184 EMBRYOGENESIS IN PLANTS 



scales. The embryo, which is now deeply embedded in the prothallus, 

 extends the whole length of the seed ; the coleorhiza has become hard ; 

 the long suspensor is coiled and packed in the micropylar region; the 

 nucellus has practically disappeared, its substance having been trans- 

 ferred to the prothallus, which functionally is now the 'endosperm' 

 surrounding the germ; and the seed coat has hardened and matured 

 in characteristic ways. The mature seed is fairly uniform throughout 

 the group. In Cycas, Macrozamia and Ceratozamia, the seeds have 

 already been shed before the embryo has formed its cotyledons. 



The cotyledon number may be variable. Where two are present, 

 they may be of unequal size. Encephalartos is reported as having three 

 cotyledons. In Ceratozamia, which usually has one, the single coty- 

 ledon always develops on the side next the ground. If, however, the 

 seeds are revolved in a klinostat throughout the development of the 

 embryo, two cotyledons are typically formed (Dorety, 1908). The basal 

 regions of the cotyledons are conjoined and form a kind of tube. The 

 lobed or divided cotyledon tips have suggested to some observers that 

 more than two cotyledons are potentially present. 



The cycad seed has no resting stage. On germination, the coleorhiza 

 bursts through the seed coat; the root-tip digests and forces its way 

 through the base of the coleorhiza and begins to grow rapidly down- 

 wards; the cotyledons begin to protrude from the seed; and the new 

 leaf formed at the apex slowly grows out. The greater part of the 

 cotyledon(s) remains within the seed, serving as an absorbing or 

 haustorial organ. 



The vascular anatomy of the seedling is quite complex. In Dioon 

 edule, both of the cotyledons have four evenly distributed vascular 

 strands; these become concurrent in the short shoot and, together 

 with whatever cauline vascular tissue there may be, constitute a four- 

 sided vascular plate. Protoxylem groups are present at each corner. 

 Conjoined with the corners of this plate, and seemingly decurrent from 

 its lower side and continuous with its protoxylems, are the four pro- 

 toxylem strands of the first root. The first and second leaves which are 

 duly formed at the apex have each four separate vascular strands 

 (Thiessen, 1908); these become conjoined in the protoxylem regions 

 of the vascular plate. The vascular strand of the young sporophyte 

 shoot is protostehc but with the enlargement of the shoot it becomes 

 an endarch solenostele (siphonostele). In Microcycas, however, the 

 vascular plate is an endarch siphonostele from the outset (Dorety, 

 1909). The anatomical developments in these cycad embryos and young 

 sporophytes thus afford many interesting points for causal investigation. 

 Here it is worth referring to some remarkable petrified embryos of 

 Bennettitales which are dicotyledonous. The seeds appear to be 



