212 The Living Plant 



the roots. Second, are various minerals, which in part are useless 

 materials absorbed along with the useful kinds, and in part are 

 by-products of chemical changes inside of the plant. For their 

 removal plants have no regular excretory system as animals 

 have, though a partial substitute exists in the fall of the leaves 

 and the bark, which thus remove crystalline matters they con- 

 tain. Other minerals are left behind as crystals in the old dead 

 cells when the living protoplasm advances into the new ones it 

 forever is building (compare figure 41). Third, are the root- 

 poisons, little known to us yet and even by some experts not be- 

 lieved to exist. They appear to be highly complex organic sub- 

 stances, slow of diffusion and drainage, and poisonous to the roots 

 which produce them though not necessarily to different kinds; 

 and this fact gives a new explanation of the advantage of rotation 

 of crops and of letting a soil lie fallow. Fourth, is extra-floral 

 nectar, apparently identical in composition and mode of forma- 

 tion with the nectar of flowers, which performs the invaluable 

 service of attracting cross-pollinating insects, as later we shall 

 note in detail. The extra-floral nectaries are very tiny structures, 

 sometimes marked by blotches of color, occurring commonly at, or 

 near, the bases of leaves in young plants (e. g. in some Ferns, 

 Horse Beans, Castor Beans and others), or with the spines (in 

 Cactus), and elsewhere. They have been supposed to attract 

 small ants which may perform some ecological service; but the 

 evidence thereon is so unsatisfactory that it seems best to place 

 this nectar for the present among the excretions, though surely 

 it is a puzzling sort. 



So, and by such means, are substances removed from plants. 

 The reader knows also in what ways they are absorbed. Between 

 absorption and removal they have to be transported, often for 

 very long distances; and this is the matter which next needs 

 attention. 



The principal substance to be transported is water, of which 

 transpiration demands so great a supply that it has to be moved 



