6 14 SECONDARV CHANGES. 



Sect. 196, The structure of the stem of Welwitschia mirabilis^ is as wonderful 

 as its form is peculiar. Moreover, by reason of the difficulty of preparation which 

 the nature of old dry specimens presents, and of the lack of young fresh specimens, 

 it remains still uninvestigated in many respects '- According to data at hand, which 

 relate almost exclusively to the secondary thickening, it is connected in the matter 

 of the arrangement of its tissues partly with many anomalous Dicotyledons, partly 

 with certain Monocotyledons with secondary thickening, and in its histological 

 structure with other Gnetaceae. 



The youngest plants, which are known, have a roundish stem, about the size of 

 a nut, which is continued downwards into a 'Strong tap root, provided with relatively 

 small lateral branches. The round stem, called by Hooker the stock, has a convex 

 uneven apical surface, the crown, on- which, in the specimens examined, there is no 

 trace of a true punctum vegelationis to be found. The blunt margin, by which the 

 crown passes over into the lateral surface of the stem, is for the most part surrounded 

 by the almost contiguous surfaces of insertion of two opposite, tongue-shaped leaves, 

 which are regarded with good reason as the two persistent cotyledons ; each of these 

 leaves is inserted at the base of a deep ring-like furrow, which is so narrow that it is 

 loosely filled up by the base of the leaf. 



It is known that the plant retains this conformation throughout life, and suffers 

 only this one important change of form, that the upper part of the stem grows 

 contimiously in width in centrifugal progression, so that it attains the form of an 

 oblong Iwo-flapped disk, in the blunt, more or less erect, marginal flaps of which are 

 situated the foliar grooves. These together with the bases of the leaves increase in 

 -girth in proportion to the stem; the leaves themselves elongate throughout life at 

 their base, and in a basipetal direction ; the stem, crown, and root increase for many 

 years in thickness, and attain colossal dimensions. 



An anatomical investigation in the smallest specimens has not been undertaken, 

 but in those which are hardly double their size, and successively in other older ones, 

 this has been done. They all resemble one another, as far as investigations go, 

 in the fundamental points of structure. The stem and tap root, with exception of 

 tjie grooves of insertion of the leaves to be described below, are covered by a 

 moderately thick, for the most part brown, fissured, bark-like, and very hard and 

 brittle cortex. This envelopes a strong internal mass of tissue, which consists of 

 vascular bundles, pale yellow thin-walled parenchyma, and those huge sclerenchy- 

 matous fibres described on p. 132, which are embedded in large numbers in the 

 parenchyma of all sorts, and which point in all directions. As regards the vascular 

 bundle-system, in the first place, in not too large specimens a large number of 

 bundles are seen, which run with a radially converging course from the insertion 

 of the leaf towards the lower end of the stem, or the upper part of the tap root. 

 They are arranged in one plane, which lies between the surface of the crown and the 

 outside of the stem, rather nearer the former than the latter ; it has thus approximately 



' J. D. Hooker, On Welwitschia, Trans. Linn. Society, London, Vol. XXIV.— Strasburger, 

 Die Coniferen u. d. Gnetaceen, p. 374. [Bertrand, Ann. Sci. Nat. Ser. V. vol. XX.] 



^ [F. O. Bower, On the Germination and Histology of the seedling of Welwitschia mirabilis, 

 Quart. Journ. Micr. Sci., vol. XXI. New series, June 1881.— On the further development of Wel- 

 witschia mirabilis, Quart Journ. Micr. Sci., vol. XXI, 1881.] 



