30 



PROTOPLASM 



microns (/u) or thousandths of a miUimeter, which is the unit of 

 microscopic dimensions, an Elodea leaf cell is about 150 by 40 

 by 30 fi. Its nucleus is 0.010 mm., or 10 /a, in diameter. The 



cellulose wall is about 0.001 mm., or 

 1 fj,, thick. Among the smallest of 

 cells is the human red corpuscle. It 

 measures 8 n across, which is 0.008 

 mm., or 0.0003 inch. It would re- 

 quire 1,500,000 red corpuscles laid 

 side by side to cover the average 

 little finger nail. 



Tissues. — Cells belong to two 

 main categories, those which form 

 tissues and those which live as 

 Fig. 19— Section of a Lyco- separate individuals. A tissue is 



podmm rhizome (under-ground ... i • i 



stem) showing tissues of several any group ot cells ot a SUTlllar kuid, 

 kinds; the large cells in the center q]^[ having (usually) a single, definite 



are the water-conducting xylem. . o i • 



function, buch tissues are epider- 

 mis, or skin; muscle; nerve; cork (Fig. 6); and xylem, or the 

 conducting tissue of plants (Fig. 19). When tissue is young and 

 undifferentiated, it is embryonic. The growing tip of a root or 

 stem is such tissue with the special function to produce cells. It 



Fig. 20. — The growing point of a fern rhizome showing the meristem tissue 

 with its large triangular initial cell (in the center), which is the mother cell of 

 all cells about it. (Figs. 19 and 20 are from G. H, Conant.) 



is known as meristem, a botanical term indicating actively divid- 

 ing and perpetual young tissue (Fig. 20). 



Nearly two centuries after Robert Hooke discovered cells in 

 cork, the Germans Schleiden and Schwann advanced the cell 



