40 



VEGETABLE HISTOLOGY. 



walls. Next to the wall is a thin layer of protoplasm, better 

 seen by staining with iodine solution. 

 The chlorophyll bands are in contact 

 with this layer of protoplasm. The 

 greater part of the interior of the cell 

 is occupied by a large vacuole contain- 

 ing cell sap, i. e., water with sub- 

 stances in solution. Each cell has a 

 usually centrally-placed, distinct pro- 

 toplasmic body known as a nucleus, 

 with radiating extensions of proto- 

 plasm passing from it to the outer 

 layer of protoplasm next the cell-wall. 

 The growth of spirogyra in length is 

 brought about by cell division. Each 

 cell is repeatedly divided into two equal 

 parts by the appearance in it of a 

 cross-partition. This process takes 

 place during the night, and special 

 precaution must be taken in order to 

 study it. This method of cell forma- 

 tion is the general mode throughout 

 the vegetable kindoni. 



The method of reproduction in 

 spirogyra is a sexual one and known 

 as conjugation. This process occurs 

 from early spring to June and July, 

 but can be induced when the plant is 

 under cultivation by allowing the 

 water in which it is growing to slowly evaporate. Two fila- 

 ments arrange themselves side by side, and the cells lying oppo- 

 site each other send out each a process or tube ; these unite and 

 the protoplasm from one cell passes over and coalesces with 

 that in the cell opposite. In Fig. 29 two such tubes about to 

 unite are shown at b, while the beginning of formation of two 

 other tubes is shown at a. 



The result of the process is a new cell called a zygospore. 

 This is set free by decay of the walls of the old cell and falls to 

 the bottom of the water and rests until proper time for growth. 



It is not an easy matter to find conjugating forms of spi- 

 rogyra, and their study is not well suited for class work. 



There are a number of species of spirogyra, and the student 

 should keep a lookout for different kinds of filaments in the 

 specimen studied. Mount some filaments in water and, under 

 low power, note the great length of the filaments, as well as of 

 the individual cells, the uniform diameter, the well-defined cell- 

 walls and the conspicuous green chlorophyll bands, the shape 

 ind relative length and breadth of the cells. 



Fig. 29. Spirogyra longata 

 (Bessey). 



