Differentiation 203 



types of differentiation seem to be related to specific lineages, almost as 

 though "determiners" were being parceled out at each division. The same 

 type of differentiation has been described by Bartoo (1930) in Schizaea. 

 In these cases differentiation is a true development, the unfolding of an 

 internally directed pattern, with each division evidently related to the 

 polarity of the cell. Cell division here seems a dominant factor in the 

 determination of pattern. It should be noted, however, that there are 

 often irregularities in this progression and that it is by no means always 

 so precise. 



In many other plants, especially those with large apical cells, differen- 

 tiation also follows a rather regular course like that just described. Chara, 

 Fontinalis, and some species of Equisetum (Fig. 4-3) are examples. In 

 others, however, such precise relationship between a specific type of dif- 

 ferentiation and cell lineage does not occur, for a particular tissue may 

 sometimes have one cell ancestry and sometimes another. The origin of 

 root, stem, leaf, and foot from the quadrants of a young fern embryo, for 

 example, is not rigidly determined. 



In higher vascular plants where an apical cell has been replaced by a 

 mass of meristematic tissue, in most cases it becomes impossible to trace 

 the origin of a group of differentiated cells from a single ancestor or to 

 determine the precise divisions at which a fundamental difference be- 

 tween two cells (or their descendants) originates. Such divisions may 

 occur, but in these there is no great difference between daughter cells 

 nor is regularity of lineage usually observable. It is probably true that in 

 very many instances differentiation is the result of factors of environment 

 or position and is not related at all to differential cell division. 



There is, however, a good deal of cellular differentiation to be seen in 

 the apex of shoot and root, either as layers or zones (p. 62 ) and these often 

 bear a close relation to the structures that develop from them. Thus in 

 periclinal chimeras ( p. 272 ) it is possible to determine with much accuracy 

 the derivation of particular tissues from particular layers at the apex. 

 Nevertheless, in forms without meristematic layering ( as in some gymno- 

 sperms) differentiation of tissues takes place equally well. The problem 

 of how the histogens become distinct from each other, in forms which 

 show them, is one which for its solution must go back to the young 

 embryo. 



In the mass of relatively undifferentiated tissue below the apex arise 

 the beginnings of vascular tissue. The distinction between the procambial 

 or provascular cells (those which are to give rise to the primary vascular 

 tissue) and the cells of the fundamental tissue begins to make its appear- 

 ance early in development near the tip of the meristem. The first differ- 

 ence to be observed here often is not a structural one but a difference in 

 the staining reaction of the cells. The earliest structural difference to be 



