140 



Practical Plant Biology. 



calyptra on the top of the embryo, the base remaining as a 



sheath the vagina round the foot. 



Under the calyptra the upper end of the embryo enlarges and 



forms the sporangium, or theca, supported on the bristle-like 



stalk or seta. The theca is pear-shaped with a slight obliquity. 



Its tapered end is continuous with the top of the seta and is bent, 



just where it joins the seta, 

 at right angles. The axial 

 tissue of the seta, which forms 

 a strand of elongated cells 

 throughout its length, is con- 

 rf tinued up into the tapered 

 base, or apophysis, of the 

 theca. This strand has ap- 

 parently the function of con- 

 veying materials absorbed by 

 the foot up the seta to the 

 developing theca. 



Externally obliquely pear- 

 shaped, the theca may be 

 compared to a thick walled 

 wine-glass contracted at its 

 rim and closed with a small 

 lid the operculum. The 

 wall of the theca is composed 

 of about five layers of cells. 

 At the bottom, however, it is 

 many layers thick, so that the 

 space enclosed by the wall 

 reaches only about half way 

 down the theca, the lower 

 half being formed by the 

 solid apophysis. In this 

 space is slung a solid cylin- 

 drical core. The core is held 

 in position there by elongate 

 cells, passing from it to the 

 inner surface of the wall of 



FIG. 29. Funaria hygrometrica, theca 

 longitudinal section, x 40. a, oper- 

 culum; 6,'peristomium ; <:, annulus; 

 d, columella ; e, sporogenous cells ; 

 /, air-space ; g, suspensory filament ; 

 z, photosynthetic cells of apophysis ; 

 k, stoma; /, epidermis. 



the theca, which traverse the thimble-shaped space separating the 

 core from the wall. 



In the outer region of the core a layer is differentiated, the cells 

 of which are richer than their neighbours in protoplasm and give 

 rise to the spores. This is the sporogenous layer, and the whole 



