Month: December 2015

A Meristem is a localized region of the cell which are dividing actively. There are various theories of the shoot apical Meristem. Apical Theory: It was proposed by Nageli. A single tetrahedral apical cell is the structural and functional unit of the cell . It controls all the growth processes of the cell. It is found in algae, bryophytes and pteridophytes. Histogen Theory: It was proposed by Hanstein. There are three Histogenic layers. Dermatogen which gives rise to epidermis. Periblem is destined to form the cortex. Pleurome forms the vascular cylinder. Tunica Corpus Theory : The peripheral zone consisting of one or more layers of regularly arranged cells is called as tunica and the cells surrounded by tunica is called as corpus. Histogenic layer theory: This theory was proposed by Dermen. There are three Histogenic layers . LI , LII and LIII layers

Volvox is a coenobial and colonial alga. Volvox colonies multiply so rapidly during the rainy season that the entire pond is covered by them and appears greenish. The colonies are oval to spherical in shape , having 500 to 60, 000 cells. The cells have a typical chlamydomonad type of structure. There is a single nucleus , cup shaped chloroplast , two contractile vacuoles , an eye spot, two flagella inserted at the anterior end. Reproduction in Volvox is of two types — asexual reproduction and sexual reproduction . Asexual reproduction occurs by means of special types of cells located at the posterior end of the colony . These special cells are called as gonidia. Sexual reproduction occurs at the end of the growing season. Male sex organ is known as antheridium and the female sex organ is known as oogonium.