Cells and Cell Principle 39 



early part of the last century shifted attention from the gross and super- 

 ficial aspects to the detailed study of cells and their inherent organiza- 

 tion. All of these, and probably many others, paved the way for the 

 formulation of the cell principle at that particular time. 



II. DETAILED STRUCTURE AND FUNCTIONS OF CELLS 



ANIMAL CELLS 



PLANT CELLS 



I. Cytosome or Cell Body (si' to som) 

 (Gr. kytos, cell; soma, body) (Fig. 

 8) 



1. Cytoplasm (si' to plazm) (Gr. kytos, 

 cell; plasma, liquid). — This is that 

 part of the living protoplasm lo- 

 cated outside the nucleus. The cy- 

 toplasm may be separated, more or 

 less distinctly, into an outer ecto- 

 plasm (Gr. ektos, outer) and an in- 

 ner endoplasm (Gr. endon, inner). 

 The cell sap or cytolymph (si' to 

 limf) (L. lympha, liquid) forms the 

 fluid, ground substance of the cyto- 

 plasm. The cytoplasm is usually 

 colorless, somewhat granular, and 

 varies in its viscosity. 



Plasma Membrane (and Cell Wall). 

 — In a few animal cells there may 

 be a cell wall, although it is rarely 

 present. Surrounding the cyto- 

 plasm there always is a thin, clear, 

 filmlike, rather rigid, plasma mem- 

 brane. The rather dense plasma 

 membrane closely adheres to the 

 cytoplasm and regulates the passage 

 of materials to and from the cell. 

 Plasma membranes are semipermea- 

 ble because certain liquids and dis- 

 solved materials can pass through 

 while others cannot. Diffusion 

 through a semipermeable membrane 

 is called osmosis (os -mo' sis) (Gr. 

 osmos, push). IDiffusion occurs 

 from the region of higher concentra- 

 tion of a substance to a region of 

 lower concentration of a substance. 



I. Cytosome or Cell Body (Fig. 9) 



1. Cytoplasm. — This varies in viscosity 

 from a thin syruplike liquid to a 

 gelatinous semisolid. It is usually 

 colorless, elastic, slightly granular, 

 and somewhat mucilaginous. Fre- 

 quently, the cytoplasm forms a layer 

 next to the cell wall with strands of 

 it extending across the internal vac- 

 uole and also surrounding the nu- 

 cleus. The watery cell sap fills the 

 vacuole in the cytoplasm and fre- 

 quently contains salts, sugars, pig- 

 ments, organic acids, etc. 



2. Cell Wall and Plasma Membrane. — 

 The cell wall is transparent, pliable, 

 semirigid, and nonliving and gives 

 strength and support to the plant 

 body. The cell wall is secreted by 

 the protoplasm and may be com- 

 posed of layers, the thickness vary- 

 ing with the tissues. Adjacent cells 

 adhere to each other because of a 

 layer common to them, known as 

 the middle lamella (la-mel'a) (L. 

 lamella, small plate). The most 

 abundant constituent of a plant cell 

 wall is cellulose (sel'ulos) (L. cell- 

 ula, small cell). Other materials in 

 various cell walls are lignin (lig' nin) 

 (L. lignum, wood), a hard organic 

 substance found especially in wood; 

 cutin (ku'tin) (L. cutis, skin), a 

 waxy substance in epidermal tissues 

 to make then somewhat impermea- 

 ble to water; suberin (su'berin) (L. 

 suber, cork), a waxy substance in 

 cork tissues to waterproof them. 

 The thin plasma membrane lies be- 

 neath the cell wall and, being semi- 

 permeable, regulates the passage of 

 materials in and out of the cell. 

 Osmosis probably occurs much as 

 in animal cells. 



