528 SECRETORY AND EXCRETORY SYSTEMS 



leaves of Laurus nobilis have a similar structure, except that the cup- 

 shaped stalk is exceedingly minute (Fig. 213 c). 



The development of these " attached " oil-vesicles has been investi- 

 gated by Bud. Miiller, working in the author's laboratory, in the case 

 of Aristolochia brasiliensis. Quite contrary to expectation, he found 

 that the stalk of the vesicle does not, as Berthold believed, originate 

 as a local expansion of the cell-wall. On the contrary, a large oil- 

 vacuole arises in the cytoplasm by the fusion of numerous smaller 

 ones and sooner or later puts forth a conical process towards the 

 cell-wall. The plasmatic membrane of the vesicle then becomes trans- 

 formed into the pellicle surrounding the oil-vesicle and its stalk, which 

 is consequently continuous with the general cell-membrane. One is 

 thus really dealing with an oil- vacuole, which originates within the 

 cytoplasm, and only secondarily becomes connected with the cell-wall 

 at a single point. The significance of this peculiar process of attach- 

 ment is quite unknown. 



2. Tannin-sacs. 261 



It has been remarked, on a previous occasion, that the compounds 

 known as tannins or tannic acids, while possibly acting as plastic materials 

 in certain circumstances, undoubtedly often represent by-products of 

 metabolism. Compounds of this class are usually deposited in sacs 

 arranged in long rows in close association with vascular strands. Such 

 tannin-sacs are found in the parenchymatous tissues of the stems and 

 petioles of many Ferns ; they also occur in the Araceae and Musaceae 

 in connection with the vascular bundles, in Phaseolus multifiorus and 

 other Leguminosae in the leptome-strands of the primary axial and 

 foliar bundles, and in Phaseolus also in the pith opposite each bundle. 

 The most remarkable tannin-sacs, however, are those which are found 

 in the cortical and medullary parenchyma of the stem of Sambucus. 

 Dippel estimates the length of one of these sacs in the fully developed 

 state at 18-20 mm. or more, and its average width at , 025- , 164 mm. 

 According to De Bary, however, it is probable that individual sacs 

 may extend through an internode, that is, over a distance of 20 cm. or 

 more. Each sac consists of a single enormously elongated spindle- 

 shaped cell. 



Closely related to tannin-sacs are the more or less tubular epi- 

 dermal tannin-containing cells observed by Engler in Saxifraga 

 Cymbalaria and allied species, and in Sedum spurium. In these 

 plants the ordinary epidermal cells are isodiametric, with wavy radial 

 walls, so that the elongated, scattered or seriated tannin-sacs are very 

 conspicuous. 



The ecological importance of tannin as a means of protection 



