

GNETEJ1. 271 



placed ovule, which is provided with one, or (in Gnetum) two, 

 integuments, and partly owing to the fact that the wood has true 

 vessels. There is only one order. 



Order. Gnetaceae. The three known genera differ very much in appear- 

 ance. Wehvitschia mirabilis (from the deserts of South Western Africa) is the 

 oldest (?) genus now living. It resembles a giant radish, in that the hypocotyl 

 is the only part of the main axis of the stem which becomes developed. It 

 attains a circumference of upwards of four metres with a length of J-f of a 

 metre. It bears only two oblong, leathery leaves (Fig. 275) which are torn into 

 segments at the apex and lie on the surface of the soil ; these are the two first 

 foliage -leaves which succeed the cotyledons, and they are remarkable for their 

 enormous length (upwards of two metres) as well as for their long duration, 

 living as long as the plant itself. In their axils are situated the 4-rowed, spike- 

 like male and scarlet-coloured female cones, upon dichotomous branches. The 

 perianth consists in the $ of 2 alternating pairs of leaves, the inner ones of which 

 are slightly united. The androecium likewise consists of 2 whorls : the external 

 (transverse) with 2, the internal with 4 stamens ; the lower halves of the 6 

 filaments uniting to form a cup. Each of the terminal anthers corresponds to 

 a sorus of 3 sporangia, the sporangia being fused together, and opening at the top 

 by one three-rayed cleft. In the centre of the $ -flower there is a sterile 

 ovule. In the -flower a perianth of two connate leaves is present. Ephedra 

 (desert plants, especially in the Mediterranean and W. Asia) at first sight re- 

 sembles an Equisetum;ihe stems are thin, long- jointed, and the leaves opposite, 

 small, and united into a bidentate sheath ; <J -perianth of two connate leaves 

 (median leaves) ; 2-8 stamens united into a column. Each anther is formed 

 of 2 sporangia (is bilocular). $ mainly, as in Wehvitschia. The seeds are sur- 

 rounded by the perianth which finally becomes red and fleshy. There are 30 

 species. Gnetum has opposite, lanceolate, pinnately-veined, leathery leaves. 

 They are mostly climbers (Lianas) from Tropical Asia and America. The <J - 

 flowers have a tubular perianth, (formed from two median leaves) which 

 surrounds a centrally-placed filament, bearing 2 anthers. In the ? -flower 

 there is a similar perianth, surrounding an ovule provided with 2 integuments . 

 The perianth becomes fleshy and envelops the hard seed. 20 species. 



From the circumstance of Welwitschia having $ flowers which, besides 

 stamens, possess also a rudiment of an ovule, Celakovsky draws the inference that 

 the earliest Gymnosperms had hermaphrodite flowers which from this structure 

 became differentiated entirely into 3 - and -flowers, with the exception of 

 Welwitschia only, in which this differentiation was only carried out in the $ - 

 flower. This theory has so far been scarcely proved. 



Fossil Gymnosperms. 



The earliest continental plants which are known belong to the COBDAITACE.E, 

 a group of plants which existed as early as the Silurian period ; they were 

 Gymnosperms, but it has not yet been determined whether they were Cycads or 

 Conifers. The CYCADS, even in the Coal period, were scarce ; they attained their 

 fullest development in Jurassic and Cretaceous periods, during which they were 

 rich in species and genera, and extended as far as the Polar regions. In addition 

 W. B T 



