6 BOTANY OF CROP PLANTS 



the higher seed plants, there are many different sorts of cells, 

 both in structure and function, and the different cells are 

 more dependent one upon another than are the cells that 

 make up the simple algal filament. Yet, even in the seed 

 plant, each cell is a unit of activity, and each is carrying on 

 its functions more or less independently of its neighbors. 

 The physiological unit of the plant is the cell. 



The Structure of the Plant Cell (Fig. i).— It must be 

 understood at the beginning that plant cells vary a good deal 

 in size and shape. However, the fundamental structure of 

 all plant cells is much the same. The plant cell consists of 

 a living mass {protoplast) of protoplasm enclosed in a non- 

 living cell wall. The wall is manufactured by the protoplast, 

 and serves as a protection to it. If we examine the proto- 

 plast, we see that it is composed of rather definite parts. 

 There is an outer, thin, and transparent living membrane 

 about the protoplast, whichis only made manifest by treating 

 the cell in a special manner. This membrane is known as the 

 protoplasmic membrane, ectoplasm or hyaloplasm. It is all- 

 important in the intake and outgo of substances. If a plant 

 tissue is immersed in a sugar or salt solution which has a 

 greater concentration than the cell sap, water is drawn from 

 the protoplast of each cell through the protoplasmic mem- 

 brane, and the protoplast shrinks, thus pulling the membrane 

 away from the cell wall and making it visible microscopic- 

 ally. Imbedded within the body of the protoplast there is 

 a darker and denser mass of protoplasm, the nucleus, sur- 

 rounded by its own living nuclear membrane. It may con- 

 tain one to several small, darker bodies, the nucleoli. The 

 protoplasm outside the nucleus is designated the cytoplasm. 

 Hence we see that the protoplast is made up of three main 

 parts: protoplasmic membrane, cytoplasm and nucleus. The 

 protoplasm has spaces within it, which are filled with cell sap. 



