IX MARATTIACEAl—ISOETACEAL 295 



(trabecule), and containing the spore mother cells (Fig. i 55, C). 

 The whole process is not unlike that in the spike (jf Ophio- 

 glossum, especially if Bower's statement is correct that the whole 

 hypodermal layer in OpJiioglossiun is to be considered as the 

 archesporium.^ 



As the sporangium grows the tissue of the leaf surrounding 

 it grows up on all sides so as to enclose it in the fovea, whose 

 edges extend more or less over the sporangium to form the 

 velum. Goebel calls attention to the analogy of the latter with 

 the integument of an ovule. 



The macrosporangium corresponds in its earlier stages 

 exactly to the microsporangium, and the difference between 

 them is first indicated by the fertile archesporial cells in the 

 former only dividing by the walls which form the tapetal cells, 

 and an inner cell of each row becomes at once the macrospore 

 mother cell. This is much larger than the others, and is very 

 conspicuous. Between the fertile rows are the trabeculae, at 

 first also only one row of cells. As the spore mother cell 

 grows, it encroaches upon, and destroys, the surrounding tapetal 

 cells, and lies in the cavity thus formed. The division into 

 four spores follows in the usual manner. 



In the development of the sporangia, especially the forma- 

 tion of the large hypodermal archesporium, and perhaps also 

 the integument, the resemblance to corresponding structures in 

 the Spermaphytes is obvious, and in these respects Isoetes 

 certainly does come nearer the latter than any other living 

 Pteridophyte. 



Bower ^ has recently made a careful study of the sporangium 

 of Lepidodendron and found structures which closely resembled 

 the trabeculae in that of Isoetes, and is inclined to regard 

 tliis as an evidence of relationship between the two genera. 



In /. laciistris the sporangium is sometimes replaced by a 

 leafy bud which may develop into a perfect plant."^ 



TJie Affinities of the Eusporangiate Filicinecs 



In attempting to discover the affinities of the members of 

 this group, many difficulties are encountered. First, and 

 perhaps most important, is the small number of forms still 



1 Bower (14). ^ Bower (15). 



^ Goebel, " Ueber Sprossbildung aus Isoetesblatter," Bot. Zeit. 1879. 



