Chapter 7. Flowers and Adaptations for Pollination and Seed Dispersal

 

The evolution of flowers, bisexual strobili that placed stamens and carpels in close proximity, led to an incredible array of adaptations for both pollination and fruit dispersal by animals and abiotic factors like wind and water.

1. The position and timing of pollen shed and stigma receptivity are carefully coordinated in flowers to avoid self-pollination while optimizing movement of pollen from flowers on one plant to flowers on another plant via animal or abioltic vectors.

2. Fragrance, color, and nectar evolved to attract animal pollinators, and are often highly specific.

3. Specialized shapes of flowers are adaptations that facilitate access to specific animals that can be relied on to carry pollen to flowers of the same species, while excluding more generalist flower feeders.

4. In some plants, an entire inflorescence may coordinate shedding of pollen and receptivity of  stigmas, acting as a "super flower."

 

Figure 7.1. When the orchid Angraecum sesquipedale was discovered in Madagascar, 

there was much speculation about what could pollinate it. The flowers have extremely 

 long nectar tubes, which could only be accessed by insects with extraordinarily long 

probosces (feeding "tongues,"), but no such insect was known at that time. 

Eventually a moth was found that fit the bill. This exemplifies the often highly specialized 

relationshups between flowers and their pollinators. Drawing from Wallace 1867

 

 

Figure 7.2. Moths in general have long, coiled probosces that can be extended to 

reach into narrow floral tubs or nectar spurs. Drawing from Gray 1879.

 

 

Figure 7.3. Flowers that are erect, open and bowl-shaped, like this field poppy, cater to 

multiple types of small flower feeders, like bees and flies, offering pollen rather than 

nectar as a reward. Generally, stamens release their pollen at a different time from when 

the stigmas are receptive to pollen. Drawing from Kerner and Oliver 1895.

 

Figure 7.4. Horizontally oriented flowers with relatively large openings facilitate relatively 

large bees, who land and crawl in, brushing against the pollen and/or stigmas, or 

by hummingbirds who stick their heads into the flower while hovering. A. Sinningia speciosa (Gesneriaceae) and Leonotis sp. (Lamiaceae). 

Drawing from LeMaout and Decaisne 1876(A) and Kerner and Oliver 1895.

 

Figure 7.5. Fuschia is a large genus found primarily in South America, who demonstrate

the classic hummingbird pollinated flower. The bright colors, particularly red, are highly 

conspicuous to the birds' visual range, and by hanging downward, the flowers exclude insects 

that need to land or crawl into the flower to feed. 

Drawing modified from Lemaout & Decaisne 1876.

 

 

Figure 7.6. Bats, like hummingbirds are able to hover while feeding, but being nocturnal, 

are attracted to white or pale-colored flowers with strong fragrance that open after dark. 

Such flowers, like these of the Saguaro cactus, tend to be large and full of pollen and 

nectar to provide a greater reward for these larger animals.

 

Figure 7.7. Stapeliads, a group of genera in the Milkweed family (Asclepiadaceae) native to

 dry regions of Africa, have bizarrely colored flowers resembling rotting animal flesh, 

and  odors to match. Carrion flies are attracted to these deceptive flowers, 

but get no meal at all from them.

 

 

Figure 7.8. Like stapeliads, the giant Rafflesia flowers of the Sumatran jungle attract flies with 

colors and odors that resemble rotting flesh. Drawing from Kerner and Oliver 1895.

 

 

Figure 7.9. The flower of the orchid Ophrys speculum looks enough like a female  

Campsocolia ciliata wasp to attract a male of that species, who attempts copulation 

with the flower. Frustrated, the male wasp eventually leaves, with pollinia (specialized 

pollen sacs of orchids) attached to its head. Chances are he'll be fooled again and deliver 

the pollen to another similarly deceptive orchid.  

Drawing from Kermer amd Oliver 1895.

 

 

Figure 7.10. The mass blooming of small flowers can function like a super-flower. In  

Hydriastele palms growing in New Guinea, flowers are unisexual, with  male and female 

flowers growing  close together along the long branches of an inflorescence (A). When 

the bract surrounding the inflorescence splits open, all the tiny female flowers, with their 

glistening sticky stigmas (B) are receptive to pollen at the same time, while all the male 

flowers remain closed. Swarms of small insects, attracted  by the fragrance, crawl among the flowers, some carrying pollen from another inflorescence, and feed on the the tissues of the 

male flowers. On the second day, all the male flowers will open simultaneously and shed their 

pollen, but the female flowers are no longer receptive. The insects now leave en masse, 

carrying pollen grains on their bodies, and likely will seek out another newly opened 

inflorescence, carrying pollen to it and repeating the cycle.

