335 
and its Mycorhiza. 
the sheath and cortex. It commences with a single tolerably 
regular and typical perky cle , the cells of which are elongated 
longitudinally and are narrow. 
Phloem — There are about 6-8 minute bundles of phloem 
lying against the pericycle. Each group consists of 1-3 
extremely delicate sieve- tubes with terminal plates, and 1-3 
slightly wider protoplasmic cells representing companion cells 
(in transverse section). 
The different bundles of phloem are separated by parenchy- 
matous conjunctive tissue, which also bounds them on their 
inner sides. 
The xylem is reduced to a single axial strand of very narrow 
annular and spiral vessels, and slightly broader, more central, 
reticulate-scalariform tracheae. In transverse section the 
strand may exhibit as few as three vessels, but never more 
than eight. 
The structure of the growing-point of this organ at once 
reveals the fact that the vegetative axis of Thismia is abso- 
lutely unique. 
The sheath is continuous over the apex, and the actual tip 
of the organ ends in a collection of sheath-cells protruding as 
conspicuous hairs 1 . The meristem lies internally (Fig. 4), and 
though it is impossible to certainly distinguish the initial cells 
sufficiently clearly to conclude that there are three distinct 
histogens, still one can recognize that in the apical meristem 
three distinct tissues are differentiated very early — the sheath , 
the tissue extending from and including the exocortex to the 
endodermis, and the central cylinder. In fact the apex 
suggests that of an ordinary succulent monocotyledonous 
root, in which the absolute distinction into calyptrogen, peri- 
blem, and plerome is not obvious at the extreme merismatic 
limit. Thus the vegetative axis is possibly a root in which the 
root-cap is preserved throughout life as a living and absorbing 
layer which further acts as an aqueous tissue and entertains 
the symbiotic fungus. Some of its cells do, however, peel 
off and die, but the majority persist. If this interpretation 
1 Compare a haustorium. 
