PTEROPSIDA 133 



Species with erect axes, the roots may emerge from the 

 cortex some distance above the ground, so forming prop- 

 roots. They are polyarch, with as many as nineteen exarch 

 protoxylems and, while the aerial portions are medullated, 

 as soon as the roots penetrate the soil the xylem extends 

 right to the centre. Those of young plants usually contain 

 a mycorrhizal fungus within the cortex (an oomycete known 

 as Stigeosporium marattiacearun). 



In all genera, the sori are borne in a *superficiar manner, 

 i.e. on the dorsal surface of the lamina, and beneath a vein 

 or a veinlet. Christensenia has circular sori irregularly dis- 

 tributed between the main veins (Fig. i9P)but, in all other 

 genera, the sorus is more or less elongated beneath a lateral 

 vein (Figs. 19Q and 19R). In Angiopteris the sporangia are 

 free from each other (Figs. 19G and 19H), but in Marattia, 

 Danaea and Christensenia they are fused into a synangium 

 (Figs. 19I-N). Danaea is peculiar in having fleshy flanges 

 of tissue (3) projecting between the adjacent synangia (or, 

 according to some, in having the synangia sunken into a 

 very fleshy pinnule). 



The first stage in the development of a sporangium is a 

 perichnal division of a single epidermal cell, of which the 

 inner half gives rise ultimately to the archesporial tissue, 

 while the outer half gives rise to part of the sporangium wall, 

 the rest of the wall being produced by the activity of 

 adjacent cells. At maturity, the sporangium wall is many 

 cells thick and there is a tapetum formed from the innermost 

 wall cells. The occurrence of numerous stomata in the 

 sporangium wall is an interesting feature rarely found else- 

 where and presumably associated with its massive structure. 

 Very large numbers of spores are produced from each 

 sporangium (e.g. 1,440 in Angiopteris, 2,500 in Marattia and 

 over 7,000 in Christensenia) and, since all the sporangia 

 within a sorus mature and dehisce simultaneously, prodigious 

 numbers of spores are shed. 



In those species with free sporangia, e.g. Angiopteris, there 



