SECT. I MORPHOLOGY 69 



brown with a solution of iodine. This colour almost wholly disappears if the 

 2)reparation be warmed, but reappears on cooling {''^). 



Ethereal Oils and Resins. — In many cases the strongly refractive drops 

 found dispersed throughout the cytoplasm are globules of some ethereal oil. It is 

 the presence of such oils in the petals of many flowers that gives to them their 

 agreeable perfume. Under certain conditions the oil globules may become crystal- 

 lised. This occurs, for example, in the petals of the Rose. In most cases, according 

 to G. Berthold and G. Haeerlandt (^^), ethereal oils or resins are formed in 

 special protuberances of the cell wall and only later are stored in the interior of the 

 cell or in special intercellular spaces formed by separation or destruction of cells. 

 According to A. Tschirch {^-) on the other hand, the protoplast first deposits a 

 mucilaginous layer within the existing cell wall ; the limits between membrane 

 and protoplasm becomes indistinct and in this zone oil-drops appear, which may 

 later fuse to one large drop. Special cells, often with corky walls and filled with 

 resin or ethereal oils, are found in the rhizomes of certain plants, as for instance 

 in those of Acorus Calamus and of Ginger {Zingiber officinale) ; also in the bark, 

 as, for example, of Cinnamon trees {Cinnamomum) ; in the leaves, as in the Sweet 

 Bay {Laurus nohilis) ; in the pericarp and seed of the Pepper {Pijyer nigrum) ; in 

 the pericarp of Anise (Illicium anisatum). 



Mucilage is often found in the cells of bulbs, as in Allium Cepa and Urginea 

 Scilla ; in the tubers of Orchids ; also in aerial organs, especially in the leaves of 

 succulents, which, living in dry places, are thus enabled to Imaintain their water- 

 supply by means of their mucilaginous cells. 



Gums are in special cases, e.g. the formation of cherry gum in the stem of the 

 Cherry tree, formed from substances derived from the contents of the cell and 

 accumulated between the limiting layer of the protoplast and the cell-wall. By 

 the further conversion of the cell-walls into gum, cavities filled with the latter are 

 produced. In other cases the gum is mainly derived from the cell walls. In all 

 cases of gummosis we have, apparently, to do with a pathological process (*^). 



Caoutchouc and Gutta-percha. — These substances are found in a number of 

 plants belonging to dift'erent groups, in particular in the Moraceae, Euphorbiaceae, 

 and SaiMtaceae. They occur in the latex of special cells in the form of small 

 globules, which, suspended in the watery sap, gives it its milky appearance. 



Enzymes and Chkomogens C'^). Enzymes are of general occurrence in living 

 protoplasm and play an important part in metabolism. Chromogens also are 

 widely distributed, and along with enzymes are concerned with the process of 

 respiration. 



Hydrocyanic Acid and derived Glucosides (^^) [e.g. amygdalin, phaseo- 

 lunatin, etc.) have been of late years shown to be widely distributed in plants. 

 Their occurrence in foliage leaves {Pajigium edule, Phaseolus lunatiis, Rosaceae) is 

 of special interest, since according to Treub it is related to the synthesis of proteid 

 substances. 



Sulphur. — The presence of sulphur in the form of small refractive grains in 

 the protoplasm of certain Bacteria, the Beggiatoae, is noteworthy. These Bacteria 

 live in water containing small quantities of sulphuretted hydrogen. According to 

 WiNOGRADSKY (^'') they obtain sulphur from this, and then oxidise it to sulphuric 

 acid according to their needs. 



The Cell Sap. — Under this term is included especially the fluid 

 which in mature cells fills the inner sap cavity. It is genei'ally more 

 watery and clearer than the fluid contained in the smaller vacuoles 



