VEGETATIVE STRUCTURE. 55 



pseudo-biilbs are placed at a considerable distance from each other, 

 whence these plants have a straggling habit ; -when thus prolonged, it is 

 usually clothed with brown scarious scales ; in the caulescent ]\raxillarias 

 (M. tenuifolia, etc.) and in species included in many other genera 

 [Odontoglossum cordatum, Oncidium flexiiosum, etc.) it is scandent ; and 

 many intermediate forms exist. In Bidhoplnjllum Beccari the rhizome 

 is enormously developed, encircling the trees to which it is attached 

 like the coils of a serpent : iu Oncidium zebrinum it is scandent and 

 often excessively elongated, the intervals between the pseudo-bulbs being 

 sometimes as much as from G tu 8 inches; : Itut these are extreme 

 cases among the cultivated ORCHiDEiE. 



Roots. — The normal roots of orchids are cylindric, often thread-like, 

 l)ranching, and of great length ; in most Cypripedes they are furnislied 

 with numerous root hairs. The aerial roots of epiphytal orchids consist 

 of a central axis, surrounded ^vith a covering of loose light tissue, 

 technically called the velamen, the cells of which when dry are filled with 

 air only ; this spongy covering, with its tliin epidermis, absorbs moisture 

 with great rapidity ; both the primary roots and their branches terminate 

 in a greenish or greenish brown sheath, the calyi^tra, which protects an 

 organ of \Qxy great importance and delicacy, the " punctum vegetationis " 

 or growing point, and the formative tissue which surrounds it. Generally, 

 the roots of epiphytal orchids are pendent, but in some species of Cym- 

 bidium iinder cultivation, the secondary roots grow upwards ; and in 

 Grammatophyllum the secondary roots with their rootlets form a dense 

 plexus of consideraljle thickness, which dies off at the end of the 

 growing season. The white thread-like much-branched roots of several 

 of the Rodriguezias, of Oncidium spliacelatum and others grow into a 

 dense tangle whicli forms a distinct feature of these plants. 



It may liere be noticed that the roots of most species of the 

 monopodial genus Phalienopsis become much flattened as they creep 

 over .surfaces in their immediate neighbourhood, and to which they clinf 

 with great tenacity. And in those Aerides, Vandas, etc., whose leaves 

 are of the " centric " type (A. ci/lindricmn, Vanda teres, etc.) the roots 

 assist the plants to cHmb to the tops of high trees in quest of more 

 air and light than they can ol)tain in the dense jungle of an Indian 

 forest.* 



Leaves. — Among the sympodial orchids occurs well-nigh every form 

 of leaf known in the ORCiiiDEiE. As tlie leaves of nil the genera 

 and their included species treated on in this work are described in 

 their respective places, a few of the most obvious characteristics need 



* Two of our native oix-liids, Epipogon aphyUus and Corullorliiza innata, botli unfortuiialely 

 fast becoming extinct in this country, have neither roots nor leaves, but are .saprophytes, living 

 on the organic matter in tlie soil under trees. Another British species, the "Bird's Nest 

 Orchis," Ncoltia Xidus-avis, plentiful in some localities, is also a saprophyte ; it is a iilant 

 with brown stems clothed with scales. 



