THE PRINCIPAL TISSUES. 



69 



of the cell-wall by the addition of successive 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 Pinus 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. 36.) 



§ II. The Principal Tissues. 



98. — There are very many kinds of tissues, distinguished 

 from each other by characters of 

 greater or less importance. 

 They all, however, 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 i-estrict- 

 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, tlie terms parenchyma and parenchymatous liave 

 often been restricted in meaning to tissues composed of cells whose 

 three dimensions are equal. 



■Fis;. 53.— Merietem-cells of stem of 

 Vicia faba in process o( division. X 

 300.— After Praiitl. 



