364 



PLANT STUDIES 



any organization of carpels which appears as a single organ 

 with one ovary is a pistil. 



The ovules (megasporangia) are developed within the 

 ovary (Fig. 325) either from the carpel wall, when they are 

 foliar, or from the stem axis which ends 

 within the ovary, when they are cauline 

 (see § 89). They are similar in struc- 

 ture to those of Gymnosperms, with in- 

 tegument and micropyle, nucellus, and 

 emhryo-sac (megaspore), except that 

 there are often two integuments, an 

 outer and an inner (Fig. 326). 



242. Modifications of the flower. — In 

 general, the flower may be regarded as 

 a modified branch bearing sporophylls 

 and usually floral leaves. Its repre- 

 sentative among the Pteridophytes and 

 Gymnosperms is the strobilus, which 

 has sporophylls but not floral leaves. 

 In Angiosperms it begins in a simple and somewhat indefi- 

 nite way, gradually becomes more complex, until finally it 

 appears as an elaborate and very efficient structure. 



The evolution of the flower has proceeded along many 

 lines, and has resulted in great diversity of structure. These 

 diversities are largely used in the classification of Angio- 

 sperms, as it is supposed that near relatives are indicated 

 by similar floral structures, as well as by other features. 

 Some of the lines of evolution may be indicated as fol- 

 lows : 



1. From naked flowers to those with distinct calyx and 

 corolla. — In the simplest flowers floral leaves do not appear, 

 and the flow r er is represented only by the sporophylls. 

 When the floral leaves first appear they are inconspicuous, 

 scale-like bodies. In higher forms they become more promi- 

 nent, but are still all alike. At last the floral leaves become 

 differentiated, the outer set (calyx) remaining scale-like or 



Fig. 326. A diagrammatic 

 section of an ovale of 

 Angiosperms, showing 

 outer integument (ai), 

 inner integument (ii), 

 micropyle (m), nucellus 

 (/:). and embryo-sac or 

 megaspore (em).— After 

 Sachs. 



