THE GYMNOSPERMAE : CYCADALES, ETC. 



711 



I 



primary root persists as a tap root. It is often short and tuberous, and may 

 be as thick as the stem itself. 



A new cluster of leaves is formed each year, but the old leaves are not 

 immediately dropped, so that most plants have leaves of at least two years 

 present at any given time. The new leaves are 

 formed at the apex in continuous succession 

 with the old leaves, and they develop during 

 a whole year under cover of the armour of 

 scales round the apex. When they push the 

 scales aside and begin to open out, growth is 

 very rapid and they may reach full size in a 

 few days. In Cycas the main rachis is incurled 

 (circinnate) and the pinnae are inrolled, as in 

 the Ferns (Fig. 713). In Zamia the main 

 rachis is incurled, but the pinnae are straight. 

 This inrolling of the developing leaf is a defi- 

 nitely Filicalean character. 



The scale leaves which cover the apex, and 

 under cover of which the foliage leaves develop, 

 are formed periodically in series with the leaves. 

 Every year, or every second year, a group of 

 new leaves is formed, and in the interval the 

 succession is kept up by the formation of scales. 

 The scales, like the leaf bases, are persistent and 

 form part of the armour of the old stems. 



A striking peculiarity oi Cycas is the formation 

 of small bunches of dwarfed apogeotropic roots, 

 just below ground level (Fig. 714). These so- 

 called " coralloid " roots are not unlike the nodular 

 roots of Almis, except that they grow upwards 

 into the air. They are stumpy, dichotomously 

 branched and closely massed, and like those of the Alder they usually contain 

 an endophyte, which in this case is a Blue-green Alga, Anabaena cycadacearum. 

 The Alga inhabits a zone of large disorganized cells in the inner cortex 

 (Fig. 715), but it does not seem to be responsible for the singular form of these 

 roots, as they are sometimes found without it. Whether any form of 

 symbiosis is involved is not known, but Bacteria are also present, which 

 suggests that Nitrogen fixation may occur, as in the root nodules of the 

 Leguminosae. 



Fig. 712. — Cycas reioluta. 

 Single leaf. The one -foot 

 rule shows the size. 



Anatomy of the Stem 



The stem anatomy of Cycas is relatively simple. The meristematic tissues 

 of the apex are not separated into distinct zones (Fig. 716). There is a 

 primary ring of narrow vascular bundles, which is usually small in comparison 

 with the diameter of the stem (Fig. 717). The greater part of the bulk, 



