ELEMENTARY STRUCTURE. 7 



tion tabular cells {fig. 7), 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 hiivc 

 various forms, as cylindrical {fig. 8), fusiform {fig. 9), fihrilli- 

 form {fig. 10), &c. ; the two former, by the mutual pressure of 

 contiguous cells, often become prismatic. 



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 can- 

 not be strictly adhered to. 



The cells, when in combination with other cells, are generally 

 bounded by plane or rounded surfaces {figs. 1 and 5) : but when in 

 combination also with the vessels of the plant, so as to form 

 what are called the vascular bundles, they are elongated, and 

 have pointed extremities {fig. 9). These differences in the con- 

 dition 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. 8); while in the latter their tapering extremi- 

 ties overlap one another, and become interposed between the 

 sides of the cells which are placed above and below them 

 {fig. 9). From this circumstance cells have been divided into 

 pareiichymatous and prosenchymatous; parenchymatous being 

 the term applied to those cells which are placed end to end ; 

 and prosenchymatous to those which are attenuated, and over- 

 lap one another when combined together to form a tissue. 

 Another distinction commonly observed between parenchyma- 

 tous and prosenchymatous cells arises from the condition of 

 their cell-walls ; thus, those of parenchymatous cells are usually 

 thin and but little incrusted; while those of prosenchymatous 

 cells are more or less thickened by the deposition upon their 

 inner surfaces of various incrusting matters. The above dis- 

 tinctions between parenchymatous and prosenchymatous cells 

 are evident enough in the extreme forms of the two di\asions, 

 but various transitional states occur which render it impossible 

 to draw, in many cases, a distinct line of demarcation between 

 them. 



The above-mentioned forms of cells are the principal ones 

 which occur when they are in combination so as to form a 

 tissue ; but it may be readily understood that many others are 

 found, which arise from various irregularities in the pressure 

 and nutrition of the cells. 



We must now briefly allude to a few of the forms which cells 

 assume when not in combination, or but partially so, under which 

 circumstances they are more or less unrestrained in their 

 development. In these cases, as in the former, the typical form of 

 the cells is to be spherical, but this form is rarely maintained as 

 they grow older, although instances of such occur frequently in 

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