SECONDARV USES OF EXCRETA, ETC. 329 



likewise belongs here) is, as a rule, of no further use in metabolism, especially when 

 it crystallises out as calcium oxalate; nevertheless, under certain circumstances, 

 calcium oxalate becomes again dissolved, and occasionally the acid is found com- 

 bined with alkalies, as in the Sorrel and the Wood-sorrel. In these cases it is 

 possible that it is not yet entirely withdrawn from metabolism. That the vegetable 

 alkaloids when once formed do not again enter into metabolism, as a rule at any 

 rate, appears probable from what is known as to their occurrence. 



The main point in determining the physiological significance of a vegetable sub- 

 stance is always whether, when it has made its appearance anywhere in the tissues, it 

 then again disappears from its receptacle during further growth and progressive meta- 

 bolism, or remains lying inert in it, in which case a further share in metabolism is 

 obviously excluded. The latter is the case with nearly all the substances contained in 

 the so-called receptacles for secretions. Of course substances which find no more use 

 in metabolism may be incidentally of the most varied significance for the maintenance 

 of the individual, as well as for reproduction. Thus, for example, the lignin which 

 permeates the cell-walls of vessels, wood-cells, and sclerenchymatous fibres, is of 

 considerable importance for the solidity of such cell-walls and the conduction of 

 water in them; and likewise the wax in the epidermis and on the cuticle affords 

 a means of protection against evaporation, and at the same time against the entrance 

 of water; and acids at the surfaces of the roots, as was shown above, aid in the 

 taking up of absorbed nutritive matters. The colouring matters of flowers are 

 assumed to play a great part in pollination, in so far that insects are allured by 

 them to the organs of fertilisation ; the large quantities of sugar and organic acids 

 which accumulate in the flesh of fruits, — e. g. Grapes, Oranges, etc., — are also of 

 course lost to metabolism, but they may be of use to wild plants in so far that 

 birds and other animals devour such fruits, and subsequently rid themselves of 

 the undigested seeds. That the formation of any substance in the plant may also be 

 essentially influenced by culture and the artificial selection of varieties is shown by 

 the ever-increasing number of the various colours of garden flowers, and the great 

 number of different flavoured varieties of orchard fruits and Grapes, all of which have 

 been produced by cultivation. 



Meanwhile, however, these are for us but matters by the way, since we are now 

 concerned only with establishing that all the different substances of the plant are 

 derived, directly or indirectly, from the starch arising in the assimilating chlorophyll ; 

 and that, in the first place, only a very limited number of products of metabolism — 

 namely the carbo-hydrates, fats, and proteid substances — come into consideration 

 as the constructive materials of the growing organs of the plant. All other 

 substances, occurring as secretions or as products of degradation, are derivatives 

 of these. 



When now starch arises in the leaves under the influence of light, it, as well as 

 the other plastic substances produced at its expense, is conveyed through the petioles 

 to the shoot-axes, to proceed in the interior of these organs partly downwards to 

 the growing apices of the roots, partly upwards to the developing buds, and to supply 

 at both places material for the growth of the organs. Hence, as occurs particularly 

 in annual summer plants, assimilation and the employment of the products of 

 assimilation for growth may proceed simultaneously, without any considerable storage 



