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R. BllAITHWAITE ON THE HISTOLOGY OF PLANTS. 



Disunited Vascular Bandies. — These lie isolated in the tissue of 

 the stem, and become separated from each other by layers of thin- 

 walled bast, with strongly-thickened parenchymatous intermediate 

 tissue. Such separated vascular bundles come nearest to those 

 of monocotyledons, and may be seen in the stem of Ranunculus, 

 Cucurhita, Bryonia, Taraxacum, &c. The wood part consists of a 

 few elongated porose wood cells, thin-walled parenchyma, and dis- 

 tinct tubular cells ; the innermost, narrow, and oldest vessels are 

 annular, then come wider and narrower spiral vessels, then netted, 

 and lastly wide vessels with bordered pores. The bast part con- 

 tains at its margin, next the bark, a bundle or crescent of bast 

 fibres, as in the Papaveracece, which sometimes approach to elon- 

 gated parenchyma, as in the stem of Cucurhita. Internal to these 

 come the bast vessels with net-like perforations, and having nar- 

 rower elongated parenchym cells intermixed. 



Not unfrequently a second bast bundle of thin-walled cells is 

 seen bordering on the wood part toward the centre of the stem, as 

 in Bryonia, Cucurhita, &c. 



The Cambium layer undergoes a complete transformation in its 

 later period of development, and loses its capacity for continued 

 formation, just as in Monocotyledons. 



Annular Vascular B undies of Herbaceous Plants. — The transition 

 from the last group to that seen in ligneous plants may be observed 

 in certain Umhelliferce, as Conium, Daucus, &c., and also in Lactuca, 

 where the bast bundles are separated yet farther apart by a layer 

 of formative parenchyma, the thickening ring; at first a closed 

 cambium cylinder, connected with the cambium of the vascular 

 bundle, and from this passing into a continuous ring, only inter- 

 rupted by narrower medullary rays, and in which isolated smaller 

 groups of cambial cells develope into vessels. 



In many plants of this group a large part of the bast vessels are 

 transformed into milk vessels, which anastomose with each other, 

 and with corresponding vessels in the bark. 



Vascular Bundles of Woody Dicotyledons. — These form in the 

 older internodes a completely closed ring, originating from isolated 

 strings of cambium, from which the separate elements are formed, 

 and which, as in herbaceous plants, unite into a closed ring by 

 further continued growth. At the end of the first year's growth 

 the vascular bundles resemble those described above, bast vessels 

 appear in the wood-mass lying between the primary bundles, nor 



