260 2 DR. ALLEMAO ON THE ORIGIN AND DEVELOPMENT OF 
Fig. 3 is the same plant, still more grown, of its natural size. ! 
Fig. 4 is the same, much magnified, as seen by the microscope. The cotyledons now 
exhibit their nervures, consisting of tracheæ considerably increased ; the gemmule, c, is 
now observed under the form of a cellular tumour without vessels; the four trachex in 
the stem descend in a parallel direction as far as the radicular bulb or bourgeon, b, con- 
stituting thus the medullary sheath : rootlets are not yet observable. 
Fig. 5 represents of its natural size the same plant now having roots, one of the leaves 
of the gemmule being at the same time fully developed. : 
Fig. 6 is the same, magnified and divided longitudinally, as seen by the microscope. 
The cotyledons remain as in the preceding case, with the exception of their having now 
acquired more nervures : the primordial leaf, f, is also seen with nervures consisting of 
tracheæ only, of which two, constituting the midrib, descend by the stem to meet the four 
cotyledonary tracheæ : in the stem or primary merithal* (radicle of authors), those 
tracheæ, a, are as yet solitary for two-thirds of the upper portion of their length, but in 
the lower third they are accompanied and invested externally by dotted vessels, b: at. 
the point d, the limit between the stem and the root, the tracheæ of the stem terminate, 
and we see the commencement of the dotted or ligneous vessels, which begin to ascend in - 
bundles through the stem outside the tracheæ, and descend through the roots without 
being accompanied by tracheæ: e is a more magnified figure of half the former vertical 
section of the mesophyte at the vital point, where at e' is shown the termination of the 
tracheæ of the stem, and where the dotted vessels are seen ascending through the stem 
and descending through the main or perpendicular root e", and also through a ramifica- 
tion of the root at e". 
From this investigation we may infer the following results :— ` 
1. The tracheæ, which are the first vessels formed, derive their origin in the stem at 
the vital point or horizontal plane in which the leaves originate, whence they extend, 
forming bundles, upwards in the leaves to constitute the nervures, which extending down- 
wards through the stem form the medullary sheath. | 
2. Roots do not exist in the embryos, but are formed in the young plant, when, freed 
from its seminal envelopes, it penetrates the earth: (there are exceptions to this rule in 
some embryos, where, from a delay in the rupture of the integuments, the roots begin to 
sprout while in the seed.) But there exists in such case the root-bud (** gommo ”) or 
`. radicular bulb, which is destined to produce it, and which bears some analogy to the 
gemmule, and may be considered as a primary spongiole, because by its means the plant 
absorbs nourishment before it has roots. 
T According to the doctrine of Gaudich aud (Recherches Générales sur l'Organographie, &c. p. 5), every germinating 
point or elementary leaf in a plant has its superior and inferior vascular system, the superior or ascending being 
resolvable into three parts or ** merithalles,” viz. the caulicular (tigellaire), the petiolar, and thé laminar (limbaire), 
or better designated as the superior, middle, and lower merithals, the line of separation between the two former being 
called by him the * mesophyte,” that between the two latter the « mesophylle :” the inferior or radicular descending 
system is separated from the lower merithal by a point which he calls the * mesocauléorhize," which is the primary 
vital knot (“nœud vital ”) of the stem, constituting its real base, and the true summit of the root.—(Note of the 
TRANSLATOR.) , l 
