108 



MINUTE STRUCTURE OF THE HOOT. 



87 



Roots which grow in the earth seldom have it much developed ; 

 but in man}- aquatics it becomes of large size, though it is always 

 thin. In some species of Pontederia the cap envelops the root 

 for the length of half a centimeter ; but it is free 

 at its upper part, and is in contact with the root 

 only at its very tip. The roots of Typhaceae 

 and Lemnaceai exhibit nearly the same struc- 

 ture. The cap consists in these cases of only 

 one or two layers of thin-walled cells. 



The aerial roots of some plants have large 

 root-caps composed of firm-walled cells. This 

 is well shown in Pandanus, where the cap con- 

 sists of man}' layers of cutinized cells. The 

 cap in all cases exfoliates on its exterior, and 

 is as constantly renewed by the cells within. 

 Nearly all of its cells contain starch-granules 

 in abundance. 



324. The peripheral tissue in the rootlet does 

 not always have the same origin ; it may in some 

 cases be regarded as true epidermis, in others as 

 the outermost portion of the cortical parenchyma. In the vast 

 majority of cases this young superficial tissue is furnished with 

 root-hairs / it is therefore designated the piliferous layer. 1 



325. The piliferous layer has no intercellular spaces (a few 

 cases of aerial roots of Orchids excepted). The hairs are con- 

 fined to a narrow zone a short distance behind the tip, although 

 in Triglochin they have been found on the edges of the cap, 

 and in Philodendron very near its edge. When first formed 

 they have delicate transparent walls, and are filled with pro- 

 toplasm. By the advance of the growing-point and with the 

 formation of new hairs, the older become less active, their walls 

 thicken and turn brown, their contents disappear, and they fall 

 off, 2;enerallv leaving a nearlv glabrous surface. 



i O -' d? v O 



326. The hairs are generally simple, but in the adventitious 

 roots of some Bromeliaceae 2 compound hairs are also found. 



Branched hairs are seen on the roots of Saxifraga sarmentosa, 

 Brassica Napus, etc. 



1 Olivier (Ann. ties Sc. nat., ser. 6, toniexi.. 1881, p. 19), according to whom 

 it is never homologous with the epidermis of the stem (p. 28). 



2 Jorgensen, Botanisk Tidsskrift, 1878, p. 144. 



FIG. 87. Seedling of Sinapis alba, showing root-hairs. 



FIG. 88. Seedling of same, showing the manner in which n'ue particles of earth cling 

 to the root-hairs. (Sachs.) 



