THE ANATOMY OF STEMS. 255 



substance, and terminating in leaves ; and its vitality 

 being, in a great degree, dependent on the herbaceous 

 part, if the central bud, or cabbage, as it is common- 

 ly called, be cut oft, the whole plant immediately dies. 

 The height to which some Palms arise, without in- 

 creasing in diameter, is very remarkable. Thus the 

 Ptychosjjerma gracilis rises more than GO feet, with a 

 stem not 4 inches in thickness. The elevation of the 

 Areca oleracea (Betel-nut Palm) is often not less than 

 180 feet; and although its diameter is greater than 

 that of the Ptychosperma, yet it is certain that it nev- 

 er increases in thickness. In tropical climates, some 

 kinds of Ferns rise with a stipe resembling that of the 

 Palms ; but this appears to be, according to Mirbel, 

 a simple fasces of petioles or leaf-stalks ; although cir- 

 cumstances occasion these to unite in the interior of 

 the stipe, and form masses of compact wood. This 

 variety of stipe remains also of the same diameter. 



The Aloes, Yucas, and the Dracaena differ in their 

 mode of growth from the palms, inasmuch as they 

 give off branches and increase in the diameter of their 

 stems. 



Such is the structure and mode of growth in mono- 

 cotyledonous stems. The positive features which 

 chiefly characterize them in point of structure, are the 

 separate vascular ligneous cords, and intermixed cel- 

 lular parenchyma ; but they are distinguished more 

 remarkably by negative qualities ; as, for example, 

 those of having no proper bark, liber, or alburnum, 

 and no medullary rays ; parts which belong exclu- 

 sively to the dicotyledonous stems. 



Dicotyledonous Stems. 



Woody Dicotyledonous stems consist of 3 dis- 

 tinct parts, the bark, the wood, and the pith. They 



