250 A TEXTBOOK OF BOTANY [Ch. V, 8 



directions of the stimuli and the relative sensitiveness of the 

 roots thereto. 



The student will be interested to read at this point the 

 fine passage which closes Darwin's book "The Power of 

 Movement in Plants." He should keep in mind, however, 

 the fact that a thread of simile and fancy runs through the 

 paragraph, to the matters of which our modern science now 

 gives a somewhat more mechanistic interpretation. 



8. The Additional, and Substitute, Functions of 

 Roots 



While the great majority of roots have the typical forms 

 and functions already described, there are some which 

 perform additional and even substitute functions, with 

 corresponding modifications of structure. 



As in case of other plant organs, roots which perform the 

 typical functions yet exhibit marked diversity of form, 

 usually in clear correspondence with different habits. Thus 

 the difference between the tap root and a mass of fibrous 

 roots (page 212) is of this nature. Again, the relative im- 

 portance of the absorbing and anchorage function varies 

 much, the latter being highly important in great trees, and 

 almost negligible in low herbs, especially such as have under- 

 ground rootstocks ; and corresponding differences in struc- 

 ture are manifest. The only roots of the low-growing Bryo- 

 phytes are the great root hairs, or rhizoids, effective in 

 absorption, but obviously having little utility, as there is 

 little need, for anchorage. The depths to which typical 

 roots descend vary also, for while those of swamp and bog 

 plants keep near the surface, obviously in relation to air 

 supply, those of some desert plants reach at least to sixty feet, 

 as in the common desert shrub called Mesquite, evidently 

 in adjustment to the water supply. And other differences 

 are revealed by intensive study, — some most reasonably 

 explained as adaptive, others as hereditary, and others as 

 structural or incidental. Yet the diversity presented by 



