stalk-cell, and body-cell, and from the body-cell are developed two spirally ciliated spermatozoids (fig. 396, M). In this respect also the monotypic genus _Microcycas_ is peculiar: it may have as many as 8 body-cells and 16 male gametes in a single pollen-tube (fig. 396, G), while in _Ceratozamia_[13] 4 gametes have been seen in one tube. The pollen-tube grows like a fungal mycelium into the nucellar tissue and the male gametes are formed in the distended proximal end 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._ _Encephalartos_, the embryogeny exhibits a close resemblance to that of _Ginkgo_[14]. The embryo is dicotyledonous[15]. 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[16]. 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[17]. Rims (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[18]. 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 with the Palaeozoic Medulloseae[19]. The occurrence of concentric cauline strands in the cortex of _Cycas_ is also a peculiarity worthy of notice. Successive bands of periderm, and occasionally a considerable amount of phelloderm[20], are formed in the peripheral region of the stem.