ELEMENTABY STEUCTUEE. 



15 



such cells have the rays at regular intervals, or all of one length, 

 but various degrees of irregularity occur, which lead to corre- 

 sponding irregular forms in such stellate cells. 



Thirdly, when the nutrition occurs chiefly in one direction we 

 have cells which are elongated, either horizontally or vertically. 

 Among the forms resulting from an extension of the cell in 

 a horizontal direction, we need only mention tabular cells {fig. 

 23), that is six-sided flattened cells, with the upper and lower 

 surfaces parallel, or nearly so. Of those cells, which are extended 

 in length or vertically, we have various forms, as cylindrical 



Fig. 23. 



Fig. 25. 



Fig. 26. 



Fig. 24. 



^<^ 



Fig. 23. Tabular cells. Fig. 24. Cylindrical cells. The small or rounded body 



in the interior of three of these cells is called a nucleus or cytoblast. 

 Fig. 25. Elongated fusiform cells. Fig. 26. Fibrilllforui cells. 



{fig. 2.^), fusiform {fig. 25), fibrilliform {fig 26) ; the two former, 

 by the mutual pressure of contiguous cells, often become 'pris- 

 matic. " 



From the above description of the forms of cells it will be seen 

 that they may be divided into the short and elongated, although, 

 as various intermediate forms occur, this division cannot be 

 strictly adhered to. 



The cells, when in combination with other cells so as to form 

 a tissue, are generally bounded by plane {figs. 20 and 21), or 

 rounded surfaces {figs. 17 and 27); but when in combination 

 also with the vessels of the plant, so as to form what are called 

 the vascular bu?idles, they are elongated, and have pointed extre- 

 mities {fig. 25). These differences in the condition of the cells 

 lead to corresponding differences in their arrangement ; thus, in 

 the former case, the cells, when arranged in lines, are placed one 

 upon another, the ends being usually flattened {fig. 24) ; while 



