STEMS OF MONOCOTYLEDONS 



153 



this intercalation takes place, as we have seen (Sec. 213), 

 at the nodes, forming the hard rings known as joints, but 

 in other monocotyledons the fibers entering the stem from 

 the leaves generally tend first downwards, towards the 

 interior (Fig. 297), then bend outward toward the sur- 

 face, where they become entwined with 

 others and form the tough, inseparable 

 cortex that gives to palmetto and bamboo 

 stems their great strength. 



This addition of fresh vascular bundles 

 as the axis lengthens will explain why the 

 lower joints of cornstalks and sugar cane 

 are so much more hard and woody than the 

 upper ones. Generally, however, mono- 

 cotyledonous stems do not increase in diam- 

 eter after a certain point, and as they can 

 contain only a limited number of vascular 

 fibers, they are incapable of supporting an 

 extended system of leaves and branches. 

 Hence this class of plants, with a few 

 exceptions, like smilax and asparagus, are 

 characterized by simple, columnar stems, and a limited 

 spread of leaves. The cabbage palmetto, banana, and 

 Spanish bayonet ( Yucca aloifolia) are familiar examples in 

 the warmer parts of our country. 



298. — A palm 

 tree, showing the 

 tall, branchless 

 trunk of monocoty- 

 ledons. 



218. Strength of the Monocotyledonous Structure. — 

 Stems of this class are admirably adapted by their struc- 

 ture to the purposes of mechanical support. It is a well- 

 known law of mechanics that a hollow cylinder is a great 

 deal stronger than the same mass would be in solid form, 

 as may easily be tested by the simple experiment of break- 

 ing in your fingers a cedar pencil and a joint of cane 

 or a stem of smilax of the same weight. In stems that 

 may be technically classed as soHd in structure, like 

 the corn and palmetto, the interior is so light compared 

 with the hard epidermis that the result is practically a 

 hollow cylinder. 



