FLOWER 



FLOWER 



1249 



petalous of five fused petals, have ten stamens iu two 

 whorls all inserted on the corolla, and two carpels 

 united into one pistil with a superior ovarj'. 



Double flowers. — Occasionally in nature and very 

 frequently in cultivation, the number of petals becomes 

 very greatly increased, often to the exclusion of the 

 stamens and pistils, so that the flower presents a full 

 rosette-Uke appearance. Such flowers are popularly 

 said to be "full" or "double." The increase in petals 

 is apparently a mutation, but is stimulated by changes 

 in nutrition due to cultivation. Most double-flowered 

 varieties tend strongly to run out. The origin of the 

 extra petals is not always the same. In most cases, as 

 in double hollyhocks and carnations, the stamens and 

 even carpels have been transformed into petals; in 

 rarer cases the extra structures are interpolated organs. 

 Double "flowers" in the sunflower, golden glow, and 

 the hke, are simply heads in which all disk-flowers are 

 converted into ray-flowers (see next paragraph). 



False flowers of the Composite (Figs. 15.35, 1536). — The 

 so-called flowers of such plants as the white daisy, sun- 

 flower, aster, goldenrod, and dandelion are found on 

 close study not to be flowers at all, but flower-clusters 

 of the type termed heads. These heads are remarkably 

 specialized for economy and division of labor. This 

 community of flowers functions as does one individual 

 flower in other cases, and the whole make-up of the 

 head simulates a flower to a remarkable degree. 

 Around the head is a caly.x-like involucre of bracts, 

 functioning like a calyx as a protection in the bud. In 



1534. Diagrams of the flower of drosera, vismia and viola. 



daisy, sunflower and others there is a corolla-like part 

 consisting of highly modified ray-flowers or ligulate 

 flowers. The central part of the head in these plants is 

 occupied by disk-flowers. The aster, goldenrod, cone- 

 flower and many others are hke the daisy, while in the 

 dandehon, chicory, hawkweed and sow thistle the head 

 consists of ligulate flowers only, and in the thistle, bone- 

 set and iron weed the head contains only disk-flowers. 

 The morphology of the less specialized disk-flower is as 

 follows: A one-ceUed, one-seeded inferior ovary is sur- 

 mounted by a variously modified calyx, which is often 

 wanting, and a tubular five-toothed gamopetalous 

 corolla. On the corolla-tube are borne five syngenesious 

 stamens, and from the summit of the ovary projects a 

 single style which is two-branched above. The ray- 

 flowers have been developed from the disk typo in the 

 course of evolution by greatly increasing the size of 

 such a tubular corolla, and by sphtting the tube down 

 one side, at the same time flattening out the slit por- 

 tion. In the sunflower, there was no great change in 

 color as the ray-flowers evolved, while in the daisy and 

 the asters the rays are of a different color from the 

 disk-flowers. Since the involucre performs for the whole 

 head the same function that the individual calyx does 

 normally for each flower, there is no longer anj- neces- 

 sity for the calyx. Therefore, following the general 

 rule that a useless structure tends either to disappear 

 or take on a new function, the calyx has become 

 obsolete in some cases while in others it has become 

 rnodified into scales, awns or bristles (pappus) which 

 aid the fruit in dissemination. In many cases the ray- 

 flowers have been sacrificed entirely for insect attrac- 

 tion and have become sterile. By this massing of the 

 flowers, more flowers may be pollinated by one insect 

 visitor, and more easily pollinated. Eflnciency and 



80 



1535. Head of composite, showing re- 

 ceptacle at e, bearing the disk-flowers. 

 The long rays are shown, and beneath 

 them the hairy involucre. — Rudbeckia. 



economy run through the whole organization of the 

 composite head to a remarkable degree. 



The biology of the flower. — The flower is a structure 

 developed by plants to promote and safegu.ard sexual 

 reproduction, primarily in land plants, and to bring 

 about cross-polhnation in these plants. The three 

 definite agents of 

 cross - pollination 

 with which the 

 flower is c o n - 

 cerned are water, 

 wind and insects. 

 The agent for 

 which the flower 

 is adapted exerts 

 a profound influ- 

 ence on the struc- 

 ture of the flower. 

 Only insect - pol- 

 hnated flowers are 

 normally showy. 

 Water- and wind- 

 pollinated flowers 

 are usually green and small, with often a total loss of 

 corolla or of both corolla and calyx. The pollen in such 

 plants is produced in abundance to make up for great 

 loss, as it is wafted indiscriminately through the air. 

 Water plants usually flower at the surface and are 

 wind- or insect-pollinated. The true water-pollinated 

 or hydrophilous plants are few in number. Naias, 

 Zannichellia, Zostera and Ruppia may be mentioned, 

 all of which belong to the Naiadaceae. In Zostera, the 

 pollen-grains are long and spiral as a further adapta- 

 tion to water-pollination. 



Wind-pollinated or anemophilous flowers (Figs. 1537, 

 1538) are very numerous. Elodes and Vallisneria (eel- 

 grass) among aquatic plants may be mentioned. Val- 

 lisneria is remarkable because the staminate flowers 

 break off before anthesis, rise to the surface, expand, 

 and are floated about by the wind, the three reflexed 

 sepals acting as floats which cannot be upset. The pis- 

 tillate flowers are attached to long peduncles which 

 extend to the surface of the water, whether it is shallow 

 or deep. The pistillate and staminate flowers are so 

 shaped that when the two float together the stamens are 

 in exactly the right place to touch the stigmas. After 

 pollination, the peduncle coils up and the fruit matures 

 under water. The catkin-bearing trees are all ane- 

 mophilous and have very much reduced flowers. The 

 willows are both wind- and insect-pollinated. Among 

 herbs the grasses, sedges, rushes, and sorrels (Rumex) 

 are wind-pollinated. Interesting in this respect is 

 Thalictrum (meadow-rue) of the Ranunculaceie, the 

 flowers of which are wholly green and insignificant 

 with large exserted anthers and abundant pollen and 

 feathery stigmas. It thus exhibits perfectly the various 

 adaptations to wind-pollination in a family that is 

 normally insect-pollinated and has showj- flowers. The 

 time of flowering of wind-pollinated flowers often shows 



1536. Parts in the head of a coreopsis. 



a distinct relation to efficiency. The wind-pollinated 

 trees and shrubs bloom in early spring before the leaves 

 interfere with the passage of pollen through the air. 

 The grasses and other herbaceous anemophilous plants 

 bloom before the tall growth of late summer has 

 maturcfl, at which time plants are mostly insect-pol- 

 linated. The pollen-grains of anemophilous plants are 

 nearly always smooth and very light, and usuaUy con- 



