490 
OPUNTIA 
used for hedge-making, others as food for cochineal-insects (see 
Nopalea). [For O. coccinellifera Steud. see Nopal ea.] 
Opuntiales. The 20th cohort of Archichlamydeae (p. 130). 
Orchidaceae. Monocotyledons (Microspermae). Over 400 gen. with 
5000 sp. cosmop., abundant in trop., rare in arctic regions. They 
agree in some general features of habit, &c., eg. they are all perennial 
herbs, but differ widely in detail, owing to the diversity of conditions 
in which they exist — land- plants, epiphytes, saprophytes, &c. Within 
the tropics they form an important feature of the vegetation, living 
chiefly as epiphytes (p. 173). Most of the temperate forms are 
terrestrial. 
The plant as a whole may be built up in one of three ways. 
It may be (1) a monopodium, the main axis growing steadily 
on, year after year, and bearing the firs, on lateral branches ; (2) an 
acranthous sympodium, the main axis being composed of annual por- 
tions of successive axes, each of which begins with scale leaves and 
ends in an infl . ; (3) a pleuranthous sympodium, where the infls. are 
borne on lateral axes, the shoot which for the current year continues 
the main axis, simply stopping short at the end of its growing period, 
and not ending in an infl. [see p. 43]. These three types of construc- 
tion are used in the classification of the order (see below). 
The saprophytic orchids are but few ; they have no green leaves ; 
below the soil, in the humus, is a fleshy rhizome, with (Neottia) or 
without roots. The rhizome is much branched, and does part or all of 
the work of absorption. Mycorhiza (p. 39) occurs in most or all. 
The terrestrial forms are all sympodial, and have usually a rhizome ; 
each annual shoot bends up into the leafy shoot of the current year. 
Many being xerophytic, and all perennial, it becomes a necessity that 
there should be a storage reservoir to last over the non-vegetative 
period of the year. In a great many sp. this reservoir takes the form of 
a thickened internode of the stem : in many sp. again, among which 
the Brit, orchids are included, the bud for the next year’s growth, i.e. 
the next part of the sympodium, is laid down at the base of the stem, 
and from it is developed a thick and fleshy adventitious root, forming 
a large tuber, which lasts over the winter. 
Coming lastly to the epiphytic orchids, which occur in great num- 
bers in the tropics, we find a great variety of forms. [See Schimper, 
Die epiphytische Vegetation Amerikas.] They are mostly of sympodial 
structure, but the few monopodial orchids also belong to this group. 
The exceedingly light seeds and the xerophytic habit of many orchids 
fit them to become epiphytes. The roots of the epiphytic forms are 
of some interest. In the first place, to fasten the plant to its support 
there are ‘clinging’ roots, insensitive to gravity, but negatively helio- 
tropic. The niche between the plant and its support and the network 
formed by the roots act as reservoirs for humus, and into this project 
‘ absorbing ’ roots, branches from the others ; these are usually, 
