244 



PLANT MORPHOLOGY 



called trabccidae, extend inward to form incomplete partitions. The 

 spores are freed by the gradual decay of the sporangium wall. 



The sporangium arises from a transverse row of initials, three or four m 

 number, and is eusporangiate in development (Fig. 20 IC). In early 

 stages the two kinds of sporangia are indistinguishable from each other. 



Fig. 201. Sporangia of Isoetes. A, cross section of microsporangium of Isoetes mdtallii, 

 X15; B, longitudinal section of megasporangium of Isoetes nuttallii, XlO; C, longitudinal 

 section of young niicrosporophyll of Isoetes howellii, showing sporogenous tissue (shaded), 

 X250; /, ligule; v, velum. 



After a large amount of potentially sporogenous tissue has been differen- 

 tiated, some of it forms the sterile trabecular The sporangium wall con- 

 sists of four layers of cells, the inner layer forming a tapetum that also 

 borders the trabeculae (Fig. 202). The tapetum does not disorganize for 

 a long while. In the microsporangium all the cells not taking part in the 

 formation of the wall, trabeculae, and tapetum become functional spore 

 mother cells. In the megasporangium the trabeculae are fewer and 

 thicker, the tapetum comprises several layers, and most of the spore 

 mother cells divide and contribute nourishment to a much smaller number 

 that enlarge and form tetrads. As in Selaginella, the megaspore wall is 

 very thick. 



