THE ROOT-TIP. 



37 



ward through the loose mass and out through the meshes of the 

 sieve, when, instead of continuing vertically, they bend toward 

 the moisture which comes from the sawdust and keep close to the 

 inclined surface in spite of gravity. 



With a view to seeing Avhether this sensitiveness to moisture 

 was localized like the sensitiveness to gravity, Darwin covered the 

 ^ tips of a number of seedlings with grease, and then 

 subjected them to an excess of moisture on one side. 

 No bending occurred so long as the tips remained 

 covered. This led him to believe that sensitive- 

 ness to moisture is confined to the same part 

 which is sensitive to gravity, and later in- 

 vestigators, using improved methods, have 

 confirmed Darwin's conclusion. The 

 lateral branches, being less controlled 

 by gravity than the main axis, are, 

 as might be expected, more re- 

 sponsive to differences in 

 moisture. So delicate is 

 this sensitiveness that 

 the roots oftentimes 

 seem to work almost 

 intelligently in their 

 search for water. 

 Thus elm roots have 

 been found filling up 

 a drain fifty yards 

 from the trunk, and 

 numerous instances of roots growing into wells and choking 

 water-pipes have been reported. 



A very common effect of this special sensitiveness is to regu- 

 late the distribution of the rootlets in accordance with the water- 

 shed from the leaves. The greater part of our trees shed the rain 

 outward like a dome or spire, so that the region of earth best 

 watered falls directly under what may be called the eaves : it is 

 just here that the tips of the rootlets occur in most profusion. In 

 the case of shrubs and herbs, which are more apt to grow close to- 

 gether, the water-shed is, of course, mostly indefinite, and as a con- 

 sequence no regularity is apparent in the distribution of the root- 

 lets ; but even among herbs quite definite water-shed is not uncom- 

 mon, and as with trees the effect upon the rootlets is well marked 

 largely in proportion to the isolation of the plants. Certain kinds 

 shed the water outwardly like the trees (Fig. 6, 1), while others 

 have the leaves so disposed as to act like a funnel and carry the 

 water toward the axial root around which the short rootlets are 

 developed (Fig. 6, 2). 



Fig. 5. Apparatus to illustrate the Mode in which the 







Influence of Gravity is overcome by the Effect of 

 Greater Moisture on one Side of the Eoot. (Sachs.) 



