388 GYMNOSPERMS 



The leaf. — There are only two leaves, the original pair of leaves of 

 the seedling persisting throughout the life of the plant, which is esti- 

 mated at more than loo years. The leaves are entire, until they 

 reach a length of 1 5-20 cm., when they begin to split at the tip (figs. 

 565, 366, and 369). Still older leaves, lying on the ground and flap- 

 ping in the wind, are split into ribbons. The leaves are thick and 

 leathery and the venation is parallel; consequently, the spUts are 

 long and straight. The cells exposed by the spHtting become suber- 

 ized so quickly that the plant suffers no damage. 



The leaves grow from the base of a groove, which extends nearly 

 half-way around the top of the crown, and functions as a moist 

 chamber. The base of the leaf remains permanently meristematic, 

 while the tip keeps wearing and dying off. In old plants the leaves 

 are about 2 meters long. 



Welwitsch did not beUeve that the plant had any leaves at all, 

 but only two cotyledons, which persisted throughout the life of the 

 plant. Hooker's material was too old either to correct or confirm 

 that view; but he adopted it tentatively, remarking that it would be 

 easy to determine the facts as soon as anyone got a seedling. These 

 have now been seen by many observers. There are two cotyledons 

 and, at right angles to them, the two leaves (fig. 380). 



The root.— As already indicated, the root is very long, and must 

 go down to the water table. People have tried to digit up, but, after 

 digging for several feet, have concluded, probably correctly, that it 

 kept on going down until it reached the water. Dioon spinulosum, 

 growing on a rock, with scarcely any soil, had roots hanging down 

 the vertical face of the rock for 12 m., when — still about 5 cm. in di- 

 ameter — they disappeared in a crack in the rock. The roots of cy- 

 cads, in regions as dry as the W elwitschia country, go down to the 

 water table and keep the plants green, while the grass is yellow, or 

 even scorched. The roots of W elwitschia must go down as deep. 



Histology. — The most spectacular feature of the histology is the 

 great display of spicular cells. They are usually branching, very 

 thick walled, lignified outside and cellulose inside, and incrusted 

 with crystals of calcium oxalate (fig. 367). These cells, branching 

 and interlocking, are formed in the stem, root, and leaf, and they 

 make free-hand sectioning impossible. One might as well try to cut 



