ORCHIDACEAE 
49 1 
Schimper asserts, negatively geotropic. Finally there are the true 
aerial roots which hang down in long festoons. The outer layers of 
cells (the epidermis and velamen) are dead and perforated, and act as 
a sponge to absorb water trickling down over them. Their internal 
tissue is green (as may be seen on wetting a root) and assimilates. 
During the dry season a great proportion of the orchids drop their 
leaves (though they may flower), and ‘hibernate’ in the condition of 
fleshy pseudobulbs. One pseudobulb, which is a thickened stem- 
internode, is usually formed each year. In this, water and other 
reserves are stored. Those epiphytic orchids which do not form these 
tubers have fleshy leaves which serve the same end. It may be noted 
that the fleshy leaved orchids, e.g. Vanilla, have usually a very feebly 
developed velamen. Lastly in this connection may be mentioned 
some of the monopodial forms which have no green leaves at all, 
assimilating either by the surface of the stem, or by the long dangling 
aerial roots (Polyrrhiza, &c.). 
The infls. are of racemose construction, very often spikes, which 
look like racemes, the long inferior ovary resembling a stalk. The 
flr. is irregular and departs very far from the ordinary Monocotyledon 
type. There are two chief divisions of O., with different firs., the 
Monandrae and the Diandrae , with i and 2 sta. respectively ; the 
great majority are monandrous. Perianth in 2 
whorls, epigynous, petaloid. The posterior 
petal is usually larger than the rest, and is 
termed the labellum ; by the twisting (resupina- 
tion) of the ovary through 180° it comes round 
to the anterior side of the flr. and forms a land- 
ing place for insects. In many O. its structure 
is exceedingly complex in connection with the 
pollination-mechanism of the flr. The essential 
organs of the flr. are all comprised in a central 
structure by which the O. can be recognised at 
a glance, viz. the column , which consists in the 
simpler cases of the combined style and sta. (to 
use the old-fashioned expression ; in reality it 
is very probably an outgrowth of the axis, bear- 
ing the anthers and stigmas at the top). In the 
monandrous forms the column exhibits one anther and two fertile 
stigmas (often ± confluent), together with a special organ, the rostel- 
lum , which represents the third stigma. The single anther is the 
anterior one of the outer whorl (if we imagine the flr. of O. derived 
from a typical 3-merous flr.); the other two of this whorl are entirely 
absent, and also all those of the inner whorl, though in some genera, 
e.g. Orchis, the anterior two are represented by staminodes upon the 
sides of the column. The two fertile stigmas are the posterior pair, 
and the third (anterior) is represented by the rostellum (in using the 
Floral diagram of Or- 
chis, before resupination 
(after Eichler, modi- 
fied) ; lab = labellum, 
STD=staminode. 
