558 Comparative Physiology. 



shrubs. Each fibre differs from those of cellular plants by the 

 possession of a bundle of vessels (woody fibre and ducts) which 

 occupy its centre. The spongiole is the growing point of the 

 root, the soft lax texture of which possesses in an eminent de- 

 gree the power of absorption. As the radical fibre elongates, 

 the spongiole becomes consolidated into the general structure of 

 the root. The extension takes place by the addition of fresh 

 tissue to the points, in the direction of least resistance, causing 

 the root to grow towards moisture, not from instinct but from 

 less resistance. The long descent of roots along bare rocks, 

 perpendicularly and horizontally in quest of earth, is, however, 

 difficult to explain. The absorbent power of the spongioles ap- 

 pears limited by the size of the pores, which, though hitherto 

 undetected, must have a sensible diameter. The most finely 

 divided particles in coloured solutions, and the most watery 

 portions of solutions of sugar, gum, &c., are always taken up 

 first. The power of selection, however, would seem to extend 

 beyond this, as of some substances in solution, some plants will 

 take one and some the other, and some neutral salts are re- 

 jected altogether. It does not appear, however, that the select- 

 ing power is employed to prevent the introduction of deleterious 

 matter. From the little at present known on the subject, it 

 seems a reasonable inference that the rejection of any particular 

 ingredient in the fluid in contact with the roots, results either 

 from the want of adaptation in the form or size of its molecules 

 to the pores of the spongiole *, or from an organic change effected 

 by it on their delicate tissue, such as is proved by the experi- 

 ments of M. Payen to occur when tannin enters into the solu- 

 tion, even in very minute proportion." 



From the similarity between lichens and algae above noticed, 

 it may be inferred that what have been reckoned instances of 

 equivocal generation may rather be due to the circumstances 

 they are placed in causing the same plant to assume a different 

 appearance ; and what have been reckoned of distinct genera and 

 species may, after all, only be different states of the same plant. 

 It is not at all probable that the action of roots, and their food, 

 on each other, can claim the high rank of instinct. That roots 

 do act on the food, however, is generally believed. In two pots 

 of charcoal exhibited for experiment in the Horticultural So- 

 ciety's gardens, the one having a growing plant inserted in it, 

 the other Avithout any plant, but treated in exactly the same 

 manner in every other respect, it was found that much more of 

 the charcoal was consumed in the one which had the growing 



* Dr. Daiibeny has found in his experiments, that salts of lime, magnesia, 

 and others that are isomorphous, are absorbed by the same plants ; while 

 they refuse such as those of strontia, which have a different crystalline 

 arrangement. 



