582 PHYSIOLOGY. 



formation ; but the same generalities apply to them as to the essential 

 oils with which they are not unfrequently associated. 



Among the resin-producing Orders may be noticed especially the Coni- 

 ferae, the Leguminosae (Copuifera, Myroxylon, &r\), Amyridaceae, Gutti- 

 ferae, Styraceas, Terebinthaceae, Liliaceae (Aloe, Xanthorrhea), &c. 



Resinous and waxy matters are found in considerable abundance on 

 the surface of the leaves or fruits of many plants. It is not clear at present 

 how far these are to be regarded as proper excretions or as chemically 

 metamorphosed epidermal structures. 



Under this head falls the waxy coat of leaves and fruits which exhibit 

 what is called a "blooin," as the' leaves of Primulaceas (P. Auricula, &c.), 

 the fruits of the Plum, &c. The w*x of the Wax Palm ( Ceroxyloii) is 

 formed in flakes upon the surface of the stem. 



"Wax and resinous matters occur on the outer coat of the pollen of 

 flowers j and the viscid surface presented by the epidermis of many plants, 

 such as Lychnis Viscaria, some Silenes, &c., is attributable to similar 

 causes. 



Latex. The so-called milky juices (latex) occurring in specially modi- 

 fied intercellular passages (p. 513) are of complex composition, containing 

 essential oils, resins, gum-resins, starch-grains, extractive matters, alka- 

 loids, proteiriaceous compounds, caoutchouc, &c. suspended in water, 

 forming a kind of emulsion. They are not opaque and milky in their 

 natural state ; but become so when exposed to air, and mostly assume a 

 transparent resinous character when their watery constituents evaporate. 

 Very different opinions have been expressed as to the nature of latex and 

 the vessels containing it. By some it has been considered a nutritive 

 fluid analogous to arterial blood, by others as of purely excrementitious 

 nature. A third notion is founded on the comparison of the fluid in 

 question with venous blood. Probably that view by which it is regarded 

 as a fluid containing, mixed with matters of a directly nutritive character, 

 others which are excrementitious in their nature (Sachs, Hanstein) is the 

 most correct. Trecul holds that the laticiferous vessels are the analogues 

 of the veins, and their contents equivalent to venous blood. He traces a 

 contact and inosculation of the laticiferous vessels with the pitted ducts 

 and other vessels. Latex from this point of view would be the residue of 

 the sap after elaboration by the cells the caput mortumn of the sap. 

 Faivre's experiments show that the latex may flow in any direction, and 

 not only through the bark, but through the wood and the pith (in Ficus 

 elastica). 



These juices abound especially in particular Orders, as in the Papa- 

 veraceae, Euphorbiaceae, roots of Cichoraceae, Apocynacese, Urticaceae, &c. 

 Amongst the most important substances obtained by evaporating them to 

 dryness are : opium from Papaver somniferum, and caoutchouc from various 

 Euphorbiaceae, Urticaceae, and Apocynaceae ; guttapercha from Isonandra 

 yutta, &c. 



Saline Matters. The saline and purely mineral excretions of plants 

 have been already referred to. They occur as incrustations of the cell- 

 membranes, as silica in the Grasses, Equisetaceae, Stellatae, &c., or car- 

 bonate of lime in Charce, Corallince, and in smaller quantities on the 

 leaves of various Saxifrages. Crystals (raphides, p. 505), either of inor- 



