80 SUMMABY OF CURKENT RESEARCHES RELATING TO 



increase of surface brought about by root-hairs as compared with that 

 of naked roots at from 5 ' 5 to 18-7. 



The root-hair has a constant tendency to grow in a downward 

 vertical direction ; when this is interfered with by any solid body, it 

 grows along this body until it can again resume its original direction. 

 A close attachment to the particles of soil is increased by the 

 mucilaginous character of the outermost cell-wall. They are formed 

 only at a certain distance from the apex of the root, in order not to 

 interfere with the hydrotropic and geotropic movement of the latter. 

 The external condition which affects more than any other the forma- 

 tion of root-hairs is the degree of moisture ; too little and too much 

 moisture are equally unfavourable, A retardation of growth from 

 too much moisture goes along with a reduction of the amount of root- 

 hairs ; a retardation of growth from too little moisture causes a local 

 increase of root-hairs, though the total quantity may be diminished. 



The suppression of root-hairs in many water-plants is not due to 

 the smaller supply of oxygen, but to other causes not altogether 

 known at present; some water-plants form root-hairs abundantly 

 when their roots penetrate into mud or soil. 



Entire suppression of the root-hairs occurs in only comparatively 

 few plants, not connected genetically, but related only in their mode 

 of life. They are nearly or altogether purposeless in such as have a 

 very abundant supply of water, as many bog- and water-plants, like 

 Butomus, Caltha, Euryale, Lemna, Nymphcea, &c., and in those which, 

 owing to their very small power of transpiration, require but very 

 little water, as many Coniferse, Agave, Phoenix, &c., from which they 

 are altogether absent. They occur, however, in some succulent plants, 

 as Crassulacese and Cactacese ; in the bulbous and tuberous Liliacese 

 they are present or absent according to their habit. Parasites usually 

 have root-hairs when they also have the power of growing inde- 

 pendently, as Euphrasia and Melampyrum ; MhinantJius, parasitic on 

 the roots of grass, is destitute of them ; many saprophytes, as Mono- 

 tropa, Neottia, and Orobanche, are entirely wanting in root-hairs. 



A great increase in the quantity of root-hairs may take place for 

 a specific purpose, and organs of different value morphologically may 

 become covered with them. They occur, for example, on the coleo- 

 rhiza of Myrtacese, Scahiosa atropurpurea, and some grasses, for the 

 purpose of fixing the seedling firmly in the soil. In Psilotum trique- 

 trum, Corallorhiza innata, and Epipogon Gmelini, they are produced 

 on the cauline organs, which perform the function of roots. 



The root-hair is almost always simply an outgrowth of an epi- 

 dermal cell. In exceptional cases the mother-cell subsequently 

 forms a sheath round it, as in the prothallium of AlsopMla australis 

 and Aspidium molle. They are developed acropetally without any 

 definite arrangement ; rarely, as in Nuphar, Elodea, &c., they arise 

 out of cells already formed. Their form does not vary greatly, 

 though they are to a certain extent affected by external circumstances, 

 contact, food-supply, &c. They may occasionally branch, and even 

 twine. The longest root-hairs observed by the author were those of 

 the Marchantiacete, 18 mm., Trianea, 8 mm., Potamogeton, 5 mm., and 

 Elodea, 4 mm. 



