PHANEROGAMIA 



171 



The two coats are usually distinct from each other, but in 

 some pollen grains they are adherent, except at- special spots, at 

 which the exine will rupture when the spore germinates. In 

 some cases there is no exine, particularly in some aquatic 

 plants. 



The microspores do not always become free from each 

 other ; in the Mimosae they are bound together by the special- 

 mother-cell walls into groupg of four or more ; in the Orchids 

 and Asclepias the whole mass is usually coherent, forming the 

 loolliniiiin. 



The Macrosporophylls and the Macrosporangia. — The 

 appearance of the macrosporophylls has already been described. 

 In the Angiosperms they are the carpels, and may be either 

 free or coherent together. In the Coniferae of the Gymnosperms 

 they are flattened, scale- 

 like leaves, each of which Fig. 925. Fig. 926. 

 bears two sporangia on 

 its upper face. In some 

 of the Cycads they are 

 small pinnate leaves, the 

 lower pinnae only bearing 

 the sporangia. In others 

 they are peltate scales 

 with the latter on their 

 under side, much resem- 

 bling the microsporo- 

 phylls. 



The development of 

 the macrosporangium or 

 ovule takes place by the 



division of certain hypodermal cells of the placenta, whether 

 the latter be upon a sporophyll or upon the axis. These 

 cells divide repeatedly periclinally till the young sporangium 

 appears as a little protrusion of tissue, whose cells are arranged 

 in radial rows {fig. 927). In the most usual case the terminal 

 cell of the central row becomes the archesporium. The growth 

 of the mass of the sporangium, known as the nucellus, has 

 already been described (Vol. I. p. 207) and its various shapes 

 mentioned. From its base arise the integuments, usually two in 

 number, which accompany it in its growth and finally surround 

 it, except at the apex, where the micropyle is left. 



While this development is proceeding changes take place in 

 the interior. The original archesporium, which is a single cell, 



Fig. 925. A mature carpel or 

 scale of the Scotcli Fir 

 ( Pinna siilvestris), with two 

 wiiigeii nakeil seeds at its 

 base. m{c. Micropyle. cJi. 



Clialaza. Fi<j. 926. A 



scale of the Larch, bearing 

 one naked winged seed ; the 

 other seed has been re- 

 naovcd. 



