xxxi] 



HABIT 



77 



daceae. Scales are absent. The pneumatophores are probably important 

 for aeration through this impervious covering. The sori appear superficially 

 upon the veins of the fertile pinnae, the receptacle being elongated and 

 slightly enlarged. There is no indusial protection, but the margins of the 

 fertile pinnae are strongly curved inwards. The sporangia are rather large 

 and long-stalked, with oblique annulus, and lateral dehiscence. They are 

 almost simultaneous in time of maturing, but in origin the sorus shows 

 a "mixed" character (see Vol. I, Fig. 250, c, where the sporangium of 

 P. pycnophylla is placed in relation with those of other Ferns). 



With such features as these the anatomy calls for very careful considera- 

 tion, and it shows relatively primitive characters. A transverse section of 

 a large stock of P. pyawpJiylla discloses a stelar structure not far removed 

 from solenostely, with V-shaped leaf-traces departing from narrow leaf-gaps, 

 each with an attendant strand of sclerenchyma on the adaxial side. The 



Fig. 543. Pneumatophores of Plagiogyria. 

 i, shows the persistent base of the leaf of 

 P. glaitca the ahaxial face bearing two 

 rows of them (/). ii, the circinate apex 

 of the leaf of P. pycnophylla with tlie 

 pneumatophores {p) alternating with the 

 pinnae, and projecting through the covering 

 of mucilaginous hairs. 



centre of the pith is occupied by a large sclerotic core with which they appear 

 to connect, while a shell of sclerenchyma covers the whole outer surface of 

 the stem (Fig. 544, yi). The stelar ring is delimited externally and internally 

 by a browm endodermis, which is continuous through the foliar gaps, and 

 completely surrounds all the vascular tracts. The protoxylem is mesarch, 

 and there are usually four groups of it in each separated leaf-trace. As the 

 trace passes obliquely out through the cortex it enlarges, taking in the leaf- 

 base a wider form. The relation of the pneumatophores (/>) to the trace is 

 seen in Fig. 544, E. Meanwhile the axillary sclerotic strands also widen 

 greatly, taking gradually a semilunar form, while in the middle of each strand 

 a large air-space appears, formed by actual involution of the outer surface 

 of the stem at the depressed leaf-axil (Fig. 544, A, inv). This resembles 

 what is seen in Anemia, and some other Ferns, and has been discussed in 



