250 BOTANY. 



a millimetre in extent, to enormous trees, 50 to 150 metres 

 high and many centuries old. 



471. We hare seen (pp. 340-1) that in the Gymno- 

 sperms the flower consists of a stem upon which are the 

 leaves which bear reproductive cells. The flower of the 

 Angiosperms is likewise a stem, bearing leaves which have 

 to do with reproduction. In this class, however, there is, 

 as a rule, a division of labor, as we may say : instead of all 

 the leaves bearing reproductive cells, some of them are 

 modified in form, color, or structure, so as to make the 

 flower more conspicuous, which is, as we shall see, to the 

 advantage of the plant. 



472. There are so many particular forms of flowers that 

 it would be impossible to notice or describe them all in this 

 place. In some cases the flower is a little stem (axis) upon 

 which are pollen-bearing or ovule-bearing leaves (stamens 

 or ovaries) ; these clusters of reproductive organs may have 

 a number of sterile leaves below them on the stem, the 

 floral leaves, or perianth. In other cases both kinds of re- 

 productive organs are in one flower, when the ovaries are 

 highest on the stem, the stamens being next, and the 

 sterile leaves (if any) lowest of all (Fig. 148). There is, 

 moreover, great diversity in the development of the sterile 

 leaves, varying from a few small green or pale leaves to 

 two or more distinct whorls of sepals (the outer) and 

 petals (the inner) which may show great differences in 

 shape, size, texture, and color. 



473. The stamens of Angiosperms often bear so little 

 resemblance to leaves that their real nature would not be 

 suspected. There is usually a slender stalk, the filament, 

 at the top of which are from one to four pollen-sacs, the 

 latter forming the anther. We may regard the filament 



