STRUCTURE OF VELAMEN 



231 



The stems of certain Mosses (Paludella squarrosa, spp. of Dicranitm, 

 Mecsca, and Mnium) are covered for the greater part of their length 

 with a dense felted mass of rhizoids. Oltmanns has shown that this 

 rhizoidal jacket represents a device for the capillary retention and 

 conduction of water. 



C. THE ABSORBING TISSUES OF AERIAL ROOTS. 122 



The aerial roots of tropical Orchids and of many epiphytic Aroids 

 are distinguished by the presence of a peculiar covering, which has 

 long been known by the names of 

 " root-sheath " ( Wurzel-hiille), velamen 

 radicum, or shortly velamen ; the 

 anatomy and development of this 

 tissue have been most closely and 

 thoroughly studied by Leitgeb, upon 

 whose observations the following 

 account is largely based. The velamen 

 usually consists of a silvery parch- 

 ment-like sheath of variable thickness. 

 A study of its development shows it 

 to be a product of the protoderm, 

 which close behind the root-tip be- 

 comes many-layered by tangential 

 division. In a few exceptional cases 

 {Vanilla planifolia, V. aphylla, Bai- 

 drocolla teres, etc.) the protoderm divides 

 by anticlinal walls only, and the 

 velamen consequently remains single- 

 layered. The number of layers com- 

 prised in the velamen ranges from a 

 single one to as many as eighteen 

 {Cyrtopodium spp.), but remains at a 

 fairly constant figure in each species. 



The cells of the velamen are 



in Uninterrupted Contact With One Sector of a T.S. through the aerial root 



. . . . of Stanhopea oculata. ir, velamen; e, exo- 



ailOthei'. Iheir Shape Varies tO a dermis; r, cortical parenchyma; e", endu- 



considerable extent. Sometimes they 



are approximately isodiametric but somewhat elongated radially, as 

 seen in transverse section ; in other cases their long axes are parallel 

 to that of the root itself. The walls are strengthened in a great 

 variety of ways, but most often by means of spiral thickening fibres 

 (Figs. 88 and 89); these fibres may run parallel to one another, or 

 they may enclose meshes of varying width, or, finally, they may be 



Fig. 88. 



