198 BOTANY PART i 



pollen grains are spores (Fig. 32) which to begin with are single cells 

 but later become multicellular (Gymnosperms), or at least contain 

 more than one nucleus (Angiosperms). The CARPELS, which are free 

 in the Gymnosperms but form closed OVARIES in the Angiosperms, 

 bear the OVULES. These are shortly-stalked oval bodies of complicated 

 structure. In each ovule a single spore is permanently enclosed, pro- 

 tected by the sterile integuments. 



The cone-like flowers of the Gymnosperms ( 97 ), composed of 

 numerous, spirally -arranged, scale -like stamens or carpels, closely 

 resemble the cones of the Pteridophyta. The flowers of the 



Angiosperms ( 97 ) have usually a quite distinct 

 appearance (Fig. 227) owing to (1) the 

 limited number of the usually whorled floral 

 leaves, (2) the frequent differentiation of 

 the outer floral leaves into firm green sepals 

 and coloured delicate petals, (3) the char- 

 acteristic form of the stamens, and (4) the 

 union of the carpels to form the pistil. All 

 these parts of the flower are arranged 

 regularly. In the typical angiospermic 



FIG. 228. -Diagram of a Liliaceous flower, five whorls, each of five floral leaVCS, 



2Ti*?i2^? regularly alternate (Fig. 228); the outer- 



to which is the bract. (After most whorl is formed of the sepals which 



STRASBURGER.) enclose and protect the other parts when 



young, the second is formed of the petals, 



the third and fourth of the stamens, and the fifth and highest by 

 the carpels ( 98 ). These foliar structures arose from the shortened, 

 and often flattened or hollowed, floral axis ; they are often united with 

 one another and with the axis in such a way as to require thorough 

 comparative and developmental study to ascertain the facts clearly. 



(b) Sexual Reproductive Cells. Gametes. 1. Different Forms 

 of Sexual Cells and Sexual Organs. A great variety in the methods 

 of sexual reproduction is shown by plants ; different as the extremes 

 are, however, they are connected by intermediate links. 



Thallophyta. In many of the lower Algae and Fungi all the 

 cells of the plant may simultaneously form sexual cells. With pro- 

 gressive organisation a division of labour is met with. As in the case 

 of the formation of asexual reproductive cells, certain cells or organs 

 with definite positions carry on the sexual reproduction. The parts 

 of 'the plant body which bear the sexual organs may be specialised in 

 relation to this. 



In the simplest types of sexual reproduction met with in the 

 lower Algae and Fungi, the sexual cells or GAMETES are usually 

 naked protoplasts of similar size and structure ; these resemble the 

 asexual swarm spores but conjugate with one another (ISOGAMY, Fig. 

 229 B). They develop, singly or in numbers, from the protoplasts 



