FLOWER 



FLOWER 



1251 



mentioned the wonderful adaptations of the orchid 

 flowers, the catapulting of the pollen of the orchid 

 Catasetum against the insect, the lever-hammering 

 stamens of Salvia, the deliberate stuffing of the Yucca 

 stigma with pollen by the Pronuba moth as she deposits 

 eggs in the ovarj', the gall flowers and caprification of 

 the fig, and many other equally extraordinary cases. 



Although most plants seem to need cross-pollina- 

 tion and to have structures adapted to this end, there 

 are some in which definite preparation is made for 

 close- or self-pollination. Thus certain plants, its violet, 

 barley, Polygala, Dalibarda (Fig. 1217) and others, 

 produce cleistogamous flowers, which are small green 

 apetalous structures often hidden by the leaves or are 

 even subterranean. The calyx of these flowers never 

 opens. The anthers lie against the stigma, and on open- 

 ing, the pollen is immediately applied to the stigma of 

 that same flower. Seeds produced by such flowers are 

 often much in excess of those produced by the showy 

 flowers of the same species. In the violet (Fig. 1540), 

 cleistogamous flowers are produced in abundance 

 through the summer after the showy flowers have 

 disappeared. Incidentally it is interesting that these 

 flowers in violets are more important in classification 

 than are the showy ones. 



Evolution of the flower. — In the Thallophyta, Bry- 

 ophyta and Pteridophj^ta there is no flower as that 

 term is here used. The sporophyte shows an increas- 

 ing complexity through these groups, but there is no 

 dilTerentiation into an organ that could popularly or 

 even technically be called a flower. Among the Gym- 

 nosperms, the cones of the Pinacea; have been hkened 

 to a flower with many carpels but with no calyx or 

 corolla, while those of the Gnetacea; are still more 

 flower-hke. The true flower, however, is a structure 

 characteristic of the Angiosperms. 



There are two prominent theories in regard to the 

 origin of the flower. First, the foliar theory holds that 

 sepals, petals, stamens and carpels are real leaves 

 modified in the course of evolution from the foliage- 

 leaves of their ancestors. Floral parts are, therefore, 

 metamorphosed leaves. The evolution in this case 

 would have been from below toward the apex of the 

 floral shoot, or from the foliage leaves toward the 

 carpels. Certain teratological conditions have been 

 cited in support of this theory, especially when petals, 

 stamens and sometimes carpels have been replaced by 

 green leaves. This has been considered merely a 

 reversion to ancestral conditions. Trillium grandi- 

 florum frequently furnishes cases of this sort. This 

 theory has been exclusively held in the past. Recently 

 another wholly different theory has been proposed by 

 Bower, and is now acceptefl by very many botanists. 

 This has been termed Bower's sterilization hypothesis. 

 It holds that the foliage-leaves together with the sepals 

 and petals are sterilized sporophylls anrl that evolution 

 has been from above downward. Specifically it holds 

 that although the simple sporophyte of the mosses 

 consisted as at present of a capsule and seta undiffer- 

 entiated into stem and leaves, in some special groups 

 of mosses, however, the spore-bearing region around 

 the columella of the capsule became .segmented into 

 transverse belts separated by sterile belts. Coincident 

 with this, the exterior of the capsule became lobed in 

 such a way that each fertile belt came to he in the axil 

 of a lobe. From this it is easy to postulate an increase 

 in size of the lobes to form the scale-leaves of the club- 

 mosses and selaginellas, and an increase in specializa- 

 •tion of the fertile belt to form the axillary sporangium 

 of these plants. It is but a step now to the angiosperm- 

 ous flower, in which some of the sterile sporcphylls 

 have become modified into petals and sepals instead of 

 leaves. The demand for a large independently growing 

 sporophyte is thought to have led to the sterilization 

 of the sporophylls. According to this theory, leaves are 

 recent rather than primitive structures. The steriliza- 



tion theory has the advantage of being more in accord 

 with modern knowledge of the evolution of organs in 

 these groups. 



Floral evolution within the angiosperms is also diffi- 

 cult to follow and botanists differ as to its course. It is 

 by many hekl that the most ancient type is the acyclic 

 type as represented by the Ranunculaceae, Magnolia- 

 cea; and the like. Another although gradually dimin- 

 ishing school holds that the simple flowers of the 

 Gramineae among the monocotyledons and the Amen- 

 tiferae among the dicotyledons are the most primitive. 

 The high specialization of other parts of these plants 

 and the hkelihood that the flowers have been simphfied 

 because of the adoption of the wind method of pollina- 

 tion, strongly suggests that these flowers are not primi- 

 tive but specialized. 



The flower from standpoint of comparative mor- 

 phologij. — The newer evolutionary morphology has 

 brought about changes in viewpoint in regard to floral 

 parts, and a new 

 terminologj' has 

 arisen. Accord- 

 ing to present 

 knowledge, there 

 is in some algae 

 and in all bry- 

 ophytes, pterid- 

 ophytes and 

 spermophytes a 

 definite alterna- 

 tion of two gen- 

 erat ions or 

 phases in thelife- 

 history of each 

 plant, separated 

 by a unicellular 

 condition of the 

 organism. One 

 of these, the 

 more primitive, 

 bears only sex- 

 cells (eggs and 

 sperms) called 

 gametes and is 

 termed the gam- 

 etophyte, , w h i 1 e 

 the other bears 

 spores only and 

 is termed the 

 sporophyte. 

 These genera- 

 tions have ex- 

 actly reversed their relative size, complexity and 

 degree of independence as evolution has progressed. 

 The originally independent carbon-assimilating gam- 

 etophyte of the mosses has become in the higher 

 plants wholly parasitic on the sporophyte and is 

 entirely lacking in green color. On the other hand the 

 sporophyte, represented in the mosses and liverworts 

 by the dependent capsule and seta stalk, has become 

 the real plant, bearing leaves and flowers in the higher 

 group. The thalloid reduced gametophyte of the ferns 

 is termed a prothallium, bearing sperm-cells in antheri- 

 dia and an egg-cell in an archegonium. This prothal- 

 lium has become differentiated in the more specialized 

 family Selaginellaceae into two types differing in size 

 and complexity of structure, and originating from spores 

 of different size. The large type of spore (megaspore or 

 macrospore) gives rise to the large female prothallium 

 which bears the archegonia; and the small spore (micros- 

 spore) gives rise to the small male prothallium bearing 

 only a single antheridium. The prothallia of both 

 sexes are very much reduced and permanently inclosed 

 within the spore wall. In the flower-bearing plants, the 

 reduction and dependence of the gametophyte have 

 been carried much farther. The male gametophyte or 



540. Common blue violet. 



The familiar flowera are shown, natural 

 size. The corolla is spurred. Later in the 

 season, cleistogamous flowers are often 

 borne on the surface of the ground. A small 

 one is shown at o. A nearly mature pod is 

 shown at b. Both a and b are one-third 

 natural size. 