 

 

Figure 7.11. Flowers can be packed even more densely into spikes of spadixes, as in  

Peperomia (A), and in members of the aroid (Araceae) (B). As in Hydriastele above, shedding

 of pollen  and stigma receptivity are separated and synchronized. 

Drawings from LeMaout and Decaisne 1876 and Kerner and Oliver 1895.

 

 

Figure 7.12. The ultimate super flowers are the composites (Asteraceae) (A), consisting of a 

head of tiny fertie flowers (B)  and outer flowers  modified to look like petals (C).  

Drawings from Brown 1935 and Gray 1879.

 

 

Figure 7.13.Most marvelous of all, perhaps, are the "inside-out" inflorescences of the fig 

(genus Ficus) that resembles a fruit. the tiny flowers within are pollenated by speciaized 

fig wasps that enter and lay their eggs. After maturing the wasps exit the fig and carry

pollen to another young fig. Drawing from Kerner and Oliver 1895.

 

Figure 7.14. Many trees of temperate climates, such as this Corylus,(Hazelnut) bloom in the 

early spring before new leaves come out. They rely on wind to carry pollen from male flowers 

in flexible catkins (A) to  female flowers with prominent stigmas (B). The resulting fruits (C), 

ripen in the summer or fall and are adapted for dispersal by animals such as squirrels and 

chipmunks. Drawing from Kerner and Oliver 1895.

 

 

Figure 7.15. Grasses are mostly adapted for wind-pollination, with feathery stigmas and 

stamens well exposed. Color, fragrance, and nectar are lacking. 

Drawing from Kerner and Oliver 1895.

 

 

Figure 7.16. A number of angiosperms have adapted for life in the water, and often use 

water itself to move pollen to stigmas. A particularly surprising mechanism is 

seen in Vallisneria. The tiny male flowers break off from their underwater rhizomes 

and float to the surface. The larger female flowers remain attached to the rhizome by a long 

stem, Surface tension around the female flower draws the male flowers in where the stamens 

make contact with the long brushy stigmas (colored purple). 

Diagram from Kerner and Oliver 1895.

Figure 7.17. Mature seeds can be dispersed in a variety of ways. Seeds released from this 

capsule of Bignona sp. are equipped with broad "wings," which help the seeds stay afloat

 in the breeze. Drawing from Kerner and Oliver 1895.

 

 

Figure 7.18. Fleshy fruits, often sweet and juicy, are adapted to be eaten by animals, 

with the seeds passing through the digestive tract unharmed, and spread as the animals

 defecate.  A. typical of the melon family (Cucurbitaceae), the cucumber fruit must be

 opened by animals and the seeds swallowed. B. the single large seed of a peach is protected 

within a large woody pit, and is dispersed as an animal chews off the fleshy tissues and drops 

the pit onto the ground. C. the flowers of strawberries and blackberries, with numerous 

single-seeded carpels borne along the central axis are virtually indistinguishable, but develop 

into different kinds of fruits: In the strawberry (Fragaria) (D) the central stalk swells into a 

berry-like structure with the achenes borne on the outside. The red color of this false fruit is

 particularly attractive to birds, who eat the fruits and pass the seeds through their digestive

 tracts. In the blackberry (Rubus) (E), each achene swells into a small fruitlet, and the entire 

structure is called an aggregate fruit. In a remarkable example of convergent evolution, 

the Mulberry (Morus) (F), forms a multiple fruit from many tiny flowers in a small, 

compact inflorescence, resulting in something superficially resembling a blackberry.

 Drawings from Brown 1935 (A,C,D,F), Lamout and Decaisne 1876 (B), 

 and 4vector.com free clipart (E).

 

 

Figure 7.19. Additional false fruits include the Cashew (A) which is the swollen stalk 

(receptacle) of the true fruit: the nut above. In apples and pears (B), the receptacle has 

already grown around the ovary in the flower before pollination. It, rather than the ovary i

tself, swells to form the edible tissue. Drawing from Kerner and Oliver 1895.

 

 

Figure 7.20. Fruits are not always soft and juicy. Some, as in Clematis (A),  

Acer (maple) (B), and Ptelea (C), are adapted for providing buoyancy for wind dispersal. 

Acorns (D) are gathered and buried by animals like squirrels, and In Hedysarum (E),

 tiny hooks on fruit segments enable them to "hitchhike" in the fur or feathers of animals. 

Drawings from Ganong 1916 (A) and Kerner and Oliver 1895 (B-E).