My Science School

          The new leaves of any plant appear pink but after some time they turn green. Do you know what the reason behind it is?

          We know that the colour of any substance depends upon its property of absorption and reflection of the different colours of light. The leaves of the plant appear coloured due to the presence of some specific substances in them. For example, the presence of chlorophyll makes them appear green while the presence of carotenes makes them yellow.

          When many substances are present in one leaf, it shows the colour of the mixture. The leaves which contain both chlorophyll and carotene appear yellowish green.

          New leaves of some plants contain a red substance called anthocyanin, which gives the pink or red colour to the tender leaves. With the passage of time, chlorophyll and carotene are formed in these leaves, which ultimately turn them green.

          The structure of chlorophyll molecules is such that when the sunlight (mixture of seven colours) falls on them, they reflect the green colour and absorb the rest. This reflected green colour reaches our eyes and gives the green appearance to the leaves.

          Plants synthesize their food with the help of chlorophyll in the presence of the sunlight. This process is called ‘photosynthesis’. Plants without chlorophyll like fungi cannot make their own food. They depend on other living beings. Such plants are called parasites.

          In this way, we see that chlorophyll is a very important substance for plants which gives them the green colour. 

          Most of the trees and plants shed their leaves during autumn and new leaves sprout on them during spring. After the defoliation they appear quite barren. The trees which shed leaves every year are called ‘deciduous trees’. However, there are other trees which do not shed all their leaves at one time, and thus always appear green. Such trees are called evergreen trees. Do you know why this happens so?

          In fact, all the trees shed leaves. But the leaves of the trees which undergo defoliation every year live only for one year. On the other hand leaves of the evergreen trees last for two to three years. As soon as a leaf withers it falls down and a new leaf sprouts in its place. Thus all leaves are not shed at a time. As a result the tree always retains a large number of leaves and this cycle continues. 

          Among the well-known evergreen trees are pines and firs. Besides these, there are many other trees which always remain green. The Italian cypress found in Europe is one such tree. The olive plant also comes under the category of evergreen plants. Its oil is very useful for us. The magnolia plants are used for decoration because their white flowers are very beautiful and do not have defoliation. Perfumes are extracted from the evergreen plants of the rosemary. The juniper plants, having short and bitter leaves which are found in North Britain, are famous for their greenness. In addition to these, other evergreen plants and trees are myrtle of Britain, oleander of North Africa, oak tree, ivy creeper, orange tree and tall palm tree. The tea plant also comes under the category of the evergreen plants.

 

          Whenever we pass through a garden during the spring the fragrance of flowers enchants us. Nature has endowed flowers with beauty, attractive colours, nectar and fragrance. Insects and flies get attracted towards the flowers and sit on them. And when they leave they carry with them pollen grains to other flowers. This dispersing of pollen grains helps in the pollination of flowers, as a result of which, seeds are produced. In this way plants and trees multiply on their own.

          Different flowers contain different oils which give specific fragrance to flowers. As these oils gradually keep on evaporating, the fragrance of the flowers spreads in the air. In fact, perfumes are prepared from the oils extracted from these flowers. There are various processes of making perfumes. In one process, flowers are kept in a pot through which steam is passed. The outgoing steam brings oil with it. This steam is passed through water. Thus the oil brought out by the steam starts floating on the water surface. This is then separated from water. There are other methods of making perfumes also. France produces the maximum amount of perfumes. It has been found that one ounce of rose perfume is extracted from 110 kgs of rose flowers.

           Flowers like rose, Raat Ki Rani, Pandarus, lavender, jasmine etc., are generally used for making perfumes.

           The anthocyanin pigment is responsible for producing red, blue and violet colours. The plastid pigment causes other colours. These pigments remain mixed in the juice of the flowers. The presence of chlorophyll and carotene make some flowers green. There are at least 200,000 kinds of flowers, ranging in size from the microscopic duckweed blossom with a diameter of 0.4 mm to the tropical Rafflesia with a diameter of 90 cm. 

