494 MISS ISABEL MCCLATCHIE ON 
constituting a second type, the tap-root tapers gently for some distance 
below the soil-level, but ultimately narrows suddenly, at the same time 
giving off a whorl of four lateral roots similar to those referred to above 
(РІ. 37. figs. 7 & 8). In a third, and relatively rare type, the tap-root 
appears to taper gradually to its apex (Pl. 37. fig. 6). In all cases, however, 
the root-system remains shallow, and is disproportionately small in com- 
parison with the aerial development of the plant. 
OCCURRENCE OP ADVENTITIOUS Roots. 
A. In the young plant. 
When the seedling is about twenty centimetres high and the stem about 
one centimetre in diameter at its base, the primary root-system' is reinforced 
by secondary roots of an adventitious nature, 
These secondary roots first arise near the soil-level and develop from the 
four xylem poles. Those subsequently formed appear among the primary 
Í 
T 
Fra, 1.—Root-system of a young plant showing primary roots and the first adventitious 
roots. fe. fistular cavity; s, subterranean adventitious root; а, aerial adven- 
titious root, 
lateral roots, still keeping to the xylem poles and showing a regular acropetal 
development. About this stage the seedling develops a fistular cavity, the 
hypocotyl thickens and appears slightly shorter, and simultaneously the 
first aerial * adventitious roots appear. А few such roots may develop before 
secondary thickening takes place in the hypocotyl; these necessarily follow 
the four lines of subterranean roots (text-fig. 1). After this, however, the 
roots show no regularity in appearance or arrangement ; they may arise at 
* The term “aerial root” is used in this paper to define roots produced above the 
soil-level. 
