VESSELS IN MONOCOTYLEDONOUS AND DICOTYLEDONOUS PLANTS. 261 
3. The fibrous, ligneous or retieulated vessels are of a formation posterior to the 
appearance of the tracheæ, 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 
tracheæ, 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 tracheæ in dicotyledonous plants; they 
exist, on the contrary, in the roots of nearly all monocotyledonous plants, where, when true 
tracheæ 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 Fourcroya 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 fecula, i, and a large quantity of raphides, 
ï, 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 tracheæ, formed like a crown about the vital point, ot horizontal plane, 
which I have supposed to be the limit between the stem and the leaves, although it is 
difficult to determine its exact placé, as each leaf has its distinct plane, the intervals 
being true merithalli. These small tracheæ, 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, igni and 
placing themselves outside the interior bundles, having a flexuose direction, as ives in 
c,d,e. In the succeeding leaves there are no simple tracheæ, but numerous rachew 
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 "ma 
