74 PART I. THE MORPHOLOGY OF PLANTS. [ 16. 



The archesporium gives rise in some cases simply and only to 

 the mother-cells of the spores, as in Riccia among the Liverworts, 

 and in the Mosses ; but more commonly it gives rise also to cells 

 which are not sporogenous. Thus, in all Hepatica3, except Riccia, 

 the archesporium gives rise to sterile cells, which, in most 

 HepaticaB, become the elaters ; and generally in the Pteridophyta 

 and Phanerogams, some at least of the tapetal cells are derived 

 from the archesporium ; in Isoetes, the tissue of the trabeculaa is 

 derived from the archesporium. 



The sporogenous cells usually divide so as to give rise each to 

 four spore-rudiments, though there are exceptions to this rule. 

 Thus, only one of the mother-cells in the macrosporangium of 

 Selaginella divides into four, the others being abortive; and in 

 that of Phanerogams the mother-cell, or cells where there is more 

 than one, does not divide, but developes directly into a single 

 macrospore (embryo-sac). In all homosporous plants, the four 

 spore-rudiments formed by each mother-cell all come to maturity ; 

 and this is the case also as regards the microsporangia of heteros- 

 porous plants. In the macrosporarigia of the heterosporous 

 Pteridophyta, the spore-rudiments all reach maturity in Sela- 

 ginella and Isoetes, so that the mature macrosporangium of 

 Selaginella contains four, and that of Isoetes many, macrospores : 

 but in the Hydropterideaa (Salvinia, Pilularia, etc.), where each 

 macrosporangium contains sixteen mother-cells, so that sixty-four 

 spore-rudiments are formed, only one of these sixty-four comes to 

 maturity, so that each macrosporangium produces only a single 

 macrospore. 



In most cases the asexually-produced spores are set free from 

 the organism producing them. An exception to this is offered by 

 the macrospore (embryo-sac) of Phanerogams, in which plants 

 the macrospore remains permanently enclosed in the macrospor- 

 angium (ovule), and the macrosporangium remains attached for 

 a considerable time to the plant bearing it. It is on account of 

 this peculiarity that seeds are produced in Phanerogams. The 

 production of seeds is the characteristic difference between 

 Phanerogams and Cryptogams. 



When spores are formed by abstriction, they are set free almost 

 as soon as they are formed. When they are produced in sporangia 

 they are usually set free by the rupture or dehiscence of the 

 sporangium. In some cases the wall of the sporangium simply 

 degenerates ; in other cases there is a special mechanism, some- 



