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 canline 
(see 89). They are similar in struc- 
ture to those of Gymnosperms, with in- 
tegument and micropyle, nucellus, and 
embryo-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 ivith distinct calyx and 
corolla. In the simplest flowers floral leaves do not appear, 
and the flower 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 ovule of 
Angiosperms, showing 
outer integument (ai), 
inner integument (ii), 
micropyle (i), nucellus 
(yfc), and embryo-sac or 
megaspore (em). After 
SACHS. 
