BY M. B. WELCH. 



479 



duets. The duct is formed therefore by the linking up of a number of small 

 cavities, each corresponding to a single oil gland, and formed by the division 

 of a single meristematic cell in certain positions in the stems and leaves. Early 

 stages in this development ai'e shown in Text-tig. 4. 



Subsequent division causes the formation of a group of cells, characterised 

 by their denser contents, arranged in an almost spherical mass. 



Text-fig. 5. 

 li. corymbosa showing 



Text-fig. 4. 



Text-fig. 4. — Transverse section of a stem apex of 

 initial stages in the development of a duct. 



Text-fig. 5. — Later stages in the development of the duct than are shown 



in Text-fig. 4, but the formation of the central cavity is just commencing. 



Separation of these cells then occurs internally, early stages being shown 

 in Text-fig. 5 a, b, finally leaving a distinct central cavity (Text-fig. 1). There 

 still remains a distinct wall of cells separating each cavity, until, as already 

 mentioned, these barriers break down forming an individual duct." The central 

 cavity has in the meantime enlarged considerably in section, evidently due to 

 natural growth of the surrounding tissues and partly due to disintegration of 

 the inner cellular tissue. Taking into account the mode of their formation, it 

 is evident that these ducts are first formed lysigenously, and later on schizo- 

 genously, a mode of formation to which Tschireh applies the term schizoly- 

 sigenous. 



In young leaves there is usually no trace of ducts until they are about 

 0.75 mm. in width, whereas oil glands appear in leaves with a diameter of 

 less than 0.2 mm. They are never found in the apex of the stem or leaf, e.g. in 

 PI. xlvi., fig. 6, the larger leaves are about 3 mm. in length and the ducts only 

 ex'tend about 1/3 of the distance from the base. As already pointed out. this 

 is also a characteristic feature in mature leaves. 



It is interesting to trace the course of these ducts in the leaf -bud ; Text- 

 fig. 6 a shows a section through the numerous, almost opposite, leaves towards 

 the apex of the bud of E. corymbosa, and, althtiugh ducts are well developed in 

 the outer leaves, there is no trace of them yet in the inner ones; Text-fig. fi b 

 is somewhat nearer the base of the bud, and shows a central stem with four 

 very small duets in very early stages. On either side are the two axillary 

 stems with no ducts, and outside these again are the petioles of the outer 



