﻿EXISTING AND FOSSIL CYCADS COMPARED. 



213 



The bundles are deeply embedded in the parenchyma in all but a few species, 

 and mainly determine the nervature, being often accompanied by or alternating with 

 a strand of hypodermal sclerenchyma. Nervation is of three characteristic kinds: 



(1) A mid-nerve without lateral nervules: Cyras. 



(2) A mid-nerve with numerous simple or once to twice dichotomizing and 



occasionally anastomosing lateral nervules: Stangeria. 



(3) Numerous equal parallel curving and from one to three times dichotomiz- 



ing nerves, the remaining genera. (Also Cycadeoidea and Cycadella.) 

 It is noteworthy that among the fossil forms belonging to diverse families of 

 the " Cycadophytes, " as the great group of eycad-leaved plants has sometimes been 

 called, there is much greater variation in nervature. Dictyozamites (fig. 138) is 

 netted-veined without a mid-nerve, or " dictyopterid, " and Otoza»iites has an odon- 

 topterid venation. 



Resume of Vegetative Characters of the Cvcadace^. 

 Trunk Types. 



(a) Tuberous. 



Zamia. 



Stangeria. 

 \ Bowenia. 

 I Microcycas. 

 ( Macrozamia (in part). 



(b) Columnar-. 



f Macrozamia (in part). 



I Encefhalartos. 



{ Dion. 



I Ceratozamia. 



y Cycas. 



Armor Types. 



(a) Armor not mark- 

 edly persistent 



Stangeria. 

 Bozi enia. 

 Zamia. 

 Microcycas. 

 Cycas (in part). 



f Dion, 



(b) Armor mainly per- J Encefhalartos. 



sistent ........} Ceratozamia. 



I Macrozamia (m part). 



[ Cycas (most species). 



Wood Zone. 



la) Monoxylic 



f Zamia. 



I Dion. 



\ Stangeria. 



I Ceratozamia . 



I Microcycas (?). 



(b) Repeated anom- 

 alously or poly- 

 xylic 



f Cycas. 

 I Macrozamia. 

 J Encephalartos. 

 (. Bowenia. 



(a) Gum canals only-. 



f Cycas. 

 I Zamia. 



Dion. 

 I Bowenia (?). 



Microcycas ( ?). 

 I Stangeria. 

 I Ceratozamia . 



Pith Types. 



fbj Gum canals and | 



asystem of can- J Encephalartos. 

 line bundles i n | Macrozamia. 

 Pith I 



FRUCTIFICATION. 



The existing cyads areall dioecious. The stamiuate fronds or microsporophylls 

 are throughout the group reduced to scale-like form and organized into typical 

 bractless cones, which are borne at the summit of the trunk, but are not always 

 terminal to the main stem, since occurring in pairs or small groups as branches. 

 (See figure of Zamia floridana.) The same is true of all the ovulate fronds, except 

 in the genus Cyras, in which the crowns of carpellary leaves are strobiliform, but 

 alternate more or less regularly with the foliar crowns. In other words, the stro- 



