Sunday, June 19, 2011

Extrafloral nectaries, a candy for ants

The nectaries are vegetal organs that produce secretions rich in sugars, fats or amino acids or a combination thereof. They can be floral nectaries secreting nectar from flowers and extrafloral more geared to reward the insects that defend the plant from herbivores. 

Tiny extrafloral nectaries in the internode of the stem of a shrub of the red variety castor, Ricinus communis var. sanguineus. In each of them can see the stoma or navel-shaped opening through which a droplet of nectar secreted when stimulated by touch from the jaws of an ant. This detail is best zoom with a double click.

The extrafloral nectaries appeared millions of years long before the first primitive plants develop flowers. It is believed that its formation was initiated by a need for plants excrete excess sap, either through the stomata or pores of the leaves or the microtubules of the glandular hairs or trichomes. Over millions of years of evolution plants learned they could use this excess sap their advantage with two very different purposes:

A-The first purpose was to fend off enemies in association with the ants in an intelligent symbiosis. It all started when the ants in his eternal wandering in search of food found droplets of sap that had to excrete excess plant. Tried it and liked it, so that gradually his visits to the source of sap became more frequent. For a simple rule of economic transaction: I give you what you want to change what you have and I will, slowly excreted sap was concentrated more and more becoming sweeter and richer in amino acids and lipids. At the same time the ants were becoming increasingly dependent on this valuable source of food until they could no longer survive without it.

 Another image of the extrafloral nectaries of castor plant. The symbiosis with ants is not complete, since their nourishment does not depend exclusively on nectar, but manages to rise to their branches and go through all the leaves and shoots, deterring phytophagous insects that are immune to castor poison. Herbivorous mammals avoid eating this plant because they instinctively know it is toxic.

Male flowers castor accompanied by small extrafloral nectaries. This plant has a anemophilous fertilization dependent wind carrying pollen from male flowers of a plant to female flowers of another plant. To avoid self-pollination the female flowers of each inflorescence open earlier than male ones, so that when castor plant emits its own pollen the female flowers are fertilized and are not receptive. At the top of the image are immature fruit of the female flowers that were fertilized a few days before the pollen of another castor.

Some plants went so far as to develop special structures so that the ants could live on them. They were having a private army of soldiers in permanent monitoring. Thus, some tropical orchids have developed cavities in their rhizome specially designed so that the ants can live comfortably in them. The nectar of the extrafloral nectaries containing a combination of nutrients ideal and complete them and no longer need to leave the orchid to get food. Thus the symbiotic ants receive food and shelter in exchange for defense services. 

Some acacias have swollen and hollowed their spines to make them the ideal habitat for the tiny ants that live in symbiosis on them. A small opening in the spine makes the role of gateway to the swarm of hard cellulose. Ants continuously patrol all the branches and leaves of the acacia to detect any enemy, from a simple phytophagous grasshoppers to rough tongue of a gazelle that likes the nutritious young leaves of the acacia. Through the issuance of special ant pheromones alert a colony of the enemy attack and go angry to defend his home and food source. The bites of their strong jaws and squirts formic acid caustic throwing with unusual force by the end of their abdomen deter the grasshopper and quiet gazelle acacia leaves. No symbiotic ants that defend the acacias are rapidly eliminated by the army of hungry mouths to inhabit the arid savannas.

B-The second purpose of the nectaries was to attract insects, birds, reptiles and mammals into the primitive flowers thus facilitating genetic exchange through pollination. To this end, closest to extrafloral nectaries of flowers came to merge with them, joining the structure of their sexual organs. Just emerging floral nectaries are therefore evolutionarily primitive post extrafloral nectaries.

 These small flowers of til or garoe, Ocotea foetens, photographed in the Bosque de los Tiles on the island of Palma, are perfectly orange floral nectaries situated between the stamens and pistil. In this way the plant gets greedy insects that are impregnated with the nectar pollen and bring it to the pistils of other flowers.

 This curious flower plant belongs to the Mediterranean is only pollinated by birds. This is the legume Anagyris foetida, highly toxic and gives off a foul odor by rubbing their leaves by hand. However, the nectar of the floral nectaries located at the bottom of the flower is not toxic and is a candy for the small birds that pollinate it.

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