82 



ANATOMY OF THE GYMXOSl'KRMS 



feature is a characteristic of 52.6 per cent of all the genera, inclu- 

 sive of Gingko, from Dammara to Sequoia, while it also appears in 

 Cupressus and Abies in part as exceptional, and in the genus Pinus 

 to the extent of 85.3 per cent. The wall presents no secondary 

 growth in thickness, either locally or generally. In the majority 

 of cases it crosses the line of the principal cell axis either at right 

 angles or diagonally, - - features which are usually of very second- 

 ary value, although in a few cases, as Taxodium, it may serve a 

 useful purpose as an associated character for differentiation from 

 closely allied genera (fig. 20). In other cases the wall is more 

 or less strongly curved. This feature is prominent in Thuya, 



Cupressus, Podo- 

 carpus, Thujopsis, and 

 Cryptomeria, as also in 

 the more highly devel- 

 oped hard pines. To 

 a less extent it also oc- 

 curs in Taxodium, and 

 it constitutes a char- 

 acter of some value for 

 differential purposes 



(fig. 21). 



The second variant 

 differs from the first 

 in that the otherwise 

 thin wall is locally thickened (fig. 22), the secondary growth 

 forming one or more beaded enlargements. This is a feature 

 which occurs exceptionally in Abies, Pseudotsuga, Picea, and 

 Pinus, but it is typical in Cupressus (66 per cent) and Juniperus 

 (72.7 per cent), where it constitutes a diagnostic element of great 

 value. It is in all cases, however, to be regarded as a transi- 

 tional form between the first and the third variant, and from this 

 point of view it also possesses a somewhat definite phylogenetic 

 value. The third variant is characteri/ol by a marked general, 

 secondary growth of the wall, which thereby becomes more or 

 less strongly thickened and traversed by numerous simple pits 



FIG. 21. THUYA GIGANTEA. Medullary ray show- 

 ing the form and disposition of the pits on the 

 lateral walls ; the thin and curved terminal walls ; 

 the cells contracted at the ends, x 280 



