43<5 



LECTURE XXVII. 



On regarding a transverse section of an organ properly taken in accordance 

 with its relations of symmetry, a characteristic pattern is as a rule presented. 

 The individual cells lie by no means without order among themselves, but exhibit 

 definite groupings and arrangements of the most various kinds, and, on a little 

 reflection, certain arrangements are always visible, which are repeated even in 

 the most different organs. It is beyond all doubt that these net-works of cells, as 

 the pattern presented to the observer may most simply be termed, are an immediate 

 expression of the processes of growth prevailing in the parts of the plant concerned ; 



FIG. 271.-A branch of Styfccnulm scapar„im FiG. 272.-Transverse section of a root of the Nettle ^JrUm). g the 



w.th two smaller branches x and y, and the rudiment group (horizontal in the figure) of five vessels which existed at a very early 

 of a third > (after Geyler). All the lines indicate stage; the pthervessels and lignified cells form a double group (vertical in the 

 cell-walls. figure), c cambium from' which the latter and the radiaUy arranged tissue 



have arisen. Externally is a layer of cork, beneath which lies a layer of 

 irregular tissue displaced by the growth in thickness (after De Bary). 



and this in a manner exactly similar to what was explained previously by a few very 

 simple examples, only that in organs which are composed of very numerous cells 

 the relation between growth and the arrangement of the cells may often be not so 

 easy to discover. 



For the preliminary guidance of the reader the following three statements may 

 here be made in the first place, 



(i) The form of the pattern presented by a network of cells depends chiefly 

 upon whether, after the successive cell-divisions, vigorous growth of single cells or 



