ROOT- HAIRS. 



109 



327. Root-hairs are best obtained for study by cultivating 

 seedlings on moist glass, or with the rootlets in water. It is well 

 to compare the forms thus obtained with those found on roots of 

 the same plant grown in loam, sand, fine clay, etc. Masters has 

 shown that the develop- 

 ment of the hairs is fa- 

 vored by many conditions, 



such as porosity of the 

 soil, moisture, etc. ; and 

 this fact should be borne 

 in mind in the examina- 

 tion of the root-hairs of 

 any plant. 



328. The walls of root- 

 hairs are only slightly cti- 

 tinized, but there is a great 

 difference in this respect 

 in different plants. 



329. The cells of the 

 superficial layer of the 

 rootlet, other than those 

 with hairs, are more or 

 less cutinized, the degree 

 of infiltration depending 

 upon their age. In some 



cases (e. g., Asphodelus) the thickening is very considerable. 



330. On a few plants 1 no root-hairs have been detected, as 

 Crocus sativus, Cicuta virosa, Abies pectinata and many other 

 gymnosperms. 



331. Roots of orchids. The newer parts of the aerial roots 

 of Orchids have an epidermis consisting of nearly spherical 

 tracheids, which, except sometimes in the outermost Ia3'ers, co- 

 here without intercellular spaces. The walls of these cells are 

 colorless, though in mass they may have a silvery lustre, and 

 when immersed in water the} 7 soon become sufficiently trans- 

 parent to permit the subjacent green tissue to be seen. 2 



1 Duchartre (Elements de Botanique, 1867, p. 214 ) cites other plants. 



See also a valuable paper by Sckwarz in Untersuchimgen aus clem bot. 

 Inst., Tubingen, 1882. 



2 According to Leitgeb, the old roots of Vanda furva are green because their 

 tracheuls contain minute Algse (De Bary, Vergl. Anat., p. 238). 



FIG. 89. Root-hairs distorted by contact with the soil. Four in the right-hand upper 

 corner, Selaginella; three in lower corner, Trifoliurn ; the others, Avena. The dark 

 points indicate the attached particle of soil, a, a, a, minute prolongations of the cell- 

 wall. (Sachs-) 



89 



