STRUCTURE OF FOSSIL PLANTS 



63 



Fig. 39. — A Cylindrical Stele, with e, inner 

 endodermis, and ph, inner phloem; w, wood; 

 p, outer phloem; E, outer endodermis. L, 

 part of the stele going out to supply a large 

 leaf, thus breaking what would otherwise 

 appear as a closed ring stele 



sheath or endodermis generally develops on the; inner 



side of the wood, outside the pith, and cuts the vascular 



tissues off from the inner 



parenchyma. A further 



step is the development 



of an inner cylinder of 



bast so that the vascular 



ring is completely double, 



with endodermis on both 



sides of the cylinder, as is 



seen in fig. 39. 



In all these cases there 

 is but one strand or cylin- 

 der, of vascular tissue in 

 the stem, but one stele, 

 and this type of anatomy 

 is known as the monostelic 

 or single-steled type. 



When from the double 

 cylinder just described a strand of tissue goes off to 

 supply a large leaf, a considerable part of the stele goes 

 out and breaks the ring. This 

 is shown in fig. 39, where l 

 is the part of the stele going 

 to a leaf, and the rest the 

 broken central cylinder. When 

 the stem is short, and leaves 

 grow thickly so that bundles 

 are constantly going out from 

 the main cylinder, this gets 

 permanently broken, and its 

 appearance when cut across at 

 any given point is that of a ^.^ ^^ _^ ^^.^^ g^^,^ ^pp^^^„^,y 



group of several steles arranged broken up into a Number of Protosteles 



in a ring, each separate stele by many Leaf Gaps 



being like the simple proto- 



stele in its structure. See fig. 40. This type of stem 



has long been known as polystelic {i.e. many-steled), and 



