FILICINEAE. — IIOMOSPOROUS FTLICTNEAE. 215 



has been already stated that if the stem of the last-named plant in its full-grown stale 

 is entirely covered with the bases of the leaf-stalks, the roots all grow from them and 

 not from the stem ; in the Tree-ferns the lower part especially of the erect stem is quite 

 covered with slender roots, and these as they grow downwards form an envelope 

 several inches thick before they enter the ground, and thus give a broad base to the 

 stem which is really much thinner in this part ; on the upper portions of the stem 

 also the roots are very numerous. They are very slender in small plants, on larger 

 stems they may be from one to three millimetres in thickness ; they are cylindrical 

 in shape, usually covered with a felt of numerous hairs, and are coloured brown or 

 black. The history of the development of the roots of Ferns has been studied by 

 Nageli and Leitgeb^ (Fig. 162). The apical cell is a three-sided pyramid with an 

 equilateral base ; the segments of the root-cap cut off by curved transverse walls are 

 first divided each into four cells, arranged crosswise in such a manner that the inter- 

 sections in the successive segments alternate at an angle of about 45°; each of the 

 four cells of a segment then divides into two outer and one inner or central cell, so 

 that the segment is now formed of four inner cells placed crosswise and eight outer 

 cells ; further divisions may then take place ; the cells in the middle of the segment 

 grow more rapidly in the direction of the axis of the root and may divide by trans- 

 verse walls, so that the segment comes to be of two or more layers in the middle. 

 The formation of a segment of the root-cap is usually followed by the formation of 

 three segments of the root before a new segment of the root-cap appears ; the root 

 segments lie in three straight longitudinal rows answering to the three-sided apical cell. 

 Each of the triangular tabular segments takes in a third of the circumference of the 

 root and is first divided by a radial longitudinal wall into two unequal halves; a 

 transverse section of the root now shows six cells (sextants), three of which meet in 

 the centre, while the other three do not quite reach the central point. Each of these six 

 cells then divides by a periclinal wall (parallel to the surface), into an inner and an 

 outer cell ; the inner belongs to the vascular bundle, which is therefore formed of six 

 cells lying about the centre, while the six outer cells are the rudiments of the cortex. 



The roots of the Ferns, like the roots of the Equisetaceae, branch monopodially ; 

 the lateral roots are formed in acropetal succession on the outside of the primordial 

 vascular bundle, usually in two rows, seldom in three or four. The mother-cells 

 of the lateral roots belong to the innermost layer of the cortex and are separated 

 by the pericambium from the vascular bundle of the primary root; the roots begin 

 to appear near the apex before the formation of the vessels. There are no adventitious 

 lateral roots, that is, roots arising behind those already formed. The mother-cell of a 

 lateral root first of all forms a three-sided pyramidal apical cell by three oblique 

 divisions, and from this cell the first segment of the root-cap proceeds. If two 

 primordial vascular bundles are formed in the lateral root, they lie right and left 

 of the primary root. The cortex of the primary root is simply broken through, 

 no root-sheath being formed. 



The hair-structiircs in the Ferns are very various in ft)rm. True root-hairs, 

 simple unsegmented tubes, are found not only on the roots, but also on underground 

 stems and on the bases of leaf-stalks, as in Pieris aquilina and the Hymenophyllaceae. 



1 Sitzungsber. d. bair. Akad. d. Wiss. 15, Dec. 1865. -[Bower, On ihe .ipex of the root in 

 Osmunda and Todea (_Q. J. M. Sc. 1SS5).] 



