248 MORPHOLOGY. 



appears, the course of which, however, I could not make out, as the piece 

 in my possession was not sufficiently large for me to trace it. Still 

 it appeared to me that the vascular bundles, /, /*, had not yet quite reached 

 the middle of the stem. It will at least suffice to draw the attention of 

 more favoured observers to this striking structure, and will give a better 

 representation of it than the miserable trash of Gaudichaud (Plate X. 

 figs. 10. to 15.). Perhaps the history of the development of Aletris fragrans 

 will afford the first some conclusions on the point : An old stem of about 

 4*25 Paris inches in diameter consists of two parts : the primary stem, 

 about 7 lines in diameter, in which the vascular bundles exhibit the 

 usual arc-like course ; and an external, much more solid zone, gradually 

 formed by the cambium-layer. The vascular bundles passing from within 

 to the leaf-cicatrices permeate this external layer in a perfectly horizon- 

 tal direction. The external layer becomes divided, however, again into 

 four zones, which produce the appearance of annual rings when seen in 

 the transverse section. The three external ones are, when taken together, 

 of about the same thickness as the fourth internal one : they differ in this, 

 that in the external ones the fibres do not ascend vertically but obliquely, 

 consequently in a spiral round the axis, and wind towards the left ; in the 

 second in like manner, but winding towards the right ; in the third, 

 again, turned towards the left, and finally becoming gradually horizontal 

 in the fourth. I may remark here, that whilst the parenchyma is 

 arranged in vertical rows in the primary stem, it appears to be in hori- 

 zontal rows between the external vascular bundles, in the manner of the 

 medullary rays. 



An essential difference presents itself here, according as the formative 

 layer is limited to the terminal bud (above the line x, y, in the woodcut fig. 

 160.), or whether there is a continuous layer in the whole circumference 

 of the stem below the rind, which is then bounded internally by it. The 

 latter occurs in the case of normally-branching stems, as, for instance, in 

 the Draccence,Aloinece,&n&Aroidece, the former in normally-simple stems, 

 as, for instance, in the Tulipacece, and Palms with undeveloped inter- 

 nodes. Beautiful investigations on this subject may be found, accompa- 

 nied by the carefully selected results of earlier observations, in Unger 

 (see his Bau und Wachsthum des Dikotyledonenstammes, Petersburg, 

 1840, page 34.). 



I must finally make mention of the singular stem-formation in the 

 tropical Orchidacea. A large portion of these, such, for instance, as are 

 commonly described as having tubers, have not very thick stems (gene- 

 rally branched), with abbreviated internodes. Those branches, however, 

 which come to blossom, produce a peculiar form that has hitherto been 

 known as tuber (knolle). Either one of the more central internodes of 

 the blossom-bearing branch swells into a disproportionate mass of very 

 varying shape, or all the lower internodes of the branch form a longer or 

 shorter, more or less thick, fleshy mass. In both, as, for instance, in 

 Epidendrum cochleatam and Bletia Tankervillice, the regular course of 

 the vascular bundles may be distinctly observed, but in the case of the 

 last-named plant (I know not whether the same holds good for all simi- 

 larly formed) there is a peculiar vascular system intended for the new 

 lateral buds. Little branches pass from the external vascular bundles, 



appears most enigmatical in the case of those which, like //, have not yet reached the 

 middle of the stem, when they cease, as in the drawing. 



