THE GYMNOSPERMAE : CYCADALES, ETC. 769 



a ridge appears below each leaf base, developing until it encircles the stem. 

 A corresponding ridge arises above each leaf base, which is thus enclosed in 

 a groove between the upper and lower ridges. The upper ridge increases 

 in height and forms the rim of a saucer-shaped depression which includes the 

 central part of the crown. As the crown grows broader the ridges, at first 

 continuous, split along a line between the leaf bases and the crown becomes 

 more or less two-lobed, with a V-shaped depression along the line of the 



split. 



The upper ridge of the leaf-groove bears the branched inflorescences, 

 which it continues to produce for several years. When it is exhausted it is 

 succeeded by a series of new ridges formed outside it, one after another in 

 centrifugal order, each bearing inflorescences for a time and then being 

 superseded. The older ridges nearer the centre soon lose their identity in 

 the thick growth of rough periderm which covers all the surface. These 

 ridges arise at the leaf bases and correspond morphologically to branches or 

 branch bases, which never develop vegetatively, but bear only the temporary 

 reproductive shoots. 



Anatomy of the Stem. 



The internal anatomy of Welwitschia, while not so peculiar as its external 

 form, displays a number of interesting features. 



In the young stem the bundles form a ring, but in the old stem the 

 vascular tissue forms a cup-shaped mass extending from one leaf groove to 

 the other. From this mass some bundles descend to the root and others 

 ascend from its edges to the reproductive shoots. Everywhere are groups 

 of sclereids, like those in the leaf, in such quantity that they make the whole 

 tissue at first solid, but later it becomes spongy as the thin-walled tissue 

 between them dies away, leaving the sclereids interlocking. 



The individual bundles are of the dicotyledonous type, very long and 

 narrow in the radial direction, like those of Cycas. Old plants show successive 

 concentric zones of bundles, another cycadean character, also shared by some 



species of Gnetum. 



The xylem consists of spiral, reticulate and pitted tracheids, the latter 

 usually uniseriate. Vessels are also present, with incomplete perforations 

 consisting of one or two simple irregular pits. The phloem is gymnospermic 

 in type and has no companion cells. 



Two crescentic groups of bundles pass upwards to the leaf bases. The 

 ridges below and above each leaf base are supplied by very much flattened 

 rings of bundles. Those in the upper ridge are said to originate from those 

 supplying the lower ridge, passing upwards between the leaf trace bundles 

 in their course from below to above the leaf base. The ridge bundles and 

 those of the cotyledonary " buds " are said to end blindly downwards and are 

 not attached to the main vascular system. 



