452 PRIMA RF ARRANGEMENT OF TISSUES, 



The same conditions of arrangement as in Pittosporum are found in the primary 

 vascular bundles of the branches and leaves of SoUya heterophylla, and Citriobatus 

 multiflorus. Bursera spinosa, however, never has sap-passages, according to van 

 Tieghem. 



CactecE. The passages containing latex in many Mamillarias (comp. pp. 202, 206) 

 traverse the whole stem, and are scattered through the parenchyma. They are rare in 

 the inner pith, but numerous in the zone of parenchyma situated between the ring of 

 wood and the inner circle of cauline bundles (comp. p. 254), in the whole cortex, and the 

 mamillae. They are branched in all directions, and all branches communicate openly 

 one with another; those which enter the mamillaj run within them near to the axile 

 strands of xylem, and give off numerous branches, which are repeatedly branched and 

 run through the chlorophyll-parenchyma straight towards the surface, many of them as 

 far as the simple hypodermal layer of collenchyma. 



In other Cactes these passages are not found. Those found by SchJeiden ^ in Opuntia 

 peruviana differ fundamentally from them. 1 have investigated them in O. robusta. 

 They lie here close to the outer limit of the phloem (not, as Schleiden states, in it) of 

 the bundles of the trace, which are connected into a net, and follow them in their lon- 

 gitudinal course. They are apparently of lysigenetic origin, being cavities in the 

 parenchyma, which attain a \j^idth of 5"'™, and are filled with swollen, sometimes still 

 recognisable cells, and numerous conglomerated crystals of calcium oxalate embedded in 

 the mucilage. 



The investigated A^iacardiacea — Schinus molle, Spondias cytherea, Pistacia vera, 

 Lentiscus, Rhus aromatica, suaveolens, Cotinus, Coriaria, virens, Toxicodendron, typhina, 

 glauca, elegans, semialata', and villosa — are, as regards the disposition of their passages 

 containing mixtures of gum-resins, distinguished by the fact that they are situated in the 

 stem and leaves in the phloem of the primary vascular bundles. Further, in addition to 

 this, there are others in the secondary bast of the stem, from which, in Rhus viminahs, 

 blindly ending branches penetrate here and there horizontally into the medullary rays 

 of the xylem: finally, in many species (Rhus toxicodendron, typhina, glauca, elegans, 

 viminalis, semialata, and Spondias cytherea) there are medullary passages. 



In the root a relatively large passage lies in the middle of each phloem portion of the 

 primary, usually 2 or 3 rayed vascular bundle. In the secondary layer of bast new ones 

 are successively added. 



The phloem portion of the primary bundles of the stem is limited from the parenchy- 

 matous outer cortex by a strong bundle of sclerenchymatous fibres of half-moon shaped 

 transverse section, and the fibrous bundles are almost in contact with one another at 

 their margins, thus forming together a ring surrounding the outer cortex. Outside this 

 there is no resin passage., but there is a thick one immediately within it in the phloem of 

 each bundle. In the secondary cortex, which appears later internally, new ones are 

 formed successively in .-the strands of bast. The medullary passages vary in number 

 according to the species ; Trecul gives as the number, e. g. in a transverse section of 

 a branch of R. semialata 58, typhina 25, viminalis 5-12 ; the larger number are situated 

 especially at the periphery of the pith, the minority scattered irregularly. According to 

 successive transverse sections it appears that a part at least of the medullary passages ends 

 blind in the pith. The cortical passages, as far as they belong to the secondary bast, are 

 connected also in the internodes by more or less numerous tangential anastomoses. In 

 the nodes the cortical passages anastomose both with one another, and also with the 

 medullary passages by means of branches, which follow the vascular bundles out into the 

 leaf; from the plexus of anastomoses the passages are continued down into the next 

 internode and into the leaf. 



' Anatomic der Cacteen (Mem. pres. Acad. S. Petersbg. tom. IV), p. 358, Taf. VII. 4. 

 ^ Trecul, Ues Vaisseaux propres dans les Terebinthinees. Comptes Rendus, tom. LXV. p. 1 7, 

 1867. — Van Tieghem, I.e. 



