A SSI MIL A TION A ND ME TA S TA SIS. J I \ 



of the protoplasm in the rapidly multiplying cells. If the colourless cells of yeast 

 are able to do this, it may be inferred, until the contrary is proved, that those cells 

 of other plants which do not contain chlorophyll can also produce albuminoids, if 

 only a carbo-hydrate or oil (or both) is conveyed to them from the leaves, and an 

 ammonium-salt or a nitrate from the roots. That the formation of albuminoids 

 probably takes place in this way within the conducting tissues of internodes and 

 petioles may be concluded from the deposition of calcium oxalate in these tissues ; 

 since in the formation of this salt sulphuric acid becomes separated from the calcium, 

 and its sulphur enters into the chemical formula of albuminoids \ 



When the cells of the leaves become emptied of their contents at the close 

 of the period of vegetation, and the deciduous parts die off, not only the last portion 

 of starch which was formed in the latter, but also the material of the chlorophyll- 

 granules, is itself absorbed and conveyed through the leaf-stalks to the reservoirs of 

 reserve-material ; all the serviceable substances contained in the leaves become in- 

 corporated in the permanent organs. The leaves change colour ; a small quantity 

 of very small shining yellow granules usually remain behind in the cells of the 

 mesophyll as a residue of the absorbed chlorophyll-granules ; and the leaves which 

 are emptied in the autumn are therefore yellow. If they are red this is in con- 

 sequence of a red sap which fills the cells in addition to the chlorophyll-granules^. 

 Enormous quantities of crystals of calcium oxalate often remain behind in the 

 deciduous leaves; the constituents of the ash which are serviceable to the plant, 

 especially phosphoric acid and potash, are conveyed with the starch and the proto- 

 plasmic substances to the persisting parts ; so that the falling leaves thus consist 

 only of a skeleton of cell-walls and of the subsidiary products, of metastasis which 

 are of no value to the plant. 



The direction of the Transport of the assimilated substances in the plant is 

 determined by the fact that it must take place from the assimilating organs to 

 the growing parts and to the reservoirs of reserve-material; while at the com- 

 mencement of every new period of vegetation its direction must be from these 

 reservoirs to the growing organs ; and since new organs are usually formed above 

 as well as below these reservoirs and the assimilating leaves, it is obvious that 

 the movements of the assimilated substances must take place at the same time in 

 opposite directions. 



The Conducting Tissue for the transport of the formative materials consists, 

 in plants with differentiated systems of tissue, of the parenchyma and the thin- 

 walled cells of the phloem of the fibro-vascular bundles. By the parenchyma of 

 the fundamental tissue, which always has an acid reaction, are conveyed the 

 carbo-hydrates and oils; by the soft bast, the albuminoids which have an alkaline 

 reaction. Small starch-grains often occur, as Briosi has recently shown, in the pro- 

 toplasm of the sieve-tubes; I had already pointed out that this accompanied the 

 absorption from the leaves in the autumn as well as very rapid growth ^ Where 



' See Sachs, Handbuch der Experimental-Physiologie, p. 345. 



^ [On the colouring matter of the leaves in autumn, see Sorby, Quart. Journ. of Science, 1871, 

 p. 64; and 1873, p. 215.] 



^ Briosi, Bot. Zeitg. 1873. It is by no means certain that the occurrence of small quantities of 

 starch in the sieve-lubes demonstrated by Briosi, and the possibility of their passaj;e through the 



