LIGNIFIED TISSUES 247 



which gives rise to the epidermal layer, periblem, whicli produces 

 the cortical tissue, and plerome, from which is formed the vascular 

 system. In the leaf, the vascular system and mesophyll are 

 formed from periblem. The root, in addition to dermatogen, 

 periblem and plerome, possesses another layer, the calyptrogen, 

 which forms the root cap. 



In certain cases it is a well-known fact that the bast or phloem 

 also becomes lignified, and in many plants the so-called " hard " and 

 "soft " bast can be readily distinguished by means of aniline 

 chloride. The formation of the bast fibres in Bcehmeria nivea has 

 been investigated by Aldaba. The bast fibres in this plant are par- 

 ticularly long, and estimations of their length have been variously 

 given as being from 150 to 580 mm. But even the higher 

 value given here does not express their true length, according to 

 Aldaba, who, using a special technique of maceration with 5 

 per cent, potash, found that the fibres were even longer than 

 580 mm. 



The fibres originate in a layer of undifferentiated parenchy- 

 matous elements a short distance behind the growing point. These 

 cells are approximately 20 jj, in length. Growth and differentiation 

 now takes j^lace simultaneously. The young, actively-growing 

 undifferentiated fibre portions are invested solely in a hyaline, 

 tenuous membrane, while their low^er portions are provided with 

 conspicuous thickened walls. The delicate membrane w^hich covers 

 the upper undifferentiated region of the fibre is a direct continua- 

 tion of the outermost lamella of the thick wall wiiich invests 

 the older part of the fibre (Fig. 83, 1). Later, tw^o membranes 

 are found to be present, an inner layer, which jackets the protoplast 

 and terminates at the lower level of the cell, and a second mem- 

 brane, which covers the elongating portion of the protoplast. 

 With elongation of the fibre, both these membranes become 

 extended apically. Fig. 83, 5 show^s the terminal portion of the 

 fibre, which has nearly attained its maximum length. In this older 

 and more highly differentiated cell, there are a series of inner 

 tenuous, hyaline membranes, which are growing upwards towards 

 the growing tip of the fibre. Each of them is in direct continuity 

 with one of the lamellae of the cell wall, wiiich mav be traced down- 



