396 PART IV. CLASSIFICATION. 



guished from the Angiosperms, in which there is always an ovary 

 and a stigma. 



The Sporangia are of two kinds, micro sporangia or pollen-sacs, 

 and macrosporangia or ovules. The development of the sporan- 

 gium is, in both, eusporangiate (see p. 53). The sporangia are, as 

 a rule, borne on the sporophylls ; bat in some few cases (microspor- 

 angia rarely, e.g. Naias ; macrosporangia of Taxus, Polygonum, 

 Primulacese, etc.) they are borne on the axis. 



The micro sporangia, or pollen-sacs, may be developed either 

 singly or in a sorus of two or more ; they may be very numerous 

 on the sporophyll, as in the Cycadacese. When borne on the 

 sporophylls, they are developed on the lower (dorsal) surface of 

 the microsporophyll in the Gymnosperms ; whereas in the Angio- 

 sperms they are usually developed both on the upper (ventral) and 

 the lower surfaces. 



The microsporangia either project freely or are embedded in the 

 placental tissue of the member bearing them. The multicellular 

 hypodermal archesporium is either a row or a layer of cells. The 

 archesporial cells undergo, as a rule, division, giving rise to the 

 sporogenous cells together with a more or less extensive transitory 

 layer of investing cells, the tapettim, which is eventually dis- 

 organised. The microsporangium is, with few exceptions, unilo- 

 cular. 



The microsporangium eventually dehisces, generally by a longi- 

 tudinal slit, less commonly by a transverse slit or by a pore. 

 The dehiscence is mainly effected by a layer of tracheidal cells, 

 differentiated as part of the wall, which are highly hygroscopic. 



The microspores, or pollen-grains, are developed from the sporo- 

 genous mother-cells of the sporangium. As a rule each mother- 

 cell divides so as to give rise to four microspores, all of which 

 develope. As a rule, also, the microspores eventually become quite 

 free from each other, but to this there are exceptions, thus, in the 

 Mimosese, while the pollen-grains are isolated in some species, 

 in other species they cohere in groups of 4, 8, 12, 16, or 32, de- 

 rived from one, two, three, or more mother cells ; again, in the 

 Orchidacese, whilst Cypripedium has isolated pollen-grains, in 

 most genera the pollen-grains are in groups of four (tetrads), and 

 cohere into a mass (or 2-8 masses), the pollinium, of varying 

 consistence. 



The microspore has, as a rule, the ordinary structure of a spore 

 (see p. 50) ; it is a nucleated cell, with a certain amount of granu- 



