98 



GYMNOSPERMS 



Altogether, the structure is such that the leaves are strong and 

 leathery. Many of them keep their fresh, green color a long time 

 after being cut otT from the plant, so that they 

 are popular for decorative purposes in cycad 

 regions, and the beautiful leaves of Cycas 

 rcvoluta have become a standard nursery stock 

 for funeral wreaths and for Palm Sunday. 



The root. — The primary root may be very 

 large, even as large as the stem; and in the 

 seedling the root is much larger than the stem 

 which, at this stage, is rather inconspicuous 

 (fig. 90). After the seedling has become thor- 

 oughly established, the stem begins to grow 

 more rapidly, and in the adult plant is usually 

 much larger than the root. 



Roots may attain a great length. A root of 

 Dioon spinidosum, at a distance of 12 meters 

 from the stem, hanging exposed over a rock, 

 was still 3 cm. in diameter when it entered a 

 crevice and could not be followed any farther. 

 The root is tetrarch. Secondary growth be- 

 gins early and more or less irregularly, so that 

 the topography, as seen in transverse sections, 

 differs markedly from that of familiar dicotyl 

 roots. 



All cycads have remarkable apogeotropic 

 roots (fig. 91). These grow up, instead of 

 down, branch dichotomously and profusely, 

 forming coralloid masses above ground. The 

 vascular structure is about the same as in 

 normal roots, but bacteria, or "bacterioids," 

 get in very near the tip and cause some distor- 

 tion, which seems to prepare the way for the 

 entrance of a blue-green alga, Anahacna. The 

 alga multiplies rapidly, so that there is a 

 blue-green zone midway between the vascular 

 cylinder and the epidermis (fig. 92). While the 



Fig. 90. — Dioon ediile: 

 seedling; all the part 

 bearing secondary roots 

 is the primary root. 

 The stem, bearing the 

 leaf and scale leaves, is 

 so small at this stage 

 that it is hidden by the 

 emergent part of the 

 cotyledons. — From 

 Chamberlain, Elements 

 of Platil Science"^ (Mc- 

 Graw-Hill Book Co.). 