          On the basis of age, plants are divided into three categories: annuals, bi-annuals and perennials. The annuals are those plants which take birth, grow up, give fruits and die within a year. Wheat, barley, gram, peas, tomato - all live for one year only. The bi-annual plants live for two years. In the first year they bear only leaves, and in the second year - flowers and fruits, and after that they die. Plants like the ‘fox-glove’ and the ‘hollyhock’ come under this category. The perennial plants live for more than two years. Generally they are called trees or bushes. They blossom and bear fruits many times. The trees of the neem, mango, jamun (black plum), guava etc. come under this category. Some perennial trees have a life of more than 4000 years. The great sequoia trees found in California are reported to have a life of more than 4000 years. The yew trees live for 3000 years. The chestnut and the oak trees live for 2000 and 1500 years respectively.

          Now the question arises: how is the age of any perennial tree determined? Scientists have devised a very simple technique to do this. This technique refers to the number of rings present in the trunk of a tree. If you cut off a slice from the tree trunk, you will notice many concentric rings in it. These rings are the record of the tree’s age. In fact, the trunk, underneath the bark, keeps on growing thicker with age. However, the wood which grows in winter and autumn is denser and harder as compared to that grown in spring and summer. This results in the formation of a ring in the trunk every year. These rings are of brown and cream colours alternatively. Thus the number of rings counted from the centre of the trunk of a tree tells its age in years. Its central portion is called the ‘path’.

          Some fruits like watermelon, guavas, grapes, apples, mangoes etc. are sweet in taste while lemon, orange, raw mango, etc. are sour. Each fruit has a distinct taste which differs in some way from the taste of any other fruit. The question arises: why does every fruit have a characteristic taste?

         In fact, the taste of any fruit depends on the compounds present in it. In general a fruit contains fructose (natural sugar), organic acids, vitamins, starch, proteins, minerals and cellulose. All these materials are in a mixed state inside the fruit and are found in different proportions in different fruits. Fruits having more fructose content taste sweeter, while those having more acids taste sour. Orange is a fruit which has almost equal quantities of fructose and acids hence it tastes both sweet and sour.

          In general, raw fruits contain more acids but on ripening, the quantity of acid in them decreases and the amount of sugar increases. Hence raw mangoes are sour, but ripe ones are sweet. Raw bananas contain more of starch, but it gets converted into fructose when the fruit ripens.

          During the process of ripening, chemical changes take place inside the fruit by which the quantity of sugar increases thus adding to the sweetness of the fruit.

          You will notice a difference of taste even in two fruits of the same kind. Two apples or two mangoes do not always taste alike. This is so because there are many varieties of the same fruit and also the variations in the quality of soil, climate, growing technique, manure, water etc. change the proportion of the compounds inside the fruit resulting in the difference of taste.

          Sour fruits like lemons; do not taste sweet even after they are ripe, because of the presence of excessive amounts of acids.

          Generally the edible-fleshy part with seeds which develops from the flowers of any plant is called a fruit, whereas trunks, leaves and flowers of herbaceous plants used as food are called vegetables. But scientifically speaking, the above definitions are not correct.

           According to botanists the part of the plant that carries seeds and produced by the ovary of a flower plant is called fruit and the remaining part used for food is called vegetable. Scientists have divided fruits into three categories (i) fruits with pulp and seeds such as apple, orange, watermelon etc. (ii) fruits with stones, such as plum, cherry, peach, etc., (iii) dry fruits such as almonds, walnuts etc.

          According to botanists, beans and peas also fall under the category of fruits. Cucumbers and marrows are also fruits. But cabbage, turnip, radish, cauliflower - all are vegetables. Onions, potatoes etc., are also vegetables.

          In USA a controversy over tomato went on for a long time. Finally the U.S. Supreme Court had to decide the issue. According to botanical sciences, tomato is a fruit but generally it is used as a vegetable. Keeping both the views in mind, the Supreme Court ruled in 1893 that tomato is a vegetable from commercial point of view. In science, however, this can be treated as a fruit. In reality, the scientific definitions of fruits and vegetables do not agree with the general usage. In our common language we treat some fruits as vegetables such as brinjal, peas etc. and some vegetables as fruits, such as raw banana.