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.

The avocado, female today, male tomorrow

The Avocado, Persea gratissima or Persea americana, is a Central American fruit tree of the Lauraceae family of grown three races: Mexican, Guatemalan and Antillean, with many natural hybrids between them have led multiple varieties. Mexican cultivars generally resist the cold better than the Guatemalan and Antillean. Under optimum growing avocado tree born of seed can exceed 20 meters, being a wild specimen reaches 30 meters. The grafted samples are much lower, which facilitates the collection of fruit. In recent decades, the cultivation of varieties of Mexican race has spread to many countries with subtropical and Mediterranean climate without frost, especially in coastal areas.

The avocado has hermaphrodite flowers that prevent selfing of a very clever way. The flowers open in two phases well separated in time. In the first phase all flowers of same tree open as female pistil receptive to pollen from other trees, but with undeveloped six stamens to pollen can not fertilize at the same pistil. These details are clearly visible in this and the following images. Recommend expanding the photos with a double click.

Avocado flowers are inconspicuous. Lack petals. The sexual organs are surrounded by six sepals yellow-green to a yarn in each of them. To attract pollinating insects have three orange nectaries situated between the stamens and ovary. With the nectar the bees produce a dark honey with a characteristic taste. In plantations of Malaga, Granada and the Canary Islands produce large amounts of avocado honey protected designation of origin.

In this other flower in female phase is clearly visible all the details. In the center is the swollen ovary of the white pistil leaving just the stigma receiving pollen. Surrounding the ovary are the three nectar of a vivid orange, followed by the six stamens in which extreme are full of pollen anthers still immature.

Finishing the day of the first stage of flowering the female stigma is sealed and the male anthers of the stamens begin to mature, so that at sunrise the next day all flowers of same tree enter the male phase of flowering and anther open for insect pollinators are impregnated with the pollen and bring it to the flower of another avocado that day is in female phase. Thus it is almost impossible for the own pollen fertilizes the stigma, thus avoiding inbreeding would lead to the degeneration of the species perpetuating regressive genes.

Here we see three flowers in male phase with mature stamens that have been stretched and lifted so that the insects gluttonous of the nectar are impregnated with the pollen from the anthers.

In some mysterious way which has not yet been explained all individuals of the same cultivar agree to bloom at the same level on alternate days. In large plantations of avocados grow different cultivars are often intermingled, as if they were planted all of the same cultivar bloom all at the same level and no flowers would be pollinated so they would not give any fruit.

In avocado plantations of a single cultivar, to get to fruition, the agriculturists use the random planting of some individuals born seed edges, ie mongrel hybrid, each of which blooms in a different phase, so that ensure good pollination, and that whatever the stage of flowering of avocado plantation edges will always be several avocados flourish in a different phase, bringing the precious pollen by bees and other insects.

In this type of fruit is well understood the importance of bees as pollinators, without which very few flowers would be pollinated and the fruits would be very rare. Pesticide spraying in full bloom would be catastrophic for the bees and productivity of avocados.

Avocado flower in the male phase. The sepals are bent down to make it exposed the stamens and facilitate impregnation with pollen from insects. In this second phase the three nectaries still produce abundant nectar to attract pollinators to flowers.

Another flower of Persea gratissima in male phase. If the ovary is fertilized with pollen from another avocado in a few days begin to grow larger and change its color from white to green. With the passage of the days its weight will double the long petiole downwards and the fruit will acquire the aspect of a testicle. Just the name of avocado, ahuacatl, comes from the ancient Aztec language Nahuatl and means testicle. The word guacamole, ahuacamolli, also comes from the Nahuatl language and means avocado salsa. During the Inca empire avocado cultivation spread south and in Quechua-speaking Incas gave the name of Palta, which is why in South America they call the fruit Palta and the tree Palto.

And here is the result of this curious flowering in two phases, a beautiful ahuacatl or palta seed always a hybrid, which if planted will result in a massive tree that will take about 12 years to give their first flower and 15 years to produce the first fruits, as during the early flowers tend to fall without reaching fruition.

The fruits may be round or oblong, with green or purple skin, smooth or rough and weight can range from about 50 grams in dwarf avocados without seed to more than one kilogram in some varieties of Puerto Rico.