IX 



MA RA TTIA CE^—ISOE TA CEJi. 



257 



single apical cell is replaced by the group of marginal initials, 

 the outer cells of the segments grow more rapidly than the 

 inner ones, and the segments project beyond the apical cell, 

 which comes to lie in a depression between the two lobes formed 

 by the outer parts of the segments, and the prothallium assumes 

 the heart -shape found in most homosporous Ferns, The 

 secondary initial cells vary in number with the width of the 

 indentation in which they lie. Seen from the surface they are 

 oblong in shape, but in vertical section are nearly semicircular 

 (Fig. 131, B). Basal segments are cut off by a wall that 

 extends the whole depth of 



the prothallium, and the seg- 

 ment is then divided by a 

 horizontal wall into a dorsal 

 and ventral cell of nearly 

 equal size. The divisions are 

 more numerous in the ventral 

 than in the dorsal cells of the 

 segment, this difference first 

 being manifest some distance 

 back of the apex. Owing to 

 this, a strongly projecting, 

 nearly hemispherical cushion- 

 like mass of tissue is formed 

 upon the ventral surface. The 

 superficial cells of both sides 

 of the prothallium have a 

 well-marked cuticle. Numer- 

 ous brown root-hairs, which, 

 like those of the simpler 

 Liverworts, are unicellular 

 and thin - walled, grow out 



from the cells of the lower surface, especially from the broad 

 midrib. The full-grown prothallium in M. Douglasii is some- 

 times a centimetre or more in length (Fig. 132), and tapers 

 from the broad heart-shaped forward end to a narrow base. 

 In Angioptevis^ it is more nearly orbicular. In both genera it 

 is dark-green in colour, looking very much like the thallus of 

 Anthoceros Icevis, and like this too is thick and fleshy in 

 texture. A broad midrib extends for nearly the whole length 



^ Farmer (3). 

 S 



Fig. 131. — Marattia Dou^/asi'i (Ba.]<^er). A, Hori- 

 zontal section of prothallium apex, with two 

 initials, Xi6o. B, Longitudinal section of a 

 similar growing point ; d, dorsal ; v, ventral 

 segment. 



