DR. J. D. HOOKER ON WELWITSCHIA. 17 
witschia, growing in open flat plains, scorched by a powerful sun, and seldom visited by 
rain, is very slowly and symmetrically developed, while Streptocarpus, growing rapidly, 
in moist localities, out of chinks in vertical rocks, where the two sides of the embryo 
and of the future plant may be exposed to very different conditions, is unsymmetrically 
developed. 
The last point to which I shall allude in connexion with the axis of Welwitschia is the 
origin of the parts I have called the stock and crown. 
The stock I assume to be what is termed the caulicle (tigellum) in germinating Dicoty- 
ledons, and to be in fact the radicle of the embryo* ; but whereas in most perennial 
dicotyledonous plants the caulicle becomes obliterated or not apparent after the first year, 
in this, owing to its assuming the functions of the trunk of the plant, it becomes the 
most important and bulky of its vegetative organs. Though I know of no exact counter- 
part to its vascular system in any other plant, such may exist; for M. Clos has pointed 
out, in his excellent papers on the structure of the tigellum (Ann. des Sc. Nat. sér. 3. 
t. xiii. p. 1, t. xviii. p. 321, t. xvi. and xvii.), that this part of the axis differs both from 
the stem and root in the disposition and number of its vascular bundles. 
The question of the theoretical origin of the crown is not so easily disposed of; nor 
can I say whether its ridges should be regarded as plumulary, or as being wholly made 
up of confluent axillary flower-branches. There being no other vertical axes developed 
in Welwitschia, these ridges, if flowering branches, can scarcely be considered as truly 
axillary ; on the other hand, any development of the embryo from between the bases of 
the cotyledons must be considered as more or less plumulary ; it is therefore easier to 
conceive the successive ridges of the crown to consist of arrested plumulary axes, with 
which the flower-buds coalesce, than to suppose them to be wholly plumulary or wholly 
floral. Iam not acquainted with the anatomy of the depressed flowering axis of Strepto- 
carpus, which may probably throw some light upon that of this plant. The disposition 
of the vascular bundles in mature plants of Welwitschia is too vague to throw much 
light on the subject. 
It would be most interesting to pursue this inquiry into the structure and development 
of the axis of Welwitschia much further, and especially to follow up the points of affinity 
between its tissues and the trunk of its allies the Cycadee and Gnetacee, on the one 
hand, and, on the other, the analogous forms of wood presented by Menispermacee and 
many other plants; but this would carry me far beyond the limits of the present essay. 
Leaf. (Plate XIV.) 
The youngest leaves I have seen are those of the specimen figured at Plate II. fig. 1; 
* In Jussieu's ‘Cours Elémentaire,’ § 488, I find it expressly stated that the radicle of Dicotyledons consists 
almost entirely of tigellum, and becomes the tigellum of the young plant. In Henfrey's ‘Elementary Course of 
Botany’ (1857), § 35, 301, &c., the radicle is called the root; in Asa Gray’s ‘Introduction to Botany ’ (1858), 
§ 158, 159, the radicle is rightly regarded as an axis, and not a root, but is further considered to be an internode, 
Of these views I adopt the first, and the first part of the last. 
VOL. XXIV. D 
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