14 
Anatomical considerations.—The parsnip root consists of a 
central core of xylem surrounded by a mass of soft spongy paren- 
chyma. The whole is bounded by a thin periderm., The rela- 
tive amount of core and soft outer flesh differs somewhat in dif- 
ferent varieties, but as a rule the fleshy tissue is largely in excess 
of the core, and it is the aim in selection to keep the central 
core as small as possible. The core is composed almost exclu- 
sively of secondary xylem, but it contains a very large amount 
of xylem parenchyma, hence it is comparatively soft. The outer 
fleshy portion consists essentially of secondary cortex; it is com- 
posed entirely of parenchymatous and other soft elements which 
are produced abundantly from the cambium. The cells are 
ensely packed with starch. Through the cortex runs a system 
of mucilage canals, which are especially found immediately 
beneath the periderm. The secretion may be seen exuding from 
roots on freshly cut surfaces as a white milky fluid, which turns 
reddish-brown on exposure to the air: this change of colour, as 
is shown by the guaiacum test, is due to the oxidation of some 
substances contained in the fluid. . 
The periderm surrounding the root is formed from a phellogen 
which arises in the inner tissues of the root, probably in the peri- 
cycle, at a very early stage. In the young growing apex of the 
root the phellogeh and three or four layers of periderm are 
clearly visible in transverse section. In older roots the periderm 
appears, owing to tangential stretching, to be less regular. In 
median longitudinal section, however, the regular arrangement 
of the cells, obviously of secondary origin, is quite clear. This 
thin layer of periderm is interrupted only in the furrows mark- 
ing the emergence of the lateral roots, where a thick pad of 
corky tissue is present. 
It is obvious to the naked eye that the cortical tissue exposed 
to the air as a result of cracking does not heal over. Anatomical 
examination shows, however, that an attempt is made on the 
part of the plant to protect itself, inasmuch as the outer and 
side walls of the exposed cells become suberised. A few cell- 
divisions also take place, but there is no trace of a definite 
phellogen or of a layer of wound-cork. This protection is quite 
inadequate to keep out micro-organisms, the inter-cellular spaces 
and cracks or fissures due to drying providing ample means of 
entry. — The soft parenchymatous tissues are, moreover, specially 
attractive to slugs, an immense amount of slug-injury being 
noticeable if large gaping slits are made with a knife. It is 
therefore not surprising that decay sets in and canker follows, and 
the cause may be said to be entirely due to the inability of the 
parsnip to form a protecting layer of wound-cork. The deep 
open clefts which occur in the roots of swedes, carrots, mangolds, 
and turnips become protected on their surfaces by a thin cork 
layer. No cork layer is formed when such deep cuts are made 
artificially in the parsnip. It would appear, therefore, that the 
hin ance to cork-formation is not due to the different method of 
vie ein but to some physiological peculiarity of the parsnip 
4 
