SACS CONTAINING RESINS AND GUM-RESINS. 151 



especially in the tuberous roots of Ipomoea Purga, where they form numerous annular 

 zones ^, 



Each single sac of a series in slightly elongated members, e.g. in the tuberous roots 

 cited, is short, being not longer, or even shorter than broad: in elongated internodes 

 they attain a considerable length, and an extended cylindrical form with Hat or slightly 

 curved ends. 



The contents of the sacs are a mass of resin, mixed to a variable extent with watery 

 fluid, and presenting therefore an appearance which varies in different cases (comp. 

 Trecul, Lc.) ; in many investigated cases it contains tannin. The walls are thin, homo- 

 geneous, and apparently soft, and show, as far as investigated, no cellulose coloration. 

 With iodine and sulphuric acid they turn yellow : long treatment with the acid does not 

 destroy them. 



On the fresh surface of section through the sac-bearing parts the contents of the sacs 

 exude as 'latex,' the more abundantly the longer and more numerous the sacs are. In 

 fresh plants it often appears that the pressure of the contiguous turgescent parenchyma, 

 which presses the milky fluid out from the cut sacs, can burst also the transverse walls, 

 which are not touched in cutting the section, and press out the contents of more deeply- 

 seated members of a series of sacs at the surface of section. I was unable to prove to 

 myself (after investigations on stems and rhizomes of Convolvulus arvensis, Calystegia 

 sepium, dahurica, Pharbitis hispida) either that there is a perforation or solution of the 

 septa within the living plant, and a formation in this manner of long sacs by the coales- 

 cence of shorter ones, or that there is a genetic connection, as stated by Vogl, between 

 long sacs and sieve-tubes. 



8. The reservoirs of the milky secretion in the Sapotaeese resemble those of the Con- 

 volvulaceas in many points. Since these are but little known, a report by Herr K. Wil- 

 helm upon an investigation of them conducted by him may be here inserted. He 

 investigated especially Bumelia tenax W., and Sideroxylon mastichodendron Jacq., with 

 which, as far as can be concluded from comparison of dried material. Isonandra Gutta 

 coincides in the main. 



The latex of these plants occurs in completely closed sacs, which are always surrounded 

 by parenchymatous elements, and differ from these fundamentally only in their contents. 

 This is literally true for the inner cortex, the reservoirs of latex here found have exactly 

 the form and size of the neighbouring parenchymatous cells. In the outer cortex and in 

 the pith the laticiferous elements are usually distinguished from the rest by their con- 

 siderable length and breadth, as wdl as by their arrangement in uniseriate strings, which 

 run longitudinally through the axis in question, and may be followed to within a short 

 distance of the punctum 'vegetotiojiis. The outer cortex and pith are thus traversed by 

 single rows of laticiferous sacs, which are arranged, at least ia the youngest parts of the 

 stem, radially and tangentially perpendicular ; new elements are constantly added to 

 these from the apical meristem. As the rows pass downwards in the stem, their originally 

 parallel arrangement is disturbed by the increase of the intermediate parenchyma : they 

 suffer tensions and fractures, — their several parts however remain nevertheless in con- 

 nection, and at the same time their character as series of distinct sacs is retained. No 

 single case was observed which necessitated or even supported the assumption that, 

 in the living plant, a coalescence of neighbouring members of tubes had taken place as a 

 typical occurrence. 



Also in the inner cortex, in the phloem of the vascular ring, it was never possible to 

 prove with certainty a coalescence of parallel laticiferous sacs, or of such as touched at 

 their ends, so as to form extensive reservoirs. In tangential sections the primary ar- 

 rangement shows no regularity: they usually lie scattered and solitary, but sometimes 

 several occur near to, or one above another, between large parenchymatous cells of 

 similar form. In radial sections they appear sometimes to form long longitudinal 



Compare Berg. Atlas, Tab. XXIII. 



