386 GYMNOSPERMS 



diameter may reach more than 4 feet, 1.2 meters. Below the surface 

 of the ground the stem tapers abruptly, and an extremely long tap 

 root extends downward to the water table ; for, in such a dry country, 

 little or no water is taken in, except by the root. 



The plants sometimes grow in clusters of two or three, or even four 

 or five, probably coming from seedlings of an older plant; and such 

 plants, coming into contact, become fused together, so that points 

 of union are indistinguishable. It might be called a natural graft. 



The stem. — The tip of the young axis is convex, but growth at the 

 apex soon stops, while it continues near the periphery, making the 

 apex depressed (fig. 366). 



HooKER^"^ called the part of the stem above the leaves the crown, 

 and the part between the crown and root, the stock. The whole plant 

 is covered by a periderm, which is thicker on the crown, and every- 

 where harder and darker than the parts beneath. On the crown of an 

 old plant it may reach a thickness of 2 centimeters. On the root it is 

 sometimes loose enough to peel off like the bark. It is thickest at the 

 top of the periphery of the crown, and diminishes to less than half 

 the maximum thickness at the bottom of the depression, and at that 

 level on the outer surface. Its thickness diminishes rapidly near the 

 leaf, and there is none at all in the leaf groove. 



The periderm grows from a phellogen, or, more probably, from a 

 series of phellogens. 



Between the leaf and the stem apex the most striking feature of 

 the crown is a series of ridges, which would be somewhat semicircular 

 in longitudinal section, extending the whole breadth of the leaf, 

 almost half of the entire circumference (fig. 366). The ridges are 

 marked by pits and scars, showing the position of the inflorescences 

 which have fallen off. The ridge is certainly not a leaf. It develops 

 from the adaxial surface of the leaf groove, but only a too rigid mor- 

 phology could make it a branch on that account. Whether there is 

 any bract at the base of the inflorescence still remains to be deter- 

 mined. 



Ridges develop on the stock, below the leaf groove, and are prob- 

 ably just as numerous as those above, but not so conspicuous. Pear- 

 son found that they occasionally bore inflorescences. However, they 

 are small and comparatively inconspicuous. 



