110 MINUTE STRUCTURE OF THE KOOT. 



332. Sometimes there are papillar outgrowths from these tra- 

 chei'ds, which are to be regarded as root-hairs. They occur 

 chiefly on younger parts of the roots which are in contact with 

 a moist support, or which are kept wet. They cling tenaciously 

 to moist surfaces, and may become much widened and flattened. 1 



333. The cortex of different plants varies greatly in thickness 

 and compactness, and in the thickness of the cell-walls. In 



90 



a few cases remarkable lacunre are to be seen (e. g., Meny- 

 anthes). 



334. The cells bounding the inner layer of the cortex have the 

 general characters described under " Endodermis ;" their radial 

 walls are generally more or less plicate, and there are no inter- 

 cellular spaces. 



335. In the primary cortex of roots all the various kinds 

 of secreting cells and receptacles for exudations described on 

 p. 97 may be looked for ; but as a rule they are less developed 

 than in the stem. Collenchyma occurs sometimes in roots ; 

 c. <?., Raphidophora. 



336. The central cylinder has, at first, a peripheral layer of 



1 Leitgeb : Die Luftwurzeln der Orchideen, Wien Akud. Denkschr., xxiv., 

 1865, p. 179. 



FIG 90. Transverse section of the central cylinder of a root of a vascular cryptogam 

 (Marattta Isevis): , Internal layer of the proper cortex; p, endodermis ; m, peripheral 

 layer of the cylinder; I, liber fascicles; r, woody fascicles ; c, conjunctive parenchyma 

 (pith and medullary rays). (Van Tieghem.) 



