CHAPTER XV. THE SPERM APH VTA. 375 



on leaves, or rarely the ovules occur at the end of the axis, 

 which is also true of some Gymnosperms ; but in Angiosperms, 

 usually, the floral branch and its leaves, both those which directly 

 bear the pollen grains and ovules, and others, called the perianth, 

 exterior to them, are much more strongly modified or highly 

 specialized, forming a flower which is more or less conspicuously 

 different from the rest of the plant, and frequently very beautiful 

 in form and coloring. The perianth is usually differentiated into 

 calyx and corolla, each of which, as we have seen in Part I, 

 performs indirectly more or less important service in the repro- 

 ductive process. The part of the axis on which the floral leaves 

 are borne, is usually very short, and the leaves themselves com- 

 pactly arranged in whorls or spirals. Moreover, the axis nearly 

 ceases its growth after these leaves are formed, and does not, 

 except in occasional monstrosities, produce buds in their axils. 

 The short, often expanded part of the axis, called the receptacle, 

 that bears the floral leaves, is frequently prolonged below into a 

 stalk or peduncle, which may either be naked or bear small 

 modified leaves, the bracteoles. In all Gymnosperms, as we 

 have seen, the flowers are either monoecious or dioecious ; but 

 while this is the case also with many Angiosperms, the majority 

 are hermaphrodite, or produce both stamens and pistils together 

 in the same flower, the carpellary leaves occupying the centre of 

 the flower, while the stamens are borne immediately below or 

 exterior to them. But perhaps the most conspicuous difference 

 between the two Classes, is the fact that, in Angiosperms, the 

 ovules are enclosed, while in Gymnosperms they are naked or 

 unenclosed. The vessel which, in the former class, contains the 

 ovules, is, as we have already learned, either a single carpellary 

 leaf or a whorl of two or more of them, which have become 

 adnate. 



Other differences, though less conspicuous and less easily 

 detected, but of still greater significance, as showing the relations 

 between the two groups, and the connection of both with the 

 Pteridophyta, are found in the pollen-grains and embryo-sacs. 

 The pollen-grain of the Gymnosperms we found to be a micro- 

 spore, which develops a very rudimentary prothallium, consisting 

 of a very small cell, and another cell, which, instead of produc- 

 ing antherozoids, gives origin to a pollen-tube. Now, in Angio- 



