ROOT-HAIRS. 



^9 



a cylinder of 6-10 mm. thick around the root (about | mm. thick), are called into 

 requisition in nutrition. We can demonstrate this at once in a neat and instructive 

 manner, if fresh, vigorously growing roots are carefully lifted out of the loose earth : 

 the particles. of earth cling so fast to the tract which is beset with root-hairs, that 

 they cannot be shaken off, whereas the growing end of the root is quite smooth and 

 dean, and the particles of earth also fall off from that part of the root which lies 

 behind the hairs, because the hairs have there died off and disappeared. A clear 

 representation of the activity of the roots in the earth is only to be obtained when 

 we picture to ourselves the richly branched root-system of a large, vigorous land- 

 plant, and reflect how the thin root-fibres travel through the soil in all directions, 

 and that each thin fibre exhausts a cylinder of earth several millimetres in 

 diameter, and how the absorbing 

 part of each root- fibre penetrates 

 continually from day to day into 

 new, fresh portions of soil, while 

 the hairs disappear from the older 

 parts of the fibres, because there 

 is nothing more there for them to 

 seek. It may be added, that only 

 by the intimate groM'ing together 

 of the root-hairs with the earth 

 particles is it possible for transpir- 

 ing land-plants still to take up 

 large quantities of water from a 

 soil apparently almost dry, in 

 order to transmit it to the green 

 leaves. 



To the most remarkable pro- 

 perties of roots belongs the short- 

 ening of the tract of a root-fibre, 

 already grown to its full length ^ 

 It begins immediately behind the 

 place where the elongation has 

 ceased, and may then continue 

 for a long time in the older 



parts. This shortening, which may amount to 10, or even 25 per cent, of the 

 original length, is brought about by the parenchymatous cortex, and the axial 

 fibro-vascular cord becomes passively drawn together, so that its vessels assume 

 a serpentine course, while the outer layers of the cortex, also passive, develope 

 transverse folds, which can be easily seen with the unaided e}e, especially in 



KIG. 12— Root-hairs of a Se/nf^ineUa—some \ 

 to tliem. The outlines show tlie impressions of the particles of 

 the tubes had closely applied themselves (strongly magnified). 



cles of soil clii 



' The shortening of roots at the termination of tlie growth in length was discovered by Fitt- 

 manti (Flora, 1819, Bd. II. p. 651), again found later by myself (Arbeiten des botan. Inst. Würz- 

 burg, I. p. 419), then brought forward anew in its biological bearing by Irmisch (Beitr. zur vergl. 

 Morph.5. Abtheilung, Aroide», Halle XI IT. 2. 1S74. p. 11). Hugo de Vries has furnished a detailed 

 investigation of the mechanics of this shortening, from the point of view of more recent knowledge 

 (Bot. Zeitung, 1879, p. 650). 



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