3(S() WILLIS : MORPHOLOGY OF THE PODOSTBMACEÆ 
obliterated by a piece of thallus growing over them. They 
may still be commonly seen at the end of October, and 
traces may be found even in January, so that we may say 
that they survive through the whole life of the plant. 
The internal anatomy of the stem can only be briefly 
touched upon ; it has already been described by Warming 
(Dicræa apicata, 1. c,). Towards the middle of the cross 
section, but a little excentrically, is a vascular bundle. 
This is not very clearly marked ofl: from the surrounding 
tissues, and shows a somewhat simple structure, like that 
of the primary axis of Willisia, consisting when mature of 
a number of irregularly arranged groups of phloem tissue 
with thicker walled cells between the groups. Details must 
be left for subsequent description. 
To return to the thallus. It grows outwards in close 
contact with the rock, to which it becomes attached by the 
usual rhizoids, or in the Nilgiri form and occasionally in 
the others by haptera also. Only the extreme marginal 
portion, for about 2 mm. or perhaps at times 5 mm. from the 
edge, is actually growing and expanding, so that the attach- 
ment can soon be made without risk of severance by further 
growth. In a very short time the thallus shows a scallop- 
shell form (PI. XXXII , fig. 14). When cut in longitudinal 
section (PI. XXXIV., fig. 2), the edge of the thallus shows a 
meristem like that of Dicræa, with a collenchy matous root- 
cap at the outer part, which is derived from a more or less 
regular transverse meristem that on the inner side gives rise 
to the thallus tissues. The root-cap is usually slightly 
raised above the rock, and hardly seems to perform any 
protective function in preserving the meristem from contact 
with the substratum, though it must be of a certain 
protective value when the thallus collides with another 
thallus or with a projecting portion of rock or other obstacle. 
It is possible also that it may have a stiffening function like 
the rim of bundles along the margin of a dicotyledonous 
leaf. The presence of this tissue is almost the only constant 
“ root ” character, whether physiological or morphological. 
