CYCADOXYLEAE 491 



probably disappeared altogether. This change is in a 

 Cycadean direction, for the recent Cycads (as well as 

 the Mesozoic Bennettiteae) have wholly lost the 

 centripetal primary wood of their vegetative stems, 

 while retaining it in their leaves, and occasionally in 

 the peduncles of their cones. It is very interesting to 

 find that the leaf- trace bundles in the cortex of 



1 



Ptychoxylon retain the Lyginodendron structure : in a 

 typical Cycad they would not possess centripetal wood 

 in this part of their course. 



The extinction of <the centripetal xylem was, no 

 doubt, correlated with the advance of secondary growth. 

 As the centrifugal wood became more and more pre- 

 dominant, with the increasing activity of the cambium, 

 the small centripetal portion (the " Cryptogamic " wood 

 of the French authors) became insignificant in com- 

 parison, and could be dispensed with, so far as the 

 stem is concerned. In the roots there was a special 

 physiological reason, connected with the absorptive 

 function, for its retention, 1 and here it has held its own 

 all through ; in the leaves, where secondary tissue- 

 formation is less marked, it persisted in many cases, as 

 in Cycads, Cordaiteae, and perhaps Coniferae, but in 

 the highly modified sporophylls, even of Cycads, it 

 tends to disappear. 2 



On the other hand, the Cycadoxyleae became 

 modified in another direction, and one peculiar to 

 themselves among the allied groups. The anomalous 

 formation of inverted medullary wood and bast, which 

 in Lyginodendron only appears as an individual and 



1 See Strasburger, Histologische Beitrage, iii. p. 140. 



2 See Worsdell, " Vascular Structure of the Sporophylls of the Cycad- 

 aceae," Annals of Botany, vol. xii. 1898. 



