OR CHID A CEAE 46 7 



Orbignya Mart, ex Endl. Palmae (iv. 2). 7 Brazil, Bolivia. 



Orbinda Nor. Inc. sed. Nomen. 



Orchadocarpa Ridl. Gesneriaceae (i). i Malay Peninsula. 



Orchard grass (Am.), Dactylis glome rat a L. 



OrcMaceras x E. G. Camus. Orchidaceae. Hybrid, Orchis-Aceras. 



Orchicoeloglossum x Aschers. et Graebn. Orchid. Hybrid, Orchis- 

 Coeloglossum. 



Orchidaceae (EP., BH.}. Monocots. (Microspermae). 450 gen., 7500 

 sp., cosmop., abundant in trop., rare in arctic regions. They agree 

 in some general features of habit, &c., e.g. 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 trop. they form an important feature of the veg., living chiefly as 

 epiphytes. Most temp, zone forms are terrestrial. 



The plant as a whole may be built up in one of three ways, 

 (i) a monopodium, the main axis growing steadily on, year after year, 

 and bearing the fls. on lat. branches ; (2) an acranthoits sympodium, 

 the main axis being composed of annual portions of successive axes, 

 each of which begins with scale 1. and ends in an infl. ; (3) a. plenran- 

 thons sympodium, where the infls. are borne on lateral axes, the shoot 

 which for the current year continues the main axis stopping short at 

 the end of its growing period, and not ending in an infl. These types 

 of construction are used in classification (see below). 



The saprophytes are few; they have no green 1. ; below the soil, 

 in the humus, is a fleshy rhiz., with (Neottia) or without roots. It is 

 much branched, and does part or all of the work of absorption. My- 

 corhiza occurs in most or all. The terrestrial forms are all sympodial, 

 and have usu. a rhiz. ; each annual shoot bends up into the leafy 

 shoot of the current year. Many being xero., and all perenn., it 

 becomes a necessity that there should be a storage reservoir to last 

 over the non-veg. period of the year. In a great many this takes the 

 form of a thickened internode of the stem : in many 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 adv. root, 

 forming a large tuber, which lasts over the winter. 



Coming lastly to the epiphytes, abundant in the trop., we find 

 great variety. [See Schimper, Die epiphytische Vegetation Amerikas.} 

 They are mostly sympodial, but the few monopodial O. also belong 

 to this group. The exceedingly light seeds and the xero. habit of 

 many O. fit them to become epiph. The roots of the epiph. forms 

 are of interest. In the first place, to fasten the pi. to its support there 

 are 'clinging' roots, insensitive to gravity, but negatively heliotropic. 

 The niche between the pi. 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 usu., Schimper asserts, 

 negatively geotropic. Finally the true aerial roots hang down in long 

 festoons. The outer layers of cells (the epidermis and velameit] are 

 dead and perforated, and act as a sponge to absorb water trickling 

 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 



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