66 



ANCIENT PLANTS 



shows both primary distinct groups in the centre, and 

 the rings of growth of later years. 



Cambium with this power of long-continued activity 

 is found in nearly all the higher plants of to-day (except 

 the Monocotyledons), but in the fern and lycopod groups 

 it is in abeyance. Certain cases from nearly every 

 family of the Pteridophytes are known, where some 

 slight development of cambium with its secondary 



thickening takes place, 

 but in the groups below 

 the Gymnosperms cam- 

 bium has almost no part 

 to play. On the other 

 hand, so far back as the 

 Carboniferous period, the 

 masses of wood in the 

 Pteridophyte trees were 

 formed by cambrum^ in 

 just the same way as 

 they are now in the 

 higher forms. Its pre- 

 sence was almost uni- 

 versal at that time in the 

 lower groups where to- 

 day there are hardly 

 any traces of it to be 

 found. 

 It will be seen trom this short outline of the vascular 

 system of plants, that there is much variety possible 

 from modifications of the fundamental protostele. It is 

 also to be noted that the plants of the Coal Measures 

 had already evolved all the main varieties of steles which 

 are known to us even now,^ and that the development 

 of secondary thickening was very widespread. In, 

 several cases the complexity of type exceeds that of 



Fig. 44.— Stem with Solid Cylinder of Wood 

 developed from the Cambium, showing three 

 "annual rings " In the centre may still be seen 

 the separate groups of the wood of the primary 

 ' ' vascular bundles " 



^ Though the Angiosperm was not then evolved, the Gymnosperm stem has 

 distinct vascular bundles arranged as are those of the Angiosperm, the difference 

 here lies in the type of wood cells. 



