82 ON THE PLANT-CELL. 



reverse, the alcohol readily passing through this substance. Similar 

 modifications in the simplest processes of cell-life must take place, on 

 account of the countless varieties of cell-membrane. In all experiments, 

 however, it is necessary to avoid the the hypothesis of the porosity of the 

 organic membrane, which can only be attended with the same bad results 

 as the notion of the existence of atoms in chemistry.* 



32. The most universally distributed medium of solution in 

 nature, water, is also the fluid which is absorbed by the plant- 

 cell, and conveys all other matters into its interior. The most 

 essential of these matters are carbonic acid and ammonia, both of 

 which are contained in water which either falls from the air or haa 

 been a long time in contact with it. Water, carbonic acid, and 

 ammonia contain carbon, hydrogen, oxygen, and nitrogen, all of 

 which are essential to the formation of the assimilated substances 

 and to the especial nourishment of the cell. But the water occa- 

 sionally conveys to the cell, in small quantities, all substances which 

 are capable of solution in water. 



In spite of the almost endless works upon the nourishment of plants, 

 nothing is in a more uncertain state than our knowledge of the food 

 necessary for plants. This has arisen from the facts having been selected 

 from, and the experiments made upon, the higher and more complicated 

 forms of plants instead of the lowest. The simplest and most natural 

 object for such researches is the Protococcus viridis, or some other simple 

 Conferva., which consists of one or only a few cells, and which floats free 

 in water, and contains the substances universally necessary for the life of 

 the cell. These plants require nothing more for their vegetation than 

 pure water, which has taken up from the atmosphere carbonic acid and 

 ammonia, and perhaps a very small quantity of inorganic salts ; the 

 necessity for which last has not been proved, but is supposed to be 

 necessary from analogy with the higher plants. The experiment is 

 easily made of supplying these plants with water containing a large 

 quantity of carbonic acid, when they will be found to grow more rapidly, 

 and thrive more luxuriously, than when placed in water to which 

 humus, humic acid, or humic acid salts have been added. This is 

 sufficient proof that these last substances are not essential to the life of 

 the cell. 



It is worthy of remark, that just as Carices, and other so-called moor- 

 plants, flourish with a certain quantity of humic acid, which is generally 

 unfavourable to vegetation, so also other plants, as the little Conferva 

 which requires tannin and grows in infusions of galls, require other 

 substances. The Mycoderma aceti grows under the influence of the 

 decomposition of vinegar. In these cases, probably, the free acid is as 

 little necessary to nutrition as in other plants, but the mode and manner 

 of the decomposition of the acid is a favouring moment for the vegetation 

 of the above-named plants. 



Few researches have been made on the nature of the nitrogenous 

 substances in the simplest plants. I have hitherto supposed that the 

 nitrogenous compounds of plants are pure protein. But if we regard 



* See Dutrochet, L' Agent immediat du Mouvement vital devoile, &c. Paris, 1826. 

 Also Poggendorff's Annalen, vol. xi. p. 138., vol. xxviii. p. 134. ; and Schweigger's 

 Journal, Iviii. p. 1. 20. [Also Draper on the Chemistry of Plants, and Matteucci on 

 the Physical Phenomena of Living Beings. TRANS.] 



