GROUP III. PTERIDOPHYTA: FILICIN^ ; EUSPORANGIATJS. 385 



.a very short time by means of an apical growing point. Its further growth is 

 intercalary by means of a zone of merismatic cells situated just above the 

 ligule. In the sporophylls the tissue below the ligule also remains merismatic 

 for a time, thus providing the necessary space for the sporangium. 



The sporangium is developed from a group of cells in the fovea. The 

 archesporium consists of a layer of hypodermal cells in the young sporangium. 

 In a microsporangium all the archesporial cells grow and divide so as to form 

 rows radiating from 



the free surface to *f 



the attachment of the 

 sporangium. Some 

 of these rows of cells 

 soon cease to grow, 

 and are not sporo- 

 penous, but remain 

 as plates of tissue, 

 termed tmbeculce, 

 which imperfectly 

 chamber the cavity 

 of the microsporang- 

 ium. Of the remain- 

 ing cells, the ma- 

 jority constitute the 

 mother-cells of the 

 microspores invested, 

 towards the wall of 

 the sporangium, by 

 sterile cells forming 

 the tapetum. In a 

 macro sporangium, 

 the fertile archespor- 

 ial cells undergo but 

 a single division, 



whilst the trabeculae M *B //W W^ V **/- ^ 



are formed as in the 

 microsporangium. 

 The large mother- 

 cells of the macro- 

 fpores are isolated, 

 and each is invested 

 by a tapetal layer. 

 Each spore-mother- 

 cell gives rise, finally, 

 to four spores. With 

 regard to the morph- 

 ology of the sporangium it may be pointed out that the segmentation of the 

 archesporium by the formation of the trabeculce, which is characteristic of 

 Isoetes, is probably to be taken as indicating a tendency towards the develope- 



C C 



FIG. 256. Isoetes lacustris (after Luerssen). A Plant, half 

 nat. size : r dichotomously branched roots. B Inner (ventral) 

 surface of base of a sporophyll : I ligule ; / fovea. C Longi- 

 tudinal section of base of a sporophyll: sp the sporangium 

 in the fovea ; tr the trabeculae ; v the velum ; I the ligule. D 

 Transverse section of the base of a sporophyll : letters as in C. 



