 CONTRIBUTIONS TO 
3. The walls of the fully-developed cells become thickened 
by the deposition of new matter in layers, a process which, in 
accordance with the old rule, a potiori fit denominatio, may be 
most aptly termed the lignification of the plant. 
If, in respect to the growth of the plant, we now hold to 
the literal sense conveyed under No. 1, then this question 
must arise,—Where are the new cells formed? Here three 
instances comprise all possible replies. Namely, the new ceils 
are either formed outside on the surface of the entire previous 
mass, or in its interior; and in that case again either in the 
intercellular spaces or in the cells themselves; guartum non 
datur. 
Mirbel, in two extremely ingenious and profound memoirs 
on the Marchantia polymorpha, which he presented to the 
French Academy in 1831 and 1832 (p. 32), has expressed the 
opinion, that all the three cases just now mentioned as possible 
do actually occur in plants. Without intending here to anti- 
cipate what follows, I must remark that only one case (the 
formation of new cells within the old ones) appears to be 
proved by his direct observations. The second case is merely 
a conclusion drawn, and the germination of the sporules of the 
Marchantie, which was to elucidate the third case, has been 
observed by me to be quite different, as I have already repre- 
sented above. 
Finally, however, we have yet to examine whether the differ- 
ence of the organs may not establish such a physiological 
difference of growth as may merit our attention. We may 
distinguish here four instances. We observe: 1. The develop- 
ment of the plants in the upward direction (im puncto vege- 
tationis, C. Fr. Wolff). 2. The elongation downwards. We 
thus comprise the formation of the necessary orgaus of the 
plant, the stem, the leaves (with their metamorphoses), and the 
root. 8. We have to keep in view the production of accidental 
organs, for example, bulbs, &c. And, 4. We find an annual 
thickening of the axile formations, the development of the 
woody stem. 
Let us now see which of the three possible modes of forma- 
tion of new cells is actually realised in each of the cases just 
enumerated. I have already explained how the new cells are 
developed in the embryonal sac ; in other words, within a large 
