PAST HISTORIES OF PLANT FAMILIES 



149 



Fig. 106. — Diagram of 

 the Arrangement of the 

 Bundles at the Node of a 

 Calamite, showing how 

 those of consecutive in- 

 ternodes alternate 



n, Region of node 



wood steadily increased as the stems grew (there appear to 

 have been no "annual rings") till there was a very large 

 quantity of secondary tissue of regular 

 texture, thi'ough which ran small me- 

 dullary rays, so that the stems became 

 increasingly like those of the higher 

 plants as they grew older. It is the 

 primary structure which is the impor- 

 tant factor in considering their affinity, 

 and that is essentially the same as in 

 the other members of the family in 

 which secondary thickening is not 

 developed. As we have seen already 

 in other groups of fossils, secondary 

 wood appears to develop on similar 

 lines whenever it is needed in any 

 group, and therefore has but little value 

 as an indication of systematic position. 

 This important fact is one, however, which has only 

 been realized as a result of the study of fossil plants. 

 The longitudinal section of the stems, when cut tan- 



gentially, is very charac- 

 teristic, as the bundles 

 run straight down to 

 each node and there 

 divide, the neighbouring 

 halves joining so that 

 the bundles of each node 

 alternate with those of 

 the ones above and be- 

 low it (see fig. 106). 



The leaves which 

 were attached at the 

 nodes were naturally 

 much larger than those 

 of the present Equisetums, though they were similarly 

 simple and undivided. Their anatomy is preserved in 

 a number of cases (see fig. 107), and was simple, with 



s p 



Fig. 107. — Leaf of Calamites in Cross Section 



f, Vascular bundles; s, cells of sheath, filled 

 with blackened contents ; /, palisade cells ; 

 e, epidermis. 



