Botany's Lush Leaves
-Drought Dormancy: Some desert trees and shrubs shed their leaves during dry periods, and then put out new leaves when rains return. This is called drought deciduous. Other desert plants live out the dry season as fleshy roots or underground stems, putting out foliage only when conditions are favorable. Arrowleaf balsamroot (Balsamorhiza sagitatta) is a good example of this; it spends much of the year as a taproot with little or no sign of its existence above ground.
Botany's Lush Leaves
-Physical Adaptations: Desert plants have many physical adaptations that allow them to survive in hot, dry climates. The thick, fleshy leaves of cacti and other succulents store water for future use. The roots of some desert plants are shallow but horizontally extensive in order to capture water more effectively when rains come. The roots of other desert plants extend deep into the ground, some (like the roots of mesquite, Prosopis spp.) even reach as deep as the water table. Palo verdes (Parkinsonia spp.) are drought deciduous trees or shrubs that have photosynthetic bark that can keep photosynthesizing even when leaves are not present. Other adaptations include small leaves, hairy leaves, dull colored leaves, and waxy leaves all of which help to reduce water loss and improve the efficiency of photosynthesis.
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Net photosynthetic CO2 uptake (P N ) and transpiration (E) rates through adaxial and abaxial leaf surfaces were measured gasometrically on the area of maize leaf blades of different insertion levels in the phase of male florescence and in the phase of spadices development. The distribution pattern of gas exchange on the area of all measured leaf blades found in the phase of male florescence (significantly lower value in the basal parts) was changed in the phase of spadices development: PN andE of abaxial surfaces of older leaves,i.e. 9th, 10th (under which the spadices were growing) and 11th, rapidly increased in the basal parts in comparison with the middle ones and positively influenced the gas exchange of the whole leaves. It appears that PN may be maintained or increased in the older leaves in the phase of spadices development to prevent shortage of photosynthates.
Mediterranean climate. The history of a separate Mediterranean climate really started 6 million years ago. The collision of India into Asia caused the rise of Tibet, and Africa's movement north caused mountain building in Spain and North Africa. The Tethys sea dried up leaving a huge salt pan. Presumably most vegetation in the area that was subject to wind blown salt dies, also the temperature was very high and the area arid. Then the Mediterranean opened and a fairly lush vegetation grew with invasions from the surrounding areas. The onset of glaciation brought climatic fluctuations which gradually settled to resemble more or less the climate we see today. This caused a gradual disappearance od plant unable to withstand periods of summer drought - these would either die out or retreat north - the south was even drier. So there was a gradual change favouring plants that could withstand quite long droughts on a soil that had never been very nutrient rich and may still have contained salt which could rise to the surface by capillary action. Added to this were the frequent volcanic eruptions causing local extinctions, but more widespread depositions of pumice etc. During this time such plants as remained would have to evolve mechanisms to cope with the increasingly harsh and unpredictable climate.
Sclerophyllous leaves are found on plants from dry, hot and nutrient poor habitats. They are usually relatively small, with small thick-walled cells, and often both faces of the leaf are the same. They are usually covered by a thick cuticle and epidermis. Some exude wax or resin, and the stomata are often sunk into the leaf and may be protected by hairs. The leaf may also roll up to protect the stomata from excessive evaporation; this is common in plants from salty areas. Most of these modification appear to have arisen to conserve water, but they are also useful in nutrient poor habitats. For instance smaller leaves require less nutrients as do leaves that last for more than one year.
Origins. It is postulated that the sclerophyllous vegetation of the Mediterranean regions has evolved from the lusher tropical vegetation that may have existed before the onset of the climatic changes. And the various adaptations we see today came about in order to cope with the increasingly harsh conditions, and those that couldn't cope simply went extinct.
The future. For a plant so limited in nutrients and water the investment in the production of fleshy fruits seems excessive. But these fruits are dispersed by birds, and birds like fleshy fruits. This is another common link to the lush tropical flora that is believed to have existed previously. Recently it has been noticed that ants play a greater part in dispersal of seed than had previously supposed. If a plant relies on ants entirely it will have no need for fleshy fruits, as the ants are interested in the lipid-rich elaiosome around or in a groove of the seed. So there may be a general trend in the sclerophyllous Mediterranean vegetation towards dry fruit dispersed by ants. This is common in other Mediterranean climates, but there may be dangers ahead as in some places the Argentine ant, Iridomyrmex humilis, has displaced the native ants. The Argentine ant is not interested in elaiosomes, so will not disperse seed. These plants may also be able to reproduce vegetatively, but after fire, seed dispersal is important. And unless the seeds are cached by harvester ants they are unprotected. So regeneration after fire is severely limited. This may lead to soil erosion and desertification. 041b061a72