290 R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 



more thickened as they approach the surface, until hardly any 

 lumen is seen in the most external. The wood part is also in form 

 of a ring, which is only interrupted on one side by a small strip of 

 bast, uniting the bast tissue on the inner and outer side of the 

 wood. 



Vascular Bundles of Monocotyledons. 



These, in the most perfect condition, comprise all three forms of 

 cells, which are arranged in two groups, constituting a wood and 

 bast part in each vascular bundle. These arise from the cambium, 

 and it is that portion next the pith which becomes transformed 

 into wood, and that on the cortical side which becomes bast. The 

 cambium quite disappears with the full development, being trans- 

 formed into permanent parenchymatous tissue. Many variations 

 occur in the development of the individual elements, and their 

 position to each other. The wood fibres are more or less thickened, 

 and always lignified and scattered ; faintly bordered pores occur in 

 them. The bast fibres resemble the wood fibres, but have more 

 pointed extremities, and are thickened in various degrees. The 

 tubular or vascular cells of the wood part consist of annular, spiral, 

 netted, or porose vessels, which communicate with each other by a 

 large pore or scalariform perforation. The bast vessels were called 

 by Von Mohl Vasa propria, and lie between the parenchyma sur- 

 rounding the vessels of the wood and the bast fibres ; besides these 

 bast vessels of the vascular bundle, there occur in certain genera, 

 especially in Liliacece, Musacece, and Aracece, wider bast vessels 

 close under the rind, with latticed partitions, which carry a peculiar 

 coloured sap, and have been regarded as milk vessels. They 

 belong to the bast part of the vascular bundle. 



The parenchym cells occur in both parts, and are usually much 

 elongated, and known from fibre cells by theii- horizontal transverse 

 partitions. 



We follow out the origin of the vascular bundles and gradual 

 onward formation of the individual elements from the cells of the 

 cambium layer, either from the germinating seed or in the buds 

 from the permanent shoots of the plant ; the young date palm, 

 Buscus, and Liliacece are well fitted for the purpose. The first 

 elements of the Vascular bundle are some bast fibres, then in the 

 wood part we observe one or two annular vessels, following 

 the formation of young wood cells. While by little and little the 

 bundle of bast fibres becomes enlarged, and the thickening pro- 



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