The Cellular Structure of Plants 21 



begin to differentiate, and they quickly change into the different 

 types of cells found in the mature plants. Some of them enlarge 

 but remain round ; others greatly elongate ; still others become 

 flattened, disk-shaped bodies. In some the walls remain thin, 

 while in others the walls are greatly thickened or are marked by 

 lines and pits and irregular thickenings. They may differ in size, 

 form, thickness of cell wall, content, or color, as will be made 

 evident by our later studies of different plant parts. 



Tissues and organs. Cells of a given kind are usually con- 

 cerned with the carrying on of some particular process, and they 

 tend to be grouped together. Such a group of cells is called a 

 tissue. The epidermis of a leaf is a tissue covering the leaf ; the 

 soft green inner part of a leaf is a tissue that is concerned with the 

 manufacture of food ; the shell of a nut is a hard tissue inclosing 

 the kernel. 



An actively working tissue needs supphes and a means of dis- 

 posing of its products. Hence, tissues are grouped together, 

 and by their cooperation carry on some general function of the 

 plant more efficiently. A number of tissues grouped and work- 

 ing together in this way form an organ. The leaf, for example, 

 is an organ especially concerned with the manufacture of foods. 

 It is made up commonly of five different tissues, each composed 

 of thousands of cells. 



Summary. Before proceeding to the study of structures and 

 processes of plants it is important that we understand (i) that 

 all plant structures are either single cells or masses of cells, 

 (2) that the protoplasm is the active living part of the cells, 

 and (3) that the processes carried on by any plant organs are 

 the combined results of processes going on in the cells of which 

 it is composed. Further details of cell structures and activities 

 will be given as they are needed to understand the processes of 

 particular parts of plants. 



