16.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 73 



from cells of deeper layers as well. These Ferns and the Hydro- 

 pteridese are henee distinguished as leptosporangiate from the rest 

 of the Vascular Plants which are said to be eusporangiate. 



The most important morphological feature in the development 

 of the multicellular sporangium is the differentiation of the sporo- 

 genous tissue, that is, of the mother-cells of the spores. These are 

 derived from a hypodermal cell or group of cells, termed the 

 archesporium, which may be distinguished at an early stage in 

 the development of the sporangium, by the highly granular pro- 

 toplasm and the large nucleus. The mother-cells of the spores 

 are usually formed by the division of the archesporial cell or cells, 

 but occasionally the archesporial cells themselves become spore- 

 mother-cells. The sporogenous cells, as they develope, become 

 more or less completely invested by a layer of highly granular 

 cells, forming a membrane termed the tapetum, which temporarily 

 separates them from the wall of the sporangium ; the tapetum 

 may be derived wholly or in part from the archesporium or from 

 the wall of the sporangium. 



The structure and form of the archesporium varies widely in 

 the different groups of plants. It may be a solid mass of cells, as 

 in all Hepaticae except Anthoceros ; or a layer of cells, as in 

 Anthoceros, Mosses (except Archidium), Isoetes, pollen-sacs of 

 some Phanerogams ; or a row of cells, as in Lycopodinse, pollen-sacs 

 of some Phanerogams ; or a single cell, as in the Leptosporangiate 

 Filicinoe, in Equisetum, and im the macrosporangia (ovules) of 

 most Phanerogams. 



The constitution of the archesporium in the Eusporangiate 

 Pteridophyta and in the Phanerogams, depends upon the form of 

 the sporangium. The young sporangium in all these plants con- 

 sists of one or more longitudinal rows of cells, covered by an 

 epidermis ; when there are several rows of cells, the lateral rows 

 radiate outwards in the apical portion of the sporangium. When 

 the sporangium is circular in transverse section, and is narrow, it 

 is only the terminal cell of the central axial row which constitutes 

 the archesporium; when the sporangium is elongated transversely, 

 the terminal cells of several of the rows lying in the plane of 

 elongation, become archesporial cells, so that the archesporium 

 consists of a row of cells ; when the sporangium is broad, the 

 terminal cells of several of the adjacent central rows become 

 archesporial cells, so that the archesporium consists of a layer of 

 cells. 



