VESSELS IN MONOCOTYLEDONOUS AND DICOTYLEDONOUS PLANTS. 261 



3. The fibrous, ligneous or reticulated vessels are of a formation posterior to the 

 appearance of the tracheae, their origin being at the vital point or horizontal plane from 

 which the roots proceed, and whence they extend in bundles upwards through the stem, 

 till they reach the extremity of the nervures of the leaves, being always exterior to the 

 tracheae, and downwards through the root till they attain its extremities, leaving almost 

 always in the centre a kind of canal filled with cellular tissue, which is true pith, and 

 which extends itself laterally, communicating with the herbaceous envelope by means of 

 medullary rays : but this pith is not enclosed by tracheae in dicotyledonous plants ; they 

 exist, on the contrary, in the roots of nearly all monocotyledonous plants, where, when true 

 tracheae do not exist, their place is supplied by mixed or scalariform vessels. I have here 

 carried my deductions beyond the points shown in the drawings, which are now pur- 

 posely curtailed ; but I have made this digression in order to explain my views : with the 

 same object several well-known facts have been repeated : all that appears here really 

 novel is the extension of two vascular systems, in opposite directions to each other, and 

 their increment at their respective extremities, by which is meant the propagation upward 

 and downward of fibres or vascular bundles. 



4. Finally, the radicular branches, as appendicular or radiated organs (fig. 6, e, e'"), 

 are in their origin perpendicular to the cauline fibres, and without continuity with them. 

 This is contrary to the theory maintained by M. Gaudichaud. 



Drawing B. — This exhibits the microscopical observations made upon a young rooting 

 bulb of Foarcroya gigantea, which tend to prove the facts before affirmed. 



Fig. 1 : young bulb, of its natural size. 



Fig. 2 shows the plane of a longitudinal section passing through the centre of the bulb. 

 Here, in the midst of an apparent confusion of vascular bundles, I obtained the result 

 shown in this figure only after numerous and patient dissections, but the result was 

 repeated frequently. The bulbous mass is formed of rather dense cellular tissue full of a 

 viscous lymph, the cells of which contain much feculaj i, and a large quantity of raphides, 

 «', or solitary prisms, i". It gives origin upwards to many sheathing and concentric leaves. 

 Of these the central one, a, which is commencing its earliest development, is composed 

 only of very slender cellular tissue : the one next in succession, exteriorly, is still cellular, 

 but beginning to receive tracheal ramifications, which are the upper extremities of 

 numerous simple tracheae, formed like a crown about the vital point, or horizontal plane, 

 which I have supposed to be the limit between the stem and the leaves, although it is 

 difficult to determine its exact place, as each leaf has its distinct plane, the intervals 

 being true merithalli. These small tracheae, b, are exceedingly slender and of a vermi- 

 cular or fusiform aspect ; they form a seat or curvature in the middle, the convexities of 

 which look toward the centre ; thence they extend upwards, penetrating the leaves in 

 great number, parallel to one another, and are prolonged downwards, crossing and 

 placing themselves outside the interior bundles, having a flexuose direction, as shown in 

 c, d, e. In the succeeding leaves there are no simple tracheae, but numerous tracheae 

 form bundles or cords, which penetrate in great numbers parallel to one another in each 

 leaf, till they reach the extremity, taking ulteriorly lateral and transverse directions 



2 m2 



