16. ASEXUAL REPRODUCTIVE ORGANS. 57 



as in most Ferns, Equisetum, and Gymnosperms (pollen-sacs) ; 

 or exclusively on the upper (ventral) surface as in the Lyco- 

 podirise, Marsileacese, macrosporangia of Coniferse and of some 

 Angiosperms (e.g. Butomus) ; or on both surfaces, as in Osmunda ; 

 or on the lateral margins, as in Ophioglossum ' and the Hymeno- 

 phyllacese, and in many Angiosperms (e.g. Leguminosse, Violacese, 

 Liliacese) ; or on the apices of segments of the sporophyll, as in the 

 Salviniacese. 



The number of the sporangia borne by a sporophyll also varies 

 widely. In some cases there is only one, as in Selaginella, Lyco- 

 podium, Isoetes ; or two, as in most Coniferse ; or four, as in most 

 Angiosperms (microsporangia) ; or many, as in the Filicinse. 



In most cases the sporangia are free on the surface of the 

 sporophyll ; but in some cases they are enclosed in a cavity formed 

 either by the infolding and junction of the margins of the sporo- 

 phyll, or by the junction of the margins of adjacent sporophylls. 

 The sporangia of the Marsileaceae are thus enclosed by the sporo- 

 phyll, as are also the macrosporangia of all Angiosperms. In the 

 latter group the resulting structure is termed the ovary. 



(b) The Hypsophylls (Fig. 27, p. 43). Under this common term 

 are included bracts and perianth-leaves. 



Bract. This term is generally applicable to the leaves, other 

 than the sporophylls and perianth-leaves, which are borne by the 

 rhachis or branches of the inflorescence : those which are borne 

 on the pedicels of individual flowers are, however, distinguished as 

 bractcoles or prophylla. 



The bract is frequently not distinguishable from a foliage-leaf ; 

 but it may be reduced to a scaly leaf ; or it may be very large and 

 even highly coloured, when it is said to be petaloid. An example 

 of the occurrence of bracts in the Pteridophyta is afforded by 

 Equisetum, where there is a whorl of small bracts, forming what 

 is known as the ring, just below the cone or flower. In some 

 Monocotyledons (e.g. Palms, Arums, etc.) there is a large bract, 

 termed a spathe, which invests the whole inflorescence ; it is 

 usually not green in colour, as in the Trumpet Lily (Zantedeschia 

 cethiopica) where it is white. In some cases the bracts are 

 arranged in whorls round the inflorescence (e.g. Composite) forming 

 an involucre. 



The bracteoles sometimes form an investment, termed an epicalyx, 

 to the flower (e.g. Malva, Camellia, etc.). 



The Perianth-leaves are leaves developed in immediate relation 



