ROOT-HAIRS ON AXIAL AND FOLIAR ORGANS 225 



which grow slowly or in very stiff soil, than they do in roots which 

 meet with little resistance. A slender root, again, requires a better 

 purchase than a stout one, because it is more easily pushed aside ; in 

 thin roots we accordingly find that the distance of the youngest hairs 

 from the apex is relatively small." Schwarz estimates the average 

 distance of the anterior limit of the piliferous zone from the apex at 

 "75 to 1 mm. for slowly-growing roots, and at 2 or 3 mm. for those 

 that grow rapidly. In very thin roots, such as those of Poa pratensis, 

 the youngest hairs may be situated only '11 mm. from the tip. 

 Certain Crassulaceae (e.g. Sedum Andersoni and Sempervivum Furikii) 

 behave in an exceptional manner, inasmuch as their roots bear hairs 

 right up to the apex ; this aberrant condition is probably connected 

 with the fact that the plants in question always grow in very hard and 

 dry soils. 



Certain seedlings display very instructive adaptations in connection 

 with the mechanical function of their root-hairs. Ecologically con- 

 sidered, one of the most important conditions for successful germination 

 is the early establishment of a satisfactory connection with the soil ; 

 the urgent nature of this necessity becomes evident, when we consider 

 that, under natural conditions, seeds usually germinate upon the surface 

 of the soil. For this reason, numerous long root-hairs are generally 

 produced during the very first stages of germination. Thus they have 

 been observed to arise in great numbers at the junction of root and 

 hypocotyl in water-plants by Warming, 120 and in Eucalyptus and other 

 Myrtaceae by Briosi. In the seedlings of Panicum miliaceum and 

 Setaria italica, the first-formed root-hairs appear, according to Schwarz, 

 on the so-called coleorhiza or sheath of the radicle ; the grain is 

 therefore actually fastened to the ground before the root emerges from 

 the coleorhiza. A similar arrangement has been observed by the 

 author in Briza minor, and is probably somewhat prevalent among 

 Grasses in general. In all such cases, of course, the hairs serve for 

 absorption as well as for fixation. 



As the preceding examples show, root-hairs may occur on organs 

 other than those from which they derive their name. This aberrant 

 location of the absorbing tissue is still more noticeable, where the place 

 of the root-system is taken by modified axial or foliar organs. In the 

 case of Psilotum triquetrum, for instance, where genuine roots are 

 altogether absent, the rhizome is furnished with root-hairs. Mention must 

 also be made of the Hymenophyllaceae, in which root-hairs appear on 

 modified axial organs and sometimes even on the leaves. Goebel 

 records the occurrence of tufts of root-hairs on the bases of the outer 

 members of the leaf-rosette in certain Orchids {Microstylis, Malaxis, 

 Sturmia). The submerged leaves of Salvinia natans, finally, which in 



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