स्चिम्पेर(Schimper 1898) के अनुसार व पादप जो कि जलाभाव (drought) की परिस्थितियों में अपनी वाष्पोत्सर्जन क्रिया को मंद करने की क्षमता रखते हैं, उनको मरुभिद् कहा जाता है जबकि जेन्टेल (Gentel 1946) के अनुसार मरुभिद् शुष्क आवासों में उगने वाले पौधे होते हैं जो अपनी विशिष्ट आंतरिक संरचना एवं कार्यिकीय विशेषताओं के कारण मृदा एवं वायुमण्डल की शुष्क परिस्थितियों को सहन करने में सक्षम होते हैं या यूँ कहें कि, मरुभिद् (Xerophytes) शुष्क आवासों या मरु प्रदेशों में पाये जाने वाले पेड़-पौधों को मरुभिद् कहते हैं. मरुदभिद् पौधों में जलाभाव या शुष्कता के प्रति सहिष्णुता या इसे सहन करने की अद्भुत क्षमता पाई जाती है.
मूल (Root) :- मरुभिद् पौधे प्रायः शुष्क या जलाभाव वाले स्थानों में पाये जाते हैं. इसलिए इनका मूल तंत्र अधिकाधिक जल का अवशोषण करने के लिए अत्यधिक विकसित एवं गहरा होता है. इन पौधों में जड़े चारों तरफ प्रसारित एवं सुविकसित होती है. इनकी जड़ें प्राय मूसला मूल (tap root) प्रकार की होती हैं, जो जमीन में बहुत अधिक गहराई तक जाती है. मृदा में इन जड़ों की शाखाओं का एक विस्तृत जाल फैला हुआ रहता हैं. सेकेरम मुन्जा (Sacchrum munja) में जड़ें, महीन रेशेदार (fibrious) एवं भूमि को ऊपरी सतहों में ही फैली हुई रहती हैं. एक वर्षीय एवं शाकीय मरुभिदों में भी जड़ों की गहराई कम होती है.
स्तंभ (Stem) :- मरुदभिदी पौधों के तनों में अनेक प्रकार के अनुकुलन लक्षण पाये जाते हैं, क्योंकि पौधे का यह भाग वातावरण के सीधे सम्पर्क में होता है. अधिकांश मरुभिद पादप काष्ठीय होते हैं. ये बहुवर्षीय शाक, झाड़ियों अथवा वृक्षों के रूप में पाये जाते हैं.
पर्ण (Leaves) :- कई पौधों में प्रारम्भ में ही पत्तियाँ विलुप्त हो जाती हैं, बहुत थोड़े समय के लिए दिखाई देती हैं. इस प्रकार के मरुभिद पौधों को आशुपाती (caducous) कहा जाता है, जैसे लेप्टाडीनिया (Leptadenia pyrotechnica) कुछ पौधों, जैसे- कैर (Cappar is decidua) में पत्तियाँ पूर्णतः अनुपस्थित होती हैं.
फल तथा बीज (Fruits and Seeds) :- मरुभिद् पौधों में पुष्पन तथा फलों का निर्माण अनुकूलन परिस्थितियों में अर्थात् जल की पर्याप्त मात्रा एवं वातावरण की उपयुक्त परिस्थितियों में होता है.
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According to Schimper (1898) plants that can slow down their transpiration activity in drought conditions, are called xerophytes, whereas according to Gentel (1946), xerophytes are plants growing in dry habitats. Due to their specific internal structure and physiological characteristics, they are capable of tolerating the dry conditions of soil and atmosphere or in other words, Xerophytes. The trees and plants found in dry habitats or desert regions are called Xerophytes. Xerophytic plants have amazing tolerance or tolerance towards water scarcity or dryness.
Root: – Desert plants are generally found in dry or water-scarce places. Therefore their root system is highly developed and deep to absorb maximum water. In these plants, the roots are spread all around and are well-developed. Their roots are usually of tap root type, which go very deep into the ground. A wide network of branches of these roots remains spread in the soil. In Sacrum Munja, the roots are fine fibrous and remain spread only in the upper surfaces of the soil. The depth of roots is also less in annual and herbaceous shrubs.
• Root hairs and root caps are well developed in the roots, due to which the roots can absorb the maximum amount of water.
• The growth rate of roots is generally found to be high in these plants. In many plants, the daily growth of roots is 10 to 50 cm.. it occurs. In phytophyte plants, which are found on top of rocks, the roots are capable of spreading over the rocks penetrating their cracks and growing.
Stem: – Many types of adaptive traits are found in the stems of xerophytic plants because this part of the plant is in direct contact with the environment. Most xerophyte plants are woody. These are found in the form of perennial herbs, bushes or trees.
• The stems of some plants like Citrullus and Sericostoma etc. are highly branched, but their branches are close together.
• The stems of some desert plants are underground, like Aloe, Agave and Sacchrum etc.
• Multicellular hairs (trichomes) are found in abundance on all three, such as in Arnebio and Calotropis. Apart from this, in some plants like Calotropis, a waxy coating is found on the surface of the stem and leaves.
