STRUCTURE OF FOSSIL PLANTS 71 



ferns, but it is a type of vascular arrangement which 

 is not common in the mature plants of the present day. 

 In the Coal Measure period, however, the protostele was 

 characteristic of one of the two main groups of ferns. In 

 different species of these ferns, the protostele assumed 

 a large variety of shapes and forms as well as the simple 



Fig. 49. — Lepidodendron, showing Part of the Hollow Ring of Primary Wood w, with 

 a relatively large amount of Secondary Tissue s, surrounding it 



cylindrical type. The central mass of wood became five- 

 rayed in some, star-shaped, and even very deeply lobed, 

 with slightly irregular arms, but in all these cases it re- 

 mained fundamentally monostelic. Frequently secondary 

 tissue developed round the protosteles of plants whose 

 living relatives have no such tissue. A case of this kind 

 is illustrated in fig. 48, which shows a simple circular 

 stele surrounded by a zone of secondary woody tissue 

 in a species of Lepidodendron. 



In many species of Lepidodendron the quantity of 



