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 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 

 Marchantice, 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 [in puncto veye- 

 tationis, 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. 3. 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 



