ASSIMILATION AND METASTASIS. 
711 
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, Bet. 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 passage through the 
