98 



PART II. ANATOMY AND HISTOLOGY. 



28 



Rue, the Myrtle, and Hypericum, where they can be discerned 

 with the naked eye as transparent dots. The development of these 

 oil-glands begins with the division of one or two cells of the young 

 leaf, a group of cells being formed, in the cytoplasm of which oil- 

 drops make their appearance. The walls between the cells (Tig. 

 76 B. C} undergo absorption, so that a cavity is formed which is 

 bounded by the closely-packed adjacent cells, and contains a large 

 oil-drop formed by the fusion of the oil-drops of the original cells. 



The most striking examples ofjschisqgenous hollow glands are 

 the various kinds of ducts, such as the resin-ducts which permeate 

 the tissues of most Coniferae and Anacardiacese ; the gum- or 

 mucilage-ducts of the Marattiacese, some species of Lycopodium, 

 Cycads, Canna, Opuntia, etc. ; the gum-resin-ducts of the Umbelli- 

 ferse, and of some Araliacese (e.g. Ivy, Fig. 77) and Compositee 



ODQQC 



PIG. 78. Sac containing a crystal, 

 from the leaf of Rhamnus Frangula : e 

 upper epjdermis; p palisade-tissue: c 

 chloroplastids ; fc the crystal. ( x 200 ) 



FIG. 79. Part of section of the petiole 

 of the Camphor-tree ( Cinnamomum 

 Camphora), showing a resin-sac 7i. 



(e.g. the Sunflower). Here the cells of the group formed by a series 

 of divisions (Fig. 77 A E), separate from each other so as to leave 

 a passage, of which they form the wall, and into which they pour 

 their secreta. 



(c) Sacs, each consisting of a single cell. To this category 

 belong the cells which contain crystals, as those in the tissues of 

 many Monocotyledons (Fig. 58), in the bast of many dicotyledonous 

 trees, in leaves (Fig. 78) and petioles (Fig. 57) : the cells which con- 

 tain mucilage, as in the parenchyma of the Lime and the_Mallo\v, 

 in the bark of Elms and Firs, in the pseudo-bulbs of Orchids, etc. : 

 the cells which contain tannin, as in many Ferns and other plants : 

 the cells which contain oil-resin, as in the Lauracese (Camphor, Fig. 

 79), the Zingiberacese, many Conifers (wood of Silver Fir), etc. 



These cells are frequently arranged in longitudinal rows : for 



