606 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



INTERNAL STRUCTURE OF STEMS. 

 By Rev. Prof. G. Henslow, M.A., V.M.H., &c. 



[Lecture to the Students at the Society's Gardens, July 2, 1902.] 



On cutting any rather soft herbaceous stem of one year across, within the 

 epidermis or skin, will be seen a mass of colourless tissue, except near the 

 circumference, where it is green. Just within this green layer will be 

 seen a circle of dots. These are the cut ends of the so-called " fibro- 

 vascular bundles," which for brevity I shall call "cords." 



In order to support such a stem various kinds of tissues are resorted 

 to, one of the commonest being called collenchyma, which might be trans- 

 lated " gluey- tissue." This consists of elongated several-sided cells, having 

 the angles greatly thickened. It is found on the periphery of the stem, 

 often forming external columns, as at the four corners of the square stem 

 of the Dead-Nettie. 



Another kind of supportive tissue on the circumference is called 

 sclerenchyma, or " hard-tissue." It consists of long fibres with very thick 

 walls. Thousands of these cells compacted together form very strong 

 strands. They may be seen externally in the vertical raised lines on the 

 stems of Docks. 



If a stem becomes woody, as in shrubs or such herbs as Fuchsias, then 

 the number of cords increases till they form a compact cylinder, separating 

 off the pith or medulla in the middle from the cortex on the outside, while 

 these cords are generally kept apart in places by means of the flat ribbon- 

 like medullary rays. This cylinder of wood prevents the shoot from 

 breaking under flexure. 



We must now consider the structure of a woody shoot. 



The tissues form two well-marked groups, the " cortical " and the 

 "central cylinder," in an ordinary stem of a dicotyledon. 



Within the epidermis is the active, growing layer out of which not 

 only are collenchyma and sclerenchyma formed in herbaceous shoots, but 

 the cork of woody ones. This layer now takes the name of phellogcn, a 

 Greek word meaning " cork-generator." When the leaf falls in autumn 

 it is because this layer is formed right across the base of the leaf-stalk, 

 and as the cells die as soon as they become cork the leaf is attached to a 

 dead layer, so that it then falls away on a puff of wind. 



The epidermis, cork, and phellogen are colourless, so that light 

 penetrates to a deeper layer which becomes green by the development of 

 chlorophyll granules under its influence. 



The inner boundary of the cortex consists of a cylinder of cells which 

 not infrequently c mtain starch held in reserve. Hence it is sometimes 

 called the starch-layer as well as cndodcrm, that is, the " within " or lining 

 skin ; just as epidermis means the " upon " or superficial skin. 



The usual difference between the stem of a dicotyledon and a 

 monocotyledon is, that in the former the cords form a compact cylinder 

 of wood having the cambium, or active layer which forms a fresh cylinder 

 of wood, outside that of the previous year ; whereas in a monocotyledonous 



