220 ABSORBING SYSTEM 



may even inhibit their formation altogether ; this effect must be 

 regarded as a pathological condition of arrested development, induced 

 by the unfavourable environment. Under normal conditions, most 

 plants are undoubtedly able to accommodate their output of root-hairs 

 very closely to the prevailing external conditions. 



An abundant production of root-hairs is only possible if all the 

 absorbing cells are capable of giving rise to these structures. Where 

 this is the case, the hairs may be exceedingly numerous. Schwarz 

 estimates that a piece of Maize root (grown in a moist chamber), 1 mm. 

 in length, bears on an average 1925 root-hairs, a number which in a 

 root 1*44 mm. in diameter represents a density of 425 hairs per 

 sq. mm. A similar piece, taken from a root of Pisum sativum, bore 

 (under the same conditions) 1094 hairs altogether, or 232 per sq. mm. 

 It should be noted that the absorbing cells only retain the capacity for 

 producing root-hairs for a limited time. Hence it is probable that 

 fresh hairs are never interpolated among pre-existing ones ; in other 

 words, root-hairs always arise, so far as is known, strictly in acropetal 

 succession. 



In certain cases, especially among water-plants, the power of forming 

 root-hairs is restricted to particular absorbing cells, which usually 

 differ markedly in appearance from the surrounding hairless cells. In 

 Nuphar luteum, Sagittaria sagittae folia, Elodea canadensis, etc., these 

 piliferous cells are much shorter than the rest, being cut off by trans- 

 verse walls at an early stage of development. In Hydromistria 

 stolonifera and Hydrocharis Morsus-Eanac, on the contrary, Kny found 

 that the piliferous cells are distinguishable, while still covered by the 

 root-cap, owing to their greater width and depth. In certain Eriocau- 

 laceae (Eaepalanthas spp.) and Juncaceae, according to van Tieghem, 

 the short piliferous cells generally become divided by a longitudinal 

 wall, each of the two daughter-cells thereupon growing out to form a 

 root-hair, so that these structures come to be associated in pairs. In 

 Distichia these " twin-hairs " are fused for half their length, but diverge 

 above so as to resemble a single forked hair. In Eybopodium, according 

 to Nageli and Leitgeb, a whole group of root-hairs arises from a small 

 common initial cell ; the latter divides into two to four daughter-cells, 

 each of which gives rise to a separate root-hair. 



Attention must next be directed to the morphology and physiology 

 of the individual root-hairs. Almost invariably it is only a limited and 

 rather sharply defined region of the outer wall of a piliferous cell that 

 grows out to form the root-hair. Frequently this portion is situated 

 at the acroscopic end of the cell ; there are, indeed, a number of plants 

 in which this relation is quite constant. Now, as a rule, it is only the 

 main root that grows straight downwards, while the smaller rootlets may 



