TISSUES AND ORGANS 



47 



the inheritable quahties or characters which make the offspring 



liice its parents. 



Tissues and Organs. — The cells which form certain parts of 



the veins, the flat blade, or other portions of a leaf, are found in 



groups or aggregations, and are more or less alike in size and 



shape. Such a collection of cells is called a tissue. Examples of 



tissues in animals are the cells covering the outside of the body, 



forming the skin or epidermal tissue ; muscle tissue, which 



produces movement ; bony tissue, which forms the framework 



to which the muscles are ^ ■ , 



^Epidermis 



Falisade kyer 

 A vein 

 Spongy tissue 

 Air spBce 



attached ; and there are 

 many others. 



Collections of tissues 

 which act together in the 

 performance of work form 

 organs. Such an organ is 

 a leaf, made of supporting 

 cells, green cells, spongy 

 cells, etc., or the human 

 arm, with its bony sup- 

 porting tissue, its nerves 

 and muscles, its blood ves- 

 sels and connective tissue. 



How Cells form Others. 

 — Cells grow to a certain 

 size and then split into 

 two new cells. In this 

 process, which is of very 

 great importance in the 

 growth of both plants and 

 animals, the nucleus divides first ; the halves separate and go to 

 opposite ends of the cell. The chromosomes divide at the same 

 time, each splitting lengthwise and the parts go in equal numbers 

 to each of the two new nuclei formed from the old nucleus. In 

 this way the matter in the chromosomes is divided equally between 

 the two new nuclei. Then the rest of the protoplasm separates, 

 and two new cells are formed. This process is known as cell 

 division- The usual method of cell division is very complicated 



A vein with 

 woody bundles 



Diagram of a small part of a leaf, partly in 

 section, greatly magnified to show cells in this 

 organ. 



