16. ASEXUAL REPRODUCTIVE ORGANS. 53 



The mature sporangium of these higher plants is either borne 

 upon a stalk (sometimes termed funicle) ; or it is sessile, and then 

 it is commonly more or less imbedded in the tissue of the parent 

 member, as in the case of the sporangia of the Ophioglossacese, and 

 of the pollen-sacs of most Phanerogams. 



The development of the sporangium begins, in all cases, at the 

 surface of the parent member. The area which bears the spor- 

 angium, especially when a number of sporangia are developed close 

 together, generally projects more or less as a cushion of tissue to 

 which the term placenta is applied. In the Ferns (except Marat- 

 tiaceae, Ophioglossacese, and Isoetacese) and in the Hydropteridese 

 (Rhizocarps), the sporangium is developed from a single superficial 

 cell : in the rest of the Pteridophyta and in the Phanerogams it is 

 developed from a group of superficial cells, and in some cases from 

 cells of deeper layers as well. These Ferns and the Hydropteridese 

 are hence 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 sporogen- 

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

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

 sporium, which may be distinguished at an early stage in the de- 

 velopment of the sporangium, by the highly granular protoplasm 

 and the large nucleus. The mother-cells of the spores are usually 

 formed by the division of the archesporial cell or cells, but oc- 

 casionally 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, form- 

 ing 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. 



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 macrosporangium 

 (ovule), and the macrosporangium remains attached for a consider- 

 able time to the plant bearing it. It is on account of this peculi- 

 arity that seeds are produced in Phanerogams. The production of 

 seeds is the characteristic difference between Phanerogams and 

 Cryptogams. The spores are usually set free by the rupture or 



