LIFE OF THE PLANT-CELL. 105 



have entirely consumed their contents, and contain nothing but 

 air : the so-termed vascular, medullary, and liber cells ; or those 

 the contents of which have become converted into an isolated ho- 

 mogeneous matter, as the cells containing nothing but essential 

 oil, resin, &c. The latter, however, are proportionately few in 

 number. 



Here we have another point, which has been either entirely neglected, 

 or only superficially and cursorily treated of in books which for the most 

 part do not even teach us any thing respecting the death of the whole 

 plant. If we place the life of the cell wholly, or at all events for the 

 greatest part, in the chemico-physical processes going on in the cell, we 

 must term the cell dead in which these processes have entirely and for 

 ever ceased. This is the case particularly in all cells which convey only 

 air, which, being themselves dead, are saved from decomposition only by 

 the living cells surrounding them, but which are instantly destroyed 

 when exposed to the decomposing action of the atmosphere ; as, for in- 

 stance, the pith and heart- wood in trees becoming hollow, and cork and 

 bark always at a certain time. There are, however, cells of this kind 

 which gradually convert their whole contents into an isolated secretion, 

 as into essential oil, as happens in the rhizomes of the Scitaminece, in 

 the leaves and stem of the Aloes, &c. In these cases the cell must be 

 regarded as dead from that moment. The after-process is neither de- 

 termined nor modified by the cell ; it is a chemical process, and consists 

 in the gradual oxidation of the essential oil, with the completion of which 

 all farther change is at an end. It is in this way that the termination 

 of the individual cell-life, even in the very interior of the most perfect 

 plants, is indicated. 



48. Only the completely formed cellulose resists the usual 

 solvent reagents ; all the other substances of which cell-walls can 

 consist, are still within the domain of the solvent or transmuting 

 chemical forces which are active in the cell. All cells, therefore, 

 whose formation is not completed admit of being again rendered 

 fluid and becoming absorbed. This is the case in all mother-cells, 

 in the spongy cellular tissue which originally fills the air-canals, 

 in the nucleus of the ovule, &c. 



It is undoubtedly a proof of superficial observation when a botanist, 

 as has been the case, denies the resorption of organised structures in 

 plants, an event which is observed with less facility in animals than in 

 plants. The enormous number of mother-cells alone affords the most 

 irresistible proof that this process does take place. But in what way 

 it is effected is as yet unknown. Probably there takes place in this case 

 a chemical change of the assimilated matter opposed to the formation of 

 cellulose ; that the former is first changed into jelly, then into gum 

 (dextrin), and finally into sugar, and as such is absorbed. I would 

 hereupon remark, that it has sometimes appeared to me as if, in the 

 nucleus of the ovule, the cytoblasts reassumed a more sharply defined 

 and younger aspect when their cells approached the period of solution. 

 A peculiar transformation of already formed cells into an amorphous 

 substance, " viscin," has been before adverted to ( 12. 6). 



49. The life of the vegetable- cell continues only so long as 

 the chemico-physical processes go on within it, and these become 



