Plant Bible

  TYPES OF EMBRYO SAC Monosporic Embryo Sac The embryo sac in which only one out of four megaspores is involved in the development of the embryo sac is called as Monosporic Embryo Sac. Since all the nuclei in such embryo sac are derived through mitoses of a single nucleus, hence all nuclei are genetically identical. On the basis of number of nuclei in the mature embryo sac, Maheshwari recognised two types - 8 - nucleate and 4- nucleate. 8- nucleate or Polygonum type: This type of embryo sac development was first discovered in Polygonum divaricatum for the first time by Strasburger, hence it is called as polygonum type. Since, this is common in Angiosperms(81%), it is called as normal type. Of the four megaspores formed from the megaspore mother cell, only one megaspore situated towards chalazal end remains functional and the remaining three situated towards micropylar end degenerate. The functional megaspore divides and gives rise to 2 nuclei, one of which moves to the mi...

  MECHANISM OF STOMATAL MOVEMENTS   1) STARCH- SUGAR HYPOTHESIS: It is proposed by sayer in 1923 IN LIGHT: IN DARK: Photosynthesis occurs Respiration occurs pH increases ↑ pH decreases ↓ Carbon dioxide Co ₂↓ Carbon dioxide Co ₂ ↑ Enzyme: phosphorylase is used Enzyme: phosphorylase is used Sugar increases ↑ so endosmosis take place Starch increases ↑ so exosmosis take place Turgor pressure increases ↑ Turgor pressure decreases ↓ Diffusion pressure deficit (DPD) increases ↑ Diffusion pressure deficit (DPD) decreases ↓ Stomata = opens Stomata = closes Starch is converted in to glucose -1- phosphate glucose -1- phosphate is converted in to Starch       2) STEWARD HYPOTHESIS: It is proposed by steward in ...

                                                                       HYBRIDIZATION Definition : Hybridization is the method of producing new crop varieties in which two or more plants of unlike genetically constitution all crossed together that result in a progeny called hybrid. Hybridization offers improvement in crop and is the only effective means of combining together the desirable characters of two or more varieties or species. The first natural hybridization was observed by Cotton Mather in maize. Steps in Hybridization: 1.Selection of Parents:   Male and female plants of the desired characters are selected. It should be tested for their homozygosity. 2. Emasculation: It is a process of removal of anthers to prevent self pollination before dehiscence of anther 3. Bagging: T...

  Factors Affecting The Rate of Respiration   A number of factors affect the rate of respiration.   Actively dividing meristematic cells have the highest respiratory rate.   The rate of respiration is relatively steady during the growth phase but generally picks up during the ripening of fruits.   This sudden increase in the rate of respiration is termed as climacteric.   It generally slows down at maturity and during senescence. Factors affecting respiration can be classified into two categories- External Factors And Internal Factors External Factors: Many external factors like temperature, oxygen, light, CO2, etc., affect the rate of respiration. Temperature: Temperature significantly affects the rate of respiration as it does with other enzymatic processes.   Usually the rate of respiration increases with increase in temperature in the range of 0-45 degree centigrade.   For every rise of 10 degrees, i.e., between 0-25 centigra...

  SELECTION Selection is the choice of certain individuals from a mixed population for a one or more desirable traits. Selection is the oldest and basic method of plant breeding. There are two main types of Selection. i. Natural Selection: This is a rule in nature and results in evolution reflected in the Darwinian principle “survival of the fittest”. It takes longer time in bringing about desired variation. ii. Artificial Selection:   It is a human involved process in having better crop from a mixed population where the individuals differ in character. The following are the three main types of artificial selection. a. Mass Selection: In mass selection a large number of plants of similar phenotype or morphological characters are selected and their seeds are mixed together to constitute a new variety. The population obtained from the selected plants would be more uniform than the original population and are not individually tested. After repeated selection for ...

                Factors affecting Photosynthesis “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the lowest factor”   “at any given point of time the lowest factor among essentials will limit the rate of photosynthesis”. Factors affecting photosynthesis are further grouped into  External or Environmental factors  Internal factors. I. External factors:  Light, carbon dioxide, temperature, water, mineral and pollutants. II. Internal factors:  Pigments, protoplasmic factor, accumulation of carbohydrates, anatomy of leaf and hormones. External factors: 1. Light Energy for photosynthesis comes only from light. Photooxidation of water and excitation of pigment molecules are directly controlled by light. Stomatal movement leading to diffusion of CO2 is indirectly controlled by light. a. Intensity of Light: Intensity of light p...

  TRANSPIRATION Definition: The loss of excess of water in the form of vapour from various aerial parts of the plant is called transpiration. Types of Transpiration Transpiration is of following three types: 1. Stomatal transpiration Stomata are microscopic structures present in high number on the lower epidermis of leaves. This is the most dominant form of transpiration and being responsible for most of the water loss (90 - 95%) in plants. 2. Lenticular transpiration In stems of woody plants and trees, the epidermis is replaced by periderm because of secondary growth. In order to provide gaseous exchange between the living cells and outer atmosphere, some pores which looks like lens-shaped raised spots are present on the surface of the stem called Lenticels. The loss of water from lenticels is very insignificant as it amounts to only 0.1% of the total. 3. Cuticular transpiration The cuticle is a waxy or resinous layer of cutin, a fatty substance covering the ep...