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 cells 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; quartum 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 anticipate 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 Marchantiæ, which was to elucidate the third case, has been observed by me to be quite different, as I have already represented above.
Finally, however, we have yet to examine whether the difference 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 development of the plants in the upward direction (in puncto vegetationis, C. Fr. Wolff). 2. The elongation downwards. We thus comprise the formation of the necessary organs 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, &. 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 formation 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
