Notes . 
151 
Conclusions drawn from Observations on Bromheadia. — Ridley is 
of opinion that B. palustris has descended from epiphytic ancestors. 
The structure of its root harmonises with this view ; its velamen is 
fundamentally similar to that of the epiphytic Bromheadia , but it lasts 
for a short time only. It is easy to perceive the use of the thick- walled 
outer layer in the case of the epiphytic form which grows in the full 
blaze of the sun. But with regard to the terrestrial B. palustris , the 
most rational explanation of the precocious development and speedy 
dissolution of this thick-walled layer with its blunt dwarf-papillae, is 
to suppose that the curious velamen is a relic handed down from an 
epiphytic ancestor. But, however this may be, the persistence of the 
velamen in the epiphyte and its early disintegration in the ground- 
orchid, together form a strong piece of corroborative evidence that 
the velamen is essentially adapted to epiphytic plants, and was evolved 
in each case subsequently to the assumption of an aerial mode of life 
by the plant. The occurrence of the velamen in two such widely 
separated families as the Orchidaceae and the Aroideae, further 
strengthens this view. And, finally, the well-known fact that an 
increase in the intensity of the sunlight to which any leaf or green 
branch is habitually exposed induces an elongation of the epidermal 
cells in a direction at right angles to the free surface, and even to 
their division into outer and inner halves, shows that in the formation 
of the velamen there is nothing exceptional or unexpected. 
General Conclusions. — The foregoing observations on Grammato- 
phyllum and on Bromheadia seem to lead to contradictory conclusions. 
For whilst, in the former, the velamen is more highly developed in the 
subterranean root, in the latter it is more highly developed in the 
aerial root. The solution of this apparent paradox is afforded by the 
consideration that the function of the velamen is not the same in all 
cases. Thus, in Grammatophyllum the velamen is essentially an absor- 
bent organ, which not only persists, but assumes a higher development, 
when the root is subterranean : whereas in Bromheadia the velamen 
is mainly protective, preventing loss of water by transpiration, the 
absorptive function being carried on by the root-hairs on the ventral 
surface of the root. Hence, in the aerial branches of the subterranean 
roots of Grammatophyllum the velamen dwindles, whilst in the sub- 
terranean roots of Bromheadia it peels off so as not to interfere with 
the process of absorption. 
