THE PRINCIPAL TISSUES. 



69 



of the cell-wall by the addition of successiye internal layers, 

 and is directly contradicted by the well-established doctrine 

 of growth by intussusception, it must be regarded as erroneous. 

 In some cases, as in the wood of PiJius sylvestris, the dif- 

 ferentiation is so great that three lamellae are formed: (1) 

 the common middle one, (2) an inner, and (3) an inter- 

 mediate one. (Fig. 16, p. 26.) 



§ II. The Pbincipal Tissues. 



98. There are yery many kinds of tissues, distinguished 



from each other by characters of 

 greater or less importance. 

 They all, howeyer, pass into 

 one another by almost insensi- 

 ble gradations ; hence by not- 

 ing all the slight differences we 

 may make a long list of tis- 

 sues ; while by noting the simi- 

 larities and gradations, all, or 

 nearly all, the forms may be re- 

 duced to one. The principal 

 varieties only will be noticed in 

 this place ; each one, as here 

 described, includes many varie- 

 ties. 



99.— Parenchyma. This is 

 the most abundant tissue in the 

 vegetable kingdom ; it is at once 

 the most important and the 

 most variable. As here restrict- 

 ed it is composed of cells whose walls are thin, colorless, or 

 nearly so, and transparent ; in outline they may be rounded, 

 cubical, polyhedral, prismatic, cylindrical, tabular, stellate, 

 and of many other forms.* When the cells are bounded by 

 plane surfaces, generally, but not always, the end planes lie 

 at right angles to the longer axis of the cells. 



* Unfortunately, the terms parenchyma and parencliymatous have 

 often been restricted in meaning to tissues composed of cellg whose 

 three dimensions are equal, 



Fig. 53.— Meristem-cella of stem of 

 Ticia faba, in prncess of division. X 

 300.— After Prantl. 



