GROUP IV. — PHANEROGAMIA : GYMNOSPERMj;. 469 



veloped at the growing-point of the stem in the place of a whorl 

 of foliage-leaves. The few sessile macrosporangia are borne 

 laterally on the lower part of the sporophyll. 



In the other Cycadaceae, the macrosporophyll is a stout scaly 

 leaf, thickened at its outer end, bearing usually two lateral ovules, 

 one on each side. 



In the Coniferoe, the simplest form of macrosporophyll is to be 

 found in Dammara ( Araucarinese) , for instance, where it is simply 

 a scaly leaf bearing a single raacrosporangium on its upper surface : 

 in other forms the superior surface of the macrosporophyll is 

 clearly marked out, by outgrowths of various kinds, into an apical 

 and a basal half, the latter alone bearing the (1-7) macrosporangia 

 (e.g. Taxodinej«, Cupressineae): in the Abietineas (Pinus, Larix, etc.) 

 the sporangiferous structure of the preceding families is developed 

 from the base oi the carpel as a placental scale, which is much 

 larger than th6 carpel itself, and bears the two macrosporangia on 

 its upper surface. In most Podocarpeae, the macrosporophyll is 

 likewise differentiated into an apical and a basal half, the latter 

 being much thickened, but here it is the apical portion which bears 

 the single macrosporangium. In the Taxeae the macrosporophylls 

 are rudimentary (e.g. Phyllocladus, Cephalotaxus) or absent (e.g. 

 Torreya, Taxus) ; even when present they do not bear the macro- 

 sporangia. 



In the Gnetaceae there are no macrosporophylls. 



The microsporangia (pollen-sacs) are borne, in nearly all cases, 

 on the lowei* (dorsal) surface of a sporophyll ; they may be 

 numerous (about 1,000) as in some Cycadaceae ; or few (2-15) in 

 the Coniferce and Gnetaceae : scattered (some Cycads),.or more 

 commonly grouped into one or more sori, with more or less well- 

 developed placental tissue ; either imbedded in the tissue of the 

 sporophyll (e.g. Abietineae), or freely suspended (e.g. Araucaria, 

 Ginkgo) : in the Cupressineae, the spomngia, when young, ai*e 

 covered by an outgrowth of the under surface of the sporophyll 

 which is comparable to the indusium of Ferns. In Gnetum, as 

 there is no microsporophyll, the two microsporangia are borne on 

 the apex of the floral axis. 



The structure of the microsporangium is simple : it is unilocu- 

 lar ; it contains, at an early stage, a mass of spore-mother-cells 

 derived from the archesporium, surrounded by a layer of tapetal 

 cells also derived from the archesporium, and by a wall consist- 

 ing of one, two, or more, layers of cells : each spore-mother-cell 



