444 



PRIMARy ARRANGEMENT OF TISSUES. 



connected on the \v;iy by occasional transverse anastomoses. Those of the peduncle only 

 occur in the hypodermal parenchyma. In the lacunar tissue of the petiole lie numerous 

 small vascular bundles at the periphery, and five arranged in a curve in the middle. Ex- 

 ternally to each peripheral bundle, and between it and the hypodermal layer of cells, lies 

 one passage; one wider one, with its epithelium abutting directly on the epidermis, 

 alternates with each pair of peripheral bundles. Around the inner bundles there is in the 

 layer of parenchyma surrounding them, one passage opposite the points of insertion of 

 each of the plates of parenchyma separating the air cavities. In the lamina of the de- 

 veloped foliage-leaves the passages appear on both sides immediately beneath the 

 epidermis: their main trunks accompany the chief vascular bundles of the leaf: their 

 very abundant branches together form a completely closed net, the meshes of which do 

 not coincide with those of the network of bundles. The linear primordial leaves of the 

 young plant have only three passages, which accompany the three vascular bundles, and 

 only come together at the apex of the leaf. Those which enter bracts and sepals are 

 only connected one with another at the base of these organs by anastomoses, and then 

 run parallel towards the apex, and end blindly short of the latter. 



In Sagittaria sagittifolia the passages are arranged in the cortex of the stolons in two 



circles, the one peripheral, the other 

 internal, near to the cylinder of vas- 

 cular bundles ; in the petiole, which has 

 a structure like that of Alisma, pas- 

 sages are found between the epi- 

 dermis and those vascular bundles 

 which do not abut directly upon it ; 

 there are others, which are arranged 

 between the bundles, and singly at the 

 points of union of the plates of paren- 

 chyma which separate the air cavities 

 (van Tieghem). 



Similar conditions to those in the 

 Alismaceae described are found in Hy- 

 drocleis Humboldtii, for the details of 

 which reference must be made to 

 Schleiden ^ and van Tieghem. Here 

 also the passages are wanting in the 

 roots. 



Among the ArcidecB, according to 

 the observations of Trecul^ and van 

 Tieghem ^, there are passages contain- 

 ing resin and ethereal oil in the genera 

 Philodendron, Homalonema, Schisma- 

 toglottis, and gum pass iges in many species of Aglaonema. The other genera of the 

 family in question, as far as investigated on this point, have no passages: this applies 

 on the one hand to all those which have true milk-tubes, and on the other to those whose 

 vascular bundles are accompanied neither by milk-tubes nor by rows of tannin-contain- 

 ing sacs, comp. p. 436. The resin passages of numerous investigated species of Philo- 

 dendron traverse all the members of the plant longitudinally as narrow canals, apparently 

 (but nothing is stated explicitly on this point) in such a way that they are all connected 

 one with another at the nodes and other points of insertion. In the lateral roots, the 

 stem, and petiole, they are scattered in the parenchyma, forming in the cortex of the 

 root 3, 4-5, or even 8 (Ph. Melinoni) more or less regular concentric rows (Fig. 192); 



Fig. 192. — Philodendron Imbe Hort. Halens ; transverse section of a 

 strong subsidiary root, sliglitly magnified. Axile vascular strand, to tile 

 rit;iit the whole cortex ; ^ outer margin of the xylem rows. The 

 obliquely-shaded radial bands, w, are the phloem groups ; / peridenn ; 

 b fibrous bundle surrounding a laticiferous intercellular passage. 



Comities Rendus, toni. LXII. p. 2ij (iS66; 



' Grundziige, 3 Aufl. I. p. 267. 



' Strucluie des Aroid^es, I.e. 



