2 A TEXT-BOOK OF BOTANY. 



to know something of their functions and habits of life as well 

 as of their internal structure. It is desirable at this point to give a 

 brief consideration to the cell, as it is the unit of plant structure. 



If we make a section of a plant and examine it by means of 

 the microscope, the cut surface presents the appearance of a 

 network, indicating that the tissue is made up of small compart- 

 ments or chambers. One of these compartments together with 

 its contents constitutes the structure known as the cell (see 

 Frontispiece). 



The cell-contents vary greatly in appearance and composi- 

 tion, but in all active or living cells there is always present the 

 substance known as protoplasm. The protoplasm is the basis 

 of all plant structures whether they belong to the lowest or high- 

 est forms ; for by its aid or from it all parts of the plant are 

 developed. Even the cell-wall is a product of protoplasmic activity. 

 The protoplasmic content of the cell consists of several intimately 

 related but more or less distinct portions, — namely, a somewhat 

 thin, semi-liquid, granular portion known as the cytoplasm ; 

 a more or less spherical body embedded in the cytoplasm called 

 the NUCLEUS ; and frequently, but not always, certain small 

 bodies which are more or less variable in shape called plastids, 

 these being also embedded in the cytoplasm (see Frontispiece). 

 The cytoplasm and nucleus are sometimes considered together 

 as a unit, which is known as the protoplast. A fuller discussion 

 of the differentiated portions of the protoplasm will be found in 

 Chapter 11. 



The lowest organisms, as the slime molds, do not have an 

 enclosing membrane, but consist of a naked mass of protoplasm. 

 With this exception plants have an outer wall or membrane. 

 They may consist of a single cell, as in the Bacteria, or a chain 

 of cells, as in the filamentous Algae, or a mass of cells, as in the 

 majority of plants, and are accordingly designated as unicellular 

 or multicellular. The cell-wall is composed for the most part 

 of cellulose, but may be modified in various ways. 



Nomenclature. — The names for describing plants have been 

 derived for the most part from studies of the higher plants, they 

 having exclusively attracted the attention of botanists at first. 

 But with the light which has been thrown on the relationship 



