230 PRIMARV ARRANGEMENT OF TISSUES. 



growing root is in contact with a solid (damp) body — Epidendron elongatum, species 

 of Stanhopea, Oncidium sphacelatum, flexuosum, Maxillaria Harrisonise. The hairs 

 attach themselves firmly to the contiguous body, while not uncommonly they are 

 considerably spread out, their free ends even branching in a palmate manner. The 

 membrane and contents of the hairs resemble those of the rest of the root-sheath of 

 the species. The membrane separates readily into spiral bands : it is easily ruptured 

 in many species (e. g. Vanda furva, Sobralia decora), and thus are formed some of 

 the holes in the outer surface. 



The tracheides of the inmost layer are for the most part more elongated than 

 the rest ; further they are either of fundamentally similar form and structure all round, 

 or are characterised by peculiarities, where they cover the thin-walled endodermal 

 cells. As regards the peculiarities of form, they appear as one-, two-, or three- 

 layered groups of small flat elements, which are evenly fitted at the points indicated 

 into the other part of the sheath. As regards their structure, the special form of 

 thickening of their walls often diff'ers from that of the surrounding tissue, but within 

 the rules above laid down for the latter. The brown coloration of the limiting walls, 

 w^hich Leitgeb states is constant, but which I failed to see in Vanda furva, Oncidium 

 sphegiferum and Acropera Loddigesii, has been above mentioned. Peculiar thicken- 

 ings are found in special cases at the points indicated : in Trigonidium Egertoni- 

 anum a strong stratified swelling intrudes slightly into the cavity of the cell on that 

 part of the wall which abuts on the thin-walled endodermal cells : in the Sobralias 

 there is a similar, very protuberant, almost spherical, stratified swelling of a dark 

 brown colour. The cells of the inmost layer of the sheath are in the latter plants in 

 all cases of fundamentally the same form : one of them overlies one of the thin- 



"^w-alled endodermal cells, or two or three of them are in contact with the latter : since 

 each of the adjoining cells has a swelling of the wall, 1-3 of the latter overlie one 

 thin-walled cell. 



The aerial roots of many epiphytic Aroidese have a sheath of Tracheides 

 derived from the dermatogen, and resembling that of the Orchids in all fundamental 

 points. In Anthurium acaule, egregium, crassinervium, and intermedium, the 

 tracheides have spiral or reticulate fibres : the sheath is 4-5 layers thick. In other 

 species of Anthurium there are 2 or several, in Homalonema caerulescens even 6 

 layers, but the walls of the tracheides are smooth and thin. A one-layered sheath 

 consisting of thin and smooth-walled elements occurs in Anthurium violaceum, Philo- 

 dendron pedatum, and other Aroideae, and again in the aerial roots of Hartwegia 

 comosa Nees. (Chlorophytum Sternbergianum, Steud.), and Hoya carnosa. In the 

 latter cases the air-containing elements (which often grow out to hairs or papillae) may 

 just as well be called diied-up cells as tracheides, or perhaps better. Still it is more 

 proper to place them here, as incomplete forms, with the sheaths of tracheides, 



}> since in those roots on which they occur (as in all others here cited) an endodermis 

 structurally similar to that of the Orchids abuts internally on the air-containing 

 sheath. 



The above description of the air-containing sheaths of roots is based on the above- 

 cited works; also on the investigations o( Oitdemnns, Ueber den Sitz der Oberhaut bei 

 den Luftwurzcln der Orchideen, Abliandl. d. k. Acad. z. Amsterdam, Math. phys. Klasse 

 IX. 1861, and especially Leitgeb, Die Luftwurzcln der Orchideen, Denkschr. d. Wiener 



