no MORPHOLOGY OF TISSUES. 



Their walls are generally very thin when they arise from the coalescence of paren- 

 chyma-cells which has already taken place in the primary meristem; they may, however, 

 become thick, and it scarcely admits of a doubt that in many cases (as Apocynaceae 

 and Euphorbiaceae) the bast-fibres themselves become transformed into laticiferous 

 vessels ; according to Hanstein it is even probable that in some Aroidese vessels of 

 the xylem assume the form and function of laticiferous vessels. The morphological 

 signification of these organs may thus be very various ; physiologically they have 

 this in common, that they contain dissolved and finely divided substances (emul- 

 sions) which find in them open courses for rapid motion. The same object is, 

 however, also obtained in the plant by the cells pouring out the substances they 

 contain into specially formed intercellular spaces, which, like the laticiferous vessels, 

 may form a connected system of channels in the plant. These also are produced 

 sometimes in the parenchymatous fundamental tissue, sometimes in the xylem, some- 

 times in the phloem of the bundles ; but they are easily distinguished from the former 

 by the peculiar arrangement of the surrounding cells. The latex contained in them 

 may be limpid, mucilaginous, or gummy {e.g. Araliacese), or there is mixed with it an 

 emulsion of resin-forming materials (as in Umbelliferae) ; or the passage contains a 

 resin-producing ethereal oil (as in ConiferEe), or other odoriferous and coloured 

 fluids of oily nature {e.g. Compositae, Umbelliferae). Glands are distinguished from 

 the latex-vessels hitherto mentioned by their not presenting channels or systems of 

 channels, but being local formations. Separate cells or roundish groups, whose par- 

 tition-walls frequently become absorbed, may take the form of glands, so that here 

 again, by the process of coalescence of cells, arise receptacles for special substances 

 (mostly strongly odoriferous, viscid, oily, or coloured). Glands may arise anywhere 

 in the tissue ; and if they belong to the epidermis may discharge their secretions 

 outwardly. 



(a) Laticiferous and Fesicular Vessels^ show, as has already been mentioned, such 

 numerous and various transitions, that it would be desirable to be able to include them 

 under a common term, such as Latex-sacs. 



The Gichoriaceae, Campanulacese, and Lobeliaceae possess very perfectly developed 

 laticiferous vessels, belonging to the fibro-vascular bundles, which they accompany 

 throughout the whole plant as reticulately anastomosing tubes, imbedded, in the case 

 of the Cichoriacese in the outer, in that of the two other families in the inner phloem- 

 layer. Their form is best recognised by boiling sections of these plants for some minutes 

 in dilute solution of potash ; the reticulations are then clearly recognised in the trans- 

 parent tissue (Fig. 94), and it is easy to separate them entirely in large pieces. In the 

 Papayaccce (Carica and Vasconcella), the laticiferous vessels, on the other hand, run 

 through the system of the fibro-vascular bundles; they, — /. e. the cells by the coalescence of 

 which they- are formed — are repeatedly produced in layers from the cambium with the 

 other elements of the xylem ; the pitted and reticulately thickened wood-vessels alter- 

 nate with them. The branches of the laticiferous vessels envelope these in all direc- 

 tions, and are sometimes firmly fixed to their surface ; but in addition horizontal 

 branches of these bundles also penetrate the medullary rays ; and these terminate 



1 J. Hanstein, Monatsberichte der Beii. Akad. 1859. — Ditto, Die Milchsaftgefasse u. verwandten 

 Organe der Rinde. Berlin 1S64. — Dippel, Verhandlungen des naturwiss. Vereins fur Rheinland u. 

 Westphalen. 22. Jahrg, vols. 1-9. — Ditto, Entstehung der Milchsaftgefasse u. deren Stellung im Ge- 

 fassbiindelsystem. Rotterdam 1865.— Vogel, Jahrb. fiir wissen. Bot. vol. V. p. 31. 



