ELEMENTARY TISSUES 29 



(4) The ground system includes the pith, which is paren- 

 chyma, the collenchyma and such other tissues as are not included 

 in the other systems. 



Organ. This term is applied to a definite part of a plant, 

 such as root, stem, leaf, tendril, flower, part of a flower or fruit, 

 which is composed of tissues and has a definite structure and defi- 

 nite function. Some organs are very simple in structure 

 and function, while others are very complex. Strictly speaking, 

 there are but three primary organs in the higher plants : the stem, 

 root and leaf. The other parts, which are usually referred to as 

 organs, such as the flowers, fruits, tendrils and thorns, are in 

 reality modifications of some of the primary organs. 



Review. The cell is the structural and physiological unit 

 of the plant. It is composed of protoplasm and possesses a 

 nucleus. The protoplasm is usually surrounded by a cell wall. 

 The cell may contain plastids, mitochondria, chlorophyll and 

 other pigments, sugar, starch, gums, proteins, fats, oils, resins, 

 acids, alkaloids, tannins, crystals and other compounds. The 

 simplest types of cells are thin walled, but may be of almost 

 any form ; they give rise to many varieties of cells. A tissue is 

 a group of cells which are alike in origin, structure and function. 

 A system is one or more tissues which serve a special function. 

 An organ is a definite part of a plant body, serving one or more 

 functions. The systems and organs will receive further con- 

 sideration in the succeeding chapters. 



LABORATORY EXERCISES. 



The greatest variety and complexity of tissues are to be found in the 

 higher forms of plants. The parenchyma is the simplest type of tissue 

 and gives rise either directly or indirectly to all other tissues. 



Exercise 1. Parenchyma tissue. Examine thin sections of the pith 

 of elder or other plant with large pith, such as stems of corn or young 



