XXVIH] ANATOMY 7 



which on bursting liberates the motile sperms with the watery 

 cell-sap. Fertilisation is succeeded by the development of a 

 homogeneous proembryo partially or completely filling the zygote 

 (fertilised egg) : by the formation of long suspensors the embryo 

 is brought into contact with the food-store of the prothallus. 

 In some Cycads, e.g. Encephalarlos, the embryogeny exhibits a 

 close resemblance to that of Ginkgo 1 . The embryo is dicotyle- 

 donous 2 . 



The single stele of the stem is characterised by a large pith 

 which in some genera (e.g. Encephalartos, Macrozamia) contains 

 an anastomosing system of collateral bundles. The vascular 

 tissue of a cycadean stem forms a cylinder of secondary xylem 

 and phloem, the primary xylem being represented only by a few, 

 usually crushed, protoxylem elements on the inner margin of 

 the reticulately pitted or scalariform tracheids. Both xylem 

 and phloem are traversed by numerous broad and deep medullary 

 rays 3 . The looser texture and more parenchymatous structure 

 of Cycadean wood afford a ready means of distinguishing it from 

 the wood of Conifers : for the Cycadean type the term manoxylic 

 is proposed and pycnoxylic for the more compact coniferous 

 wood 4 . Kims (or 'bars') of Sanio, of which much has been said 

 in discussions on the phylogeny of Conifers, have recently been 

 described in the petiolar xylem of Cycas revoluta : the rims are 

 short and 'cling closely to the borders of the pits,' features which 

 also characterise the rims found in the cones of the Araucarineae 

 and in the root- and cone- wood of certain Pines 5 . In some 

 Cycads the secondary xylem and phloem form a single cylinder, 

 but in others (Cycas, Encephalartos, Macrozamia, Bowenia) the 

 cambium is succeeded by one or several concentric cylinders 

 of meristem which have their origin in the pericycle. The 

 spasmodic occurrence of separate arcs of inversely orientated 

 secondary xylem and phloem between the normal cylinders is 

 a feature of importance from the point of view of comparison 



1 Saxton (10 4 ). 



2 For an explanation of the occurrence of a single cotyledon in Ceratozamia, 

 see Dorety (08). 



3 WorsdeU (00); (01); Chamberlain (11). 



4 fji.av6s, porous, loose in texture; TTVKVOS, compact. 



5 Sifton (15). 



