490 



BOTANY 



~? 



velamen (p. 42), which greedily absorbs water ; while, in many cases, the stems 

 are tuberously swollen and serve as water-reservoirs, accumulating water in their 

 cells and transmitting it to the leaves in dry weather. Terrestrial Orchids, on the 

 other hand, are more numerous outside of the tropical zone, particularly in the 

 drier regions of Southern Africa and the countries adjoining the Mediterranean, 

 which are especially characterised by the profusion of their tuberous and bulbous 

 plants {cf. Ziliaceae). 



Officinal. — The unripe fruit of Vanilla planifolia (Fig. 436) is the officinal 

 Fructus Vanillae. The "Vanilla is indigenous to Mexico, but is now culti- 

 vated in all tropical countries. It climbs by means of its aerial roots, like the 

 Ivy. The ovate, not the divided, tubers of several species of Orchis and allied 

 plants found in Europe and Asia Minor are used as Salep, Tubeka Salep. 



The small, wholly tropical family Burmanniaceae forms a connecting link 

 between the Gynandrae and the Amaryllidaceae and other epigynous Liliiflorae. 

 In common with the latter, its flowers have usually an actinomorphic perianth and 

 a free andrcecium with both whorls present or with one whorl suppressed. The 

 zygomorphism of many forms, the often unilocular ovary, the very numerous and 

 small seeds with unsegmented embryos, indicate on the other hand a relationship 

 with the Orchids. 



Sub-Class II 



Dicotyledones 



Flowers generally constructed after the pentamerojis, — para- 

 cyclic type. Seeds with or without albumen ; embryo with two 



COTYLEDONS. Herbs and woody 

 plants with open vascular 

 bundles exhibiting, in cross-sec- 

 tions of the stems, a circular 

 arrangement, and also almost 

 always with a cambium which 

 ^fv intersects the bundles (Fig. 437). 

 -d> Leaves usually with reticulate 

 venation (Fig. 438). 



The seeds are variously con- 

 structed, sometimes with, some- 

 times without albumen. The 

 embryo may be large or small ; 

 in some parasites and sapro- 

 phytes it is unsegmented, but 

 otherwise it is differentiated into 

 radicle, hypocotyl, and two coty- 

 ledons. On germination, the cotyledons remain in some cases enclosed 

 within the seed, in others they become green and unfold above the 

 surface of the soil. 



The primary root is usually retained, and may be distinguished 

 from the lateral roots by its larger size and more vertical growth. 



Fig. 437. — Transverse section of a young stem of 

 Aristolochia Sipho (cf. p. 109). 



