260 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 tracheae considerably increased ; the gemmule, c, is 

 now observed under the form of a cellular tumour without vessels ; the four tracheae 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 

 tracheae only, of which two, constituting the midrib, descend by the stem to meet the four 

 cotyledonary tracheae : in the stem or primary merithal* (radicle of authors), those 

 tracheae, 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 tracheae 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 tracheae, and descend through the roots without 

 being accompanied by tracheae : 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 

 tracheae 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 <?'". 



From this investigation we may infer the following results : — 



1. The tracheae, 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. 



* According to the doctrine of Gaudich aud (Recherches Generates 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 " merilhalles," viz. the caulicular (tigellaire), the petiolar, and the 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 " mesocauleorhize," which is the primary 

 vital knot (" nceud vital ") of the stem, constituting its real base, and the true summit of the root. — (Note of the 

 Translator.) 



