FERNS. 



363 



the central cells of the layer grow more quickly in an axial direction, and may 

 become divided by transverse septa, by which the layer is made to consist of two 

 or more strata in the middle. The formation of a layer is generally followed by 

 that of three root-segments before a further new layer is formed ; these segments, 

 corresponding to the faces of the three-sided apical cell, lie in three straight longi- 

 tudinal rows. Each of these triangular tabular segments includes a third of the 

 circumference of the root, and is first divided by a radial longitudinal wall into two 

 unequal portions. The transverse section of the root now shows six cells, three 

 of which meet in the centre, while the other three do not reach quite so far. 

 Each of these six cells is then divided by a tangential wall (parallel to the surface) 

 into an inner and an outer cell ; the inner ones form the fibro-vascular bundle, while 

 the six outer cells form the rudiment of the cortex. If the root becomes thick, 

 the six cortical cells divide by radial walls ; if it remains slender, this division 

 does not take place. The six or twelve cortical cells are now divided by a tan- 

 gential longitudinal wall, and the fibro-vascular bundle is enclosed by two layers of 

 cells, the outer of which forms the epidermis, the inner the fundamental tissue of 

 the cortex. The epidermis usually continues to consist of one layer only, dividing 

 only by walls vertical to the surface ; but in some Ferns {e. g. Polypodium, Blech- 

 num, and Cystopteris) the layer of epidermal cells is doubled. The layer of cells 

 between the epidermis and the central bundle becomes double, an outer and inner 

 cortex resulting from further divisions. In most Ferns, however, the distinction 

 between the two layers cannot be made out in the fully grown root ; thotigh in 

 some the inner cortex consists of thick-walled long cells, the outer cortex of thin- 

 walled short ones. 



The Fibro-vascular Bundles consist at first, as has been mentioned, of six cells 

 in transverse section ; these are each divided simultaneously by a tangential wall into 

 an outer tabular and an inner cell. From the further divisions of the outer cell 

 proceeds a tissue which Nageli and Leitgeb call Pericambium, and the cells of which 

 are characterised in the fully developed root by their thin walls and by their granular 

 and mucilaginous contents. They are broad, but short. From the six inner cells 

 proceeds the prolongation of the true fibro-vascular bundle ; they divide in all 

 directions, the divisions advancing in centrifugal succession ; the peripheral cells 

 are considerably smaller, after the completion of the division, than the inner ones. 

 The formation of vessels begins with their production at two or three points of 

 the circumference lying diametrically opposite one another on the inner side of the 

 pericambium ; it proceeds either at first right and left (tangentially), or centri- 

 petally, a diametral row of vessels being thus formed. In slender roots this may 

 proceed no further than the production of the first vessel ; in thicker roots one or 

 more broad vessels lie in the centre, which only become woody at a later period. 

 The peripheral cells and the narrow ones that lie between the vascular bundles 

 form the phloem-layer of the bundle by the thickening of their walls. The roots 

 of Ferns branch in a monopodial manner only ; the lateral rootlets arise in acropetal 

 succession on the outside of the primary vascular bundle, and are therefore usually 

 arranged in two rows, rarely in three or four. The mother-cells of the lateral roodets 

 belong to the innermost layer of the cortex, and are separated from the vascular 

 bundle of the primary root by the pericambium ; the roodets originate very near 



