510 MISS ISABEL MCCLATCHIE ON 
(c)—Roots were slit through longitudinally for a distance of one 
centimetre. 
Result. 
Where the slit was at right angles to the soil the rootlets developed in two 
rows, one down either side and parallel with the soil (text-fig. 17 ¢). Where 
the slit was parallel with the soil, rootlets appeared on the underside of the 
root only (text-fig. 17 d). 
STRUCTURE. 
r 
Tap-root. 
"m ig IDE "M r 
The tap-root is tetrarch, with xylem to the centre, but rarely develops 
much secondary wood (РІ. 38. fig. 10). 
Lateral roots. 
1 
The lateral roots are generally tetrarch or pentarch, but may be occasion- 
ally triarch; they have usually а solid xylem core and develop some 
secondary wood. In both main and lateral primary roots the vessels become 
closed by the formation of tyloses, these in the former sometimes beino 
. . . - . UP 
present in quite young seedlings. Both show a considerable amount of cork 
development. This is produced by the division of the whole of the cortical 
cells external to the endodermis into series of cambiform elements which 
. . А . E 
then become suberised. A true pericyclie phellogen Is never formed 
(PI. 38. fig. 11). 
Adventitious roots. 
The adventitious roots are polyarch with eight to fifteen xylem poles 
arranged round a wide pith. The primary xylem is feeble and secondary 
thickening takes place very early, but there is never a great quantity of 
wood present (P1. 38. fig. 12). The xylem consists largely of tracheids with 
annular, spiral, or reticulate thickenings, but the lignification is feeble. and 
consequently makes staining difficult. The endodermis is well marked 
and the cortex regular. Root-hairs are found on the younger portions of 
the subterranean roots, but they are absent from the normal aerial roots 
which develop a subepidermal protecting cork layer at an early stage ; this 
cork is particularly well developed in short stunted roots. HN 
Several plants were found growing in the long grass which bordered the 
Balsam bed. In these the aerial roots from the cotyledonary node hune 
down into a moist atmosphere, and during the rainy weather many cases were 
noted in which the red pigmented roots were covered with a silvery coating 
of root-hairs for a distance of abont two centimetres. In these plants 
also the aerial roots attained a greater length and remained more slender. | 
The root-cap is of the commonest type found in dicotyledons, i. e., it is a 
dermacaly ptrogen *. 
* Haberlandt, * Physiological Plant Anatomy, 
