I50 GYMNOSPERMS 



Thiessen^'^ found that the vascular cylinder of the stem is very 

 short, so that it may be called a vascular plate rather than a cylinder 

 (fig. 1 68). The plate is squarish and has a protoxylem group at each 

 corner, from each of which a strand extends downward, forming the 

 protoxylem of the tetrarch root. Four strands extend in the op- 

 ix)site direction, soon forking and entering the cotyledons, so that 

 each cotyledon gets four strands. For each of the first leaves four 

 strands leave the vascular plate at or near its four protoxylem 

 points. 



An interesting feature is the change from the endarch to the exarch 

 condition. The cotyledonary bundles, as they leave the vascular 

 plate, are endarch; but, farther along in the cotyledon, they become 

 mesarch. The leaf traces also, when leaving the vascular plate, are 

 endarch; but, in the leaf base, centripetal xylem appears, so that the 

 bundle becomes mesarch. From this point the centripetal xylem in- 

 creases and the centrifugal decreases until the bundle becomes en- 

 tirely exarch before it reaches the region of leaflets. Beyond this point 

 the bundles are exarch. 



The vascular cylinder of the embryo is a protostele; but in older 

 stages it gradually becomes an endarch siphonostele, except in Mi- 

 crocycas.^^^ 



Sister Helen Angela'^' also found that in Microcycas the vascu- 

 lar strands of cotyledons and leaves are endarch near the base and 

 exarch in the upper portions. The vascular plate, from its first ap- 

 pearance, is an endarch siphonostele, probably the only siphonostele 

 in early stages of a cycad seedling (fig. 169). 



Matte, ^'' and others who have studied the embryogeny of the 

 cycads, agree that the stem consists of leaf bases. This would mean 

 that the adult stem is a mass of leaf bases, in which secondary growth 

 has produced the familiar trunk. This interpretation is suggestive 

 and may have a wider application. 



The seedling usually has just one leaf. Leaf after leaf appears at 

 irregular intervals, and it is likely to be several years before leaves 

 begin to appear in crowns. It is rare for a cycad to produce a cone 

 before the tenth year, and some are probably much older before the 

 coning stage is reached. The demarcation between seedling and adult 

 plant is as indefinite as that between baby and boy — and between 

 boy and man. 



