CHAPTER XV. 



SPBRMAPHYTBS. 



CLASS II. ANGIOSPERMS. 



THE angiosperms include an enormous assemblage of plants, 

 all those ordinarily called " flowering plants " belonging here. 

 There is almost infinite variety shown in the form and struc- 

 ture of the tissues and organs, this being particularly the case 

 with the flowers. As already stated, the ovules, instead of 

 being borne on open carpels, are enclosed in a cavity formed 

 by a single closed carpel or several united carpels. To the 

 organ so formed the name "pistil " is usually applied, and this 

 is known as " simple " or " compound," as it is composed of 

 one or of two or more carpels. The leaves bearing the pollen 

 spores are also much modified, and form the so-called "sta- 

 mens." In addition to the spore-bearing leaves there are 

 usually other modified leaves surrounding them, these being 

 often brilliantly colored and rendering the flower very conspic- 

 uous. To these leaves surrounding the sporophylls, the general 

 name of "perianth" or "perigone" is given. The perigone 

 has a twofold purpose, serving both to protect the sporophylls, 

 and, at least in bright-colored flowers, to attract insects which, 

 as we shall see, are important agents in. transferring pollen 

 from one flower to another. 



When we compare the embryo sac (macrospore) of the 

 angiosperms with that of the gymnosperms a great difference 

 is noticed, there being much more difference than between the 

 latter and the higher pteridophytes. Unfortunately there are 

 very few plants where the structure of the embryo sac can be 

 readily seen without very skilful manipulation. 



143 



