730 A TEXTBOOK OF THEORETICAL BOTANY 



period of free nuclear division continues longest at the base of the oospore, 

 and it is from this region that the suspensor and embryo arise. Following 

 this period there comes a simultaneous formation of cell walls throughout 

 the oospore, but all the central cells break down leaving a large cavity with 

 one or two cell layers round the outside. The first differentiation is that of 

 the suspensor cells near the base of the oospore, which elongate immensely, 

 pushing the lowest group of cells through the oospore membrane and right 

 down to the base of the prothallus. All the archegonia may be fertilized, 

 and each oospore may produce a suspensor. These all twist together and 

 form a compound structure supporting the one embryo which develops to 

 maturity. The coiled suspensors may be pulled out to a length of several 

 centimetres and their function seems to be simply to thrust the embryo into 

 the middle of the nutritive prothallus tissue. The tough oospore membrane 

 remains persistent for long after the embryo has grown and may be pulled 

 out of the ovule with the suspensors. Development is slow, and when mature 

 the embryo reaches the whole length of the seed. The suspensor is pushed 

 back and packed against the micropylar end. 



There are two cotyledons, sometimes unequal, and the embryo is perfectly 

 straight. The mature seed coat consists of the hard, stony integument with 

 its outer fleshy covering, the inner flesh having dried up. It encloses the 

 female prothallus, which functions as a nutritive endosperm, and the straight 

 embryo. The base of the embryo at the root end is marked by the develop- 

 ment of a massive hard pad, the coleorhiza, partly formed of the suspensors, 

 which appears even before the root itself is differentiated. It acts as a 

 protection while the expansion of the embryo is breaking open the hard shell 

 of the seed at germination, and it then turns papery and is pierced by the 

 emerging root. 



Germination 



Cycad seeds have no resting period. They germinate directly, and never 

 in any case keep their vitality for more than a few months. This points to a 

 primitive condition of the seed as compared to some of the highly persistent 

 seeds of Angiosperms. The cotyledons remain in the seed coat, below 

 ground, and continue active as long as there is any endosperm left for them 

 to absorb ; when this is finished they dry up. 



The seedling stem is very short indeed, and its vascular system forms a 

 plate rather than a column. At each of its four corners is a protoxylem, from 

 which strands descend to form the vascular poles of the tetrarch root. Two 

 of these may be suppressed in cases where the root is only diarch, and in 

 any case two of them usually develop further than the other pair, so that the 

 root becomes diarch near its tip. From the same protoxylems four strands 

 pass upwards and forthwith divide. They go to the two cotyledons, each 

 of which receives four. It will be realized that this seedling structure is an 

 autonomous organization, complete in itself, and the solid vascular plate is 

 the only truly cauline stele the plant possesses at any stage. Above this level 



