658 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of the formation of the secondary vascular biinclles in Yucca, Dracaena, 

 and Aloe. The phenomena are the same in essential points in the 

 three genera. 



Tne stem of any of these plants may be divided into three regions. 

 The central portion consists of a loose tissue in which are scattered a 

 comparatively small number of vascular bundles, becoming more 

 numerous towards their periphery. This portion is surrounded by a 

 usually annular zfine, sliarply differentiated and of closer and firmer 

 structure, in which are a much larger number of vascular bundles ; these 

 are usually arranged in concentric circles, giving the impression of the 

 annual rings of a dicotyledonous stem ; and they are even traversed by 

 radial stri^ resembling the medullary rays ; these are in connection 

 with the leaf-traces in the central cylinder. The outermost zone, the 

 cortex, is again composed of loose tissue, and is bounded on the outside 

 by a more or less developed layer of cork. This annular woody zone 

 continues to increase in thickness below, gradually disappearing towards 

 the summit, so tljat the entire mass resembles a truncated cone, the 

 result of a secondary increase in thickness, and surrounded by a zone 

 of meristem from which are formed new parenchyme and vascular 

 bundles. The leaf-trace-bundles bend out from the central cylinder to 

 the cortex. In each bundle the phloem lies towards the centre of the 

 stem, and is half surrounded by the annular and spiral vessels imbedded 

 in parenchyme ; and these are sometimes accompanied by tracheides. 

 In the leaf-trace-bundles the true vessels gradually disappear. The 

 secondary vascular bundles of the cone contain only tracheides and 

 parenchyme, never true vessels. 



The segments of the initial cells from which the thickening-ring 

 originates do not appear to divide in accordance with any general law; 

 nor do they form a ring, as in 1 )icotyledons and Gymnosperms. The 

 multiplication (if the thickening-rings also does not take place in the 

 same way ; and the divisions in the thickening-ring cannot be traced 

 back to initial cells, i. e. to cambium-mother-cells which are capable of 

 dividing without limit, forming alternately wood and cortex. 



With regard to the development of the secondary bundles, the 

 author does not agree with Kny that the tracheides are the result of 

 the coalescence of cells ; but, on the contrary, he believes that they can 

 originate in no other way than from the development of single cells in 

 the rudiments of the bundles. 



Primary Cortex in Dicotyledons.*— According to Herr Tcdin, a 

 cork-layer is usually formed during the first year in woody Dicotyledons, 

 either in the epiderm or in that portion of the primary cortex next the 

 epiderm; less often in the inner portion of this tissue, or in the 

 bast. When the cork-layer has an outer position, the cortical tissue or 

 primary cortex is usually more strongly developed, and has thicker 

 walls, than when it occupies a more central position ; the first case 

 presenting a resemblance to those plants which do not form cork during 

 the first year. When the cork-layer has a more internal position, the 

 primary cortex has usually ceased to be a living tissue at the close of 

 the period of growth, when it splits and peels off. When the cork- 

 layer has a more external position, or is wholly wanting during the first 



* SB. Bot. Ver. Lund, Feb. 25, 1888. See Bot. Centralbl., xxxvii. (1889) 

 jip. 300 and 380, and xxxviii. (1889) p. 727. 



