22 THE MORPHOLOGY OF PTERIDOPHYTES 



xylem and secondary phloem are formed. Cambial cells 

 possess the power of cell division even though the surround- 

 ing tissues may have lost it; they may either have retained 

 this power throughout the lapse of time since they were laid 

 down in the apical meristem, or they may have regained it 

 after a period of temporary differentiation. While relatively 

 uncommon in living pteridophytes, a vascular cambium was 

 widely present in coal-age times, when many members of the 

 group grew to the dimensions of trees. Just as, at the present 

 day, all trees develop bark on the outside of the trunk and 

 branches by the activity of a cork cambium, so also did these 

 fossil pteridophytes. In some, the activity of this meristem 

 was such that the main bulk of the trunk was made up of the 

 periderm which it produced. 



Any attempt to interpret modern pteridophytes must 

 clearly take into account their forerunners, now extinct, in 

 the fossil record. This involves some understanding of the 

 ways in which fossils came to be formed and of the extent 

 to which they may be expected to provide information useful 

 to the morphologist. A fossil may be defined as ^anything 

 which gives evidence that an organism once hved'. Such a 

 wide definition is necessary to allow the inclusion of casts, 

 which are no more than impressions left in the sand by some 

 organism. Yet, despite the fact that casts exhibit nothing of 

 the original tissues of the organisms, they are nevertheless 

 valuable in showing their shape. At the other extreme are 

 petrifactions, in which the tissues are so well preserved by 

 mineral substances that almost every detail of the cell walls 

 is visible under the microscope. Between these two extremes 

 are fossils in which decay had proceeded, to a greater or 

 lesser degree, before their structure became permanent in 



the rocks. 



Under certain anaerobic conditions (e.g. in bog peat and 

 marine muds), and in the absence of any petrifying mineral, 

 plant tissues slowly turn to coal, in which little structure can 

 be discerned, apart from the cuticles of leaves and spores. 



