XVII.] 



BOTANY. 



107. The relation of the stamen to the leaf is not 

 so clear as are those of the sepals, petals, and carpels; 

 nevertheless the transition from petal to stamen is 

 obvious in the white waterlily, and in manv double 

 flowers, as the rose (Fig. 49). 



FIG. 50. a, pollen grains of orange ; b, pollen grains of buttercup upon the 

 stigma with their tubes descending to the ovule ; both very much 

 enlarged. 



108. The pollen grains are usually globose, or ellip- 

 soid, or rounded with obtuse angles ; they are generally 

 free, but sometimes escape from the mother-cell con- 

 nected in fours (rhododendron). In orchis they escape 

 as club-shaped masses (Fig. 57). The surface of the 

 granules is smooth, sculptured, or prickly, and this and 

 their size and shape are wonderfully constant in each 

 kind of plant, and through many allied plants. 



A pollen-grain consists of two cellulose coats and 

 fluid protoplasmic contents. When placed on the 

 stigma (Par. 112), one or more tubes formed of the 

 inner cellulose coat are pushed through slits or holes 

 in the outer, and descend through the stigma and 



