NATURE OF PLANTS 117 



that varies in different plants, exposing the microspores to the 

 air (Fig. 75, B). The pistil or carpel is quite different in char- 

 acter. At or near the top is a more or less modified part, the 

 stigma, below which is an elongated stalk, the style, that broadens 

 out into a base or ovary. By longitudinal and transverse 

 sections of a pistil we gain a better idea of its real character 

 (Fig. 74, D-F). The ovary is now seen to contain a cavity 

 from the walls of which one or more rather spherical bodies, 

 the ovules, are developed. The structure of the ovule is very 

 complicated. If a thin section is made through the center of 

 one of them it will be seen to contain usually but one large spore, 

 therefore called the megaspore (Fig. 76, mg). This spore is 



A 



FIG. 75. Structure of the anther: A, diagram of the anther cut across, 

 showing the four cavities, sporangia (sing, sporangium), filled with spores. 

 B, cross-section of a mature anther. The tissue about the spores has broken 

 down, thus forming two cavities and at the left the breaking open of the wall 

 of the anther is shown. 



developed in a mass of tissue, the nucellus, and the whole is 

 surrounded by one or two coats, the integument, which grows 

 up from the base of the ovule but never quite surrounds it, 

 thus leaving a small opening or micropyle (Fig. 76, mi). The 

 megaspore, unlike the microspore, is never set free but is retained 

 permanently in the ovule. 



It is not known how the sporophylls have originated but there 

 are indications that they may have arisen through the modifica- 

 tion of leaves for the purpose of forming spores instead of con- 

 structing foods. It is for this reason that these organs are 

 called sporophylls, or spore-forming leaves, from phylon, a leaf. 

 Those forming small spores are sometimes called microsporo- 

 phylls instead of stamens and those forming large spores mega- 

 sporophylls instead of pistils or carpels. Very many modifica- 

 tions of the sporophylls will be noted later in the work. 



