338 



LECTURE XX. 



mixed solutions of tannin and red coloui-ing matter. Chemical analysis would 

 in both cases detect small quantities of various other chemical compounds. It 

 is clear that the substances mentioned, 'resin and tannin, as well as the vegetable 

 acids which are never wanting in growing organs, must also have arisen from the 

 reserve-materials ; and since it is known that the quantity of proteid substances 

 is not lessened in germination, it follows that the tannin and resin, vegetable acids, 

 and other non-nitrogenous constituents also of the seedling must have arisen in 

 the Pine from the fat, and in the Bean from the grains of starch. Among these 

 materials, however, the resin and tannin present yet further the remarkable peculiarity 

 that they do not subsequently disappear from the reservoirs in which they are 

 collected during germination; they, as well as the calcium oxalate, remain lying 

 unemployed where they have once arisen. What has here been said of the Pine and 

 Bean, for the sake of example, obtains moreover generally. The resins, ethereal 

 , oils, most tannins, as well as various (probably not all) gum-like contents of 

 the reservoirs of secretions described previously, are substances which are withdrawn 

 from metabolism, and find no further use in the nutrition and in the growth of the 

 organs ; and they are thereby essentially distinguished from the three groups of con- 

 structive materials. It is probable that the vegetable alkaloids are also to be counted 

 among these useless secretions, and the colouring matters so frequently appearing 

 in the vegetating organs certainly are. How and why all these substances, which are 

 very different in diflferent plants, originate is not known ; for us, however, the im- 

 portant point remains that they simply find no further use in metabolism. 



Finally, it is to be mentioned here that some organic substances do not arise 

 in the metabolism which accompanies growth, but are formed by the later meta- 

 morphoses of previously organised parts of cells, and then likewise find no further 

 use in metabolism. To these belong, chiefly, bassorin, gum-tragacanth, gum- 

 arabic, linseed-mucilage, and similar bodies, all of which arise by means of sub- 

 sequent chemical metamorphosis of the cellulose of certain cell-walls : like the 

 secretions, they may, under certain circumstances, be of use for the plant, but they 

 take no further part in metabolism itself. That protoplasmic structures also suffer 

 subsequent degradation, and either wholly or in part find no further use in meta- 

 bolism, follows from what has been said in the last lecture on the destruction of 

 chlorophyll ; and it appears also that old cell-nuclei, as well as the thin protoplasmic 

 utricles of old parenchyma cells, often remain as inert masses until the destruction 

 of the organ. The share in metabolism taken by some other organic compounds, 

 such as pectinaceous substances, the so-called glucosides, and some still dubious 

 tannins, is still very doubtful. The same is also true of the vegetable acids, malic 

 acid, citric acid, tartaric acid, formic acid, acetic acid, etc. ; the wide distribution 

 of which is no doubt usually the cause of the acid reaction of the cell-sap in the 

 parenchyma. According to a view recently expressed by Hugo de Vries^, the 

 significance of these acids in growth is chiefly mechanical, in so far that they increase 

 the turgescence of the growing cells, and so contribute to the extension of the 

 growing cell-walls. It has already been mentioned above that the oxalic acid (which 



' On the significance of acids in turgescence and in the growth of vegetable cells, cp. De Vries, 

 Bot Zeitung, 1879, p. 847. 



