CYCAS. 389 



very conspicuous zone containing Nostoc chains ; it would 

 appear that the coralloid growth is due to hypertrophy 

 set up by Bacteria, the Nostoc slipping in afterwards and 

 inhabiting the middle cortical tissue as an endophyte 

 since these roots come to the light, the arrangement is 

 probably a symbiosis. 



548. Stamens. Each male flower (cone) consists of an 

 axis bearing spirally arranged stamens. 



Note that each stamen consists of a hard thick scale 

 with an expanded head and bearing on its lower surface a 

 large number of sessile egg-shaped pollen-sacs (micro- 

 sporangia), which are arranged more or less definitely 

 in small groups (sori) ; the heads of the scales are 

 hexagonal, and they fit closely together until the pollen 

 is ripe, then they separate and each pollen- sac opens by 

 a slit. 



549. Carpels. Note that each carpel (megasporo- 

 phyll) is a pinnate leaf, much smaller than the foliage- 

 leaves ; the leaflets are only developed on the upper part 

 of the leaf, and are narrow, thick, and woolly, while on the 

 margins of the lower portion there are large ovules in 

 place of leaflets. 



The ovule, which becomes very large before fertilisation, 

 contains when mature a large mass of endosperm 

 (female prothallus), surrounded by a thin layer of 

 nucellus tissue, which is covered by a thick integument 

 consisting of a fleshy outer layer and a hard inner layer. 



