62 THE EVOLUTION OF PLANTS 



In most Cycads each leaflet is traversed by a 

 number of more or less parallel veins; in Stangeria 

 there is a well-marked midrib, and the veins 

 which pass off from it on either side are forked 

 like those of a Fern-frond; in Cycas itself, each 

 leaflet has a midrib only, with no lateral veins 

 at all. 



In many Cycads the stem is clothed with an 

 armour, made up of the bases of the old leaves, 

 which continues to protect the trunk for many 

 years, as in Encephalartos, Dioon, Ceratozamia, 

 Macrozamia (fig. 2), and most species of Cycas. 

 In the rest of the family the armour is not so per- 

 sistent. In most species scale-leaves are produced 

 at intervals, and the armour is then composed of 

 alternating zones of the larger and smaller leaf- 

 bases, corresponding respectively to the green 

 leaves and the scale-leaves. 



The stem has a very large pith, the diameter 

 of which may be half that of the stem as a whole; 

 in this respect the Cycads differ conspicuously 

 from Conifers and from most Dicotyledons. 

 Otherwise the structure of the stem, at least in 

 its simplest form, is of the usual Gymnospermous 

 type, which, as we have seen, is common to Di- 

 cotyledons also. The zone of wood and bast is 

 often poorly developed, the stem having a succu- 

 lent character; thus in a specimen of Dioon edule, 

 a Mexican species, the stem, though 120 years 