• In some plants like Cocoloba, Ruscus and Opuntia, the stem gets transformed and becomes flat and green like a leaf, this is called phylloclade. In Asparagus, multicellular hairs (trichomes) are found in abundance on all three axils of the stem, like in Arnebio and Calotropis. Apart from this, in some plants like Calotropis, a waxy coating is found on the surface of the stem and leaves. Axillary branches also get transformed into green-coloured and acicular structures. These are also called Parnabh festivals.
Leaves: – In many plants, leaves disappear in the beginning and remain visible for a very short time. This type of desert plant is called caducous, like Leptadenia pyrotechnica. Leaves are completely absent in some plants like Cappar is decidua.
• In many deserts, the size of the leaves is small, and in the plants that have large leaves, the surface of the leaves is shiny and smooth, due to which intense light is reflected. Due to this, the temperature of the leaf decreases and the rate of transpiration also decreases. The leaves of some plants like Tamarix are sharp and needle-like.
• In some plants like Opunita, leaves get transformed into thorns. Whereas in some other examples like Ruscus, Asparagus, Casuarina and Muehlenbeckia, the leaves degenerate into scales.
• A covering of wax and silica is found on the leaves of many plants, such as in Calotropis.
• In xerophyte plants, the size of the leaf lamina generally becomes smaller, such as Prosopis and Acacia. The leaves of some plants like Parkinsonia are extremely small in shape, but their rachis is flat and thick. It works to protect the leaves from strong sunlight.
• High-speed winds and storms often blow in dry and desert areas. Therefore, in desert plants found in such places as Arnebia, multicellular hairs (trichomes) are found on the surface of leaves. These hairs cover the epidermis and stomata and reduce the rate of transpiration.
• In some monocotyledonous xerophyte plants and grasses like Poa, Ammophila and Agropyron, at times of water scarcity or dryness, the leaves fold into a round shape or a tube.
• The leaves of some xeric plants like Banksia and Cycas become thick, sometimes fleshy.
Fruits and Seeds: – Flowering and formation of fruits in desert plants take place under favourable conditions i.e. sufficient amount of water and suitable environmental conditions.
• Apart from this, the germination of seeds in these plants occurs at higher temperatures, and the seeds also show tolerance towards higher temperatures.
• Generally, in these plants, the upper coverings of fruits and seeds are hard, which protects the seeds and embryos, and they are also permeable to water.
• Anatomical adaptations – Along with morphological adaptation traits, various types of specific traits are also found in the internal structures of desert plants, due to which minimum expenditure of water is ensured in these plants. On studying the internal structure, different types of adaptations are found in the roots, stem and leaves of these plants which are as follows…
1. A thick layer of lignin and cutin develops on the epidermis. In some xerophytes like Calotropis, the deposition of wax or silica is found on the epidermis. Many types of multicellular hairs (trichomes) etc. are found on the outer skin. The outer surface of various plant parts is shiny due to which the sunlight falling on them gets reflected.
2. Their epidermis cells are small in size and compactly arranged, the outer walls of these cells are lignified…like Nerium. More than one layer present on the outside protects these aerial parts from strong sunlight and reduces the speed of transpiration.
3. Due to the presence of motor or hinge or bulliform cells present in the upper epidermis in the leaves of some special types of desert grasses, such as Poa and Psamma, these leaves are wrapped and round. It happens. Due to this, they remain safe from the strong sunlight and there is no transpiration from the stomata present at this stage.
4. In some plants, like Casuarina and Ephedra, the external outline of the stem is differentiated into ridges and grooves. Here sunken stomata are also found in the grooves. Multicellular hairs (trichomes) are also found around the stomata in these grooves or cavities.
5. Cortical tissue (cortex) is also found well developed in desert animals. Resin and latex vessels are mostly found in these cortical tissues, such as in the Pinus and Calotropis.
6. In fleshy or pulpy plants (succulents) like Opuntia, Aloe Euphorbia etc., a special type of thin-walled parenchyma cells are developed to store water.
7. In drought-resistant xerophyte plants, the cells are usually of small size, and the inter-cellular spaces between them are also found in less quantity. Mechanical tissues are abundant in their internal structure.
8. Starch particles are often present in the endodermal cells of xerophyte plants, that is why the endodermis here is also called starch sheath.
9. In xerophyte plants the transport tissues are completely well developed, but in them xylem is more developed than the phloem. The vessels of xylem tissue are large and long and a relatively large amount of thickening lignin is found on their walls.
10. Due to secondary growth, well-developed annual rings, cork and barks are found in xerophyte plants.
11. In the leaves of xerophyte plants, columnar columnar parenchyma and spongy parenchyma are well differentiated in the leaf mesophyll. Pillar parenchyma is more developed than spongy parenchyma, such as in Nerium, where pillar parenchyma is closer to the upper and lower epidermis and spongy parenchyma is located between these two tissues. Folded mesophyll cells are found in the needles of Pinus.