202 The Phenomena of Morphogenesis 



thickenings in protoxylem and regenerating xylem cells are directly op- 

 posite those in adjacent ones. 



In all these examples of the differentiation of histological pattern it 

 is evident that the pattern as a whole transcends cellular boundaries and 

 involves an extensive and correlated series of changes. This poses in most 

 direct fashion the problem of pattern in general. 



More Complex Patterns. In most cases an anatomical pattern consists 

 of more than one type of tissue and thus is much more complex than 

 the ones just described. In plants that grow by a large apical cell at 

 the meristem, differentiation of the various tissues from particular cells 

 cut off from this apical cell may be traced. Thus in Selaginella (p. 58), 



^° 





N C s 



H 



Fig. 8-14. Continuity in the differentiation of various wall thickenings. Left, rings 

 and spirals in protoxylem of Zea. Center, thickenings in reticulate vessel elements 

 that have developed from parenchyma cells in bundle regeneration. Right, lignified 

 bands (reseau de soutien) in cortex of air root of an orchid. In all these cases the 

 thickenings form a continuous pattern across cell boundaries. {From Sinnott and 

 Bloch. ) 



the apical cell, by an unequal division parallel to one of its faces, pro- 

 duces on its apical side a large cell, the continuing apical initial, and 

 on its basal one a cell which, seen in section, has parallel anticlinal walls 

 (Barclay, 1931; Fig. 4-4). This cell divides into two by a wall at right 

 angles to the first division. Each daughter cell divides again into two 

 in the same way. Thus a row of four cells is produced. Proceeding down 

 the shoot axis, in a longitudinal section, one can observe the fate of simi- 

 lar rows of four cells which had been cut off by previous divisions of the 

 apical cell. The outermost of the four becomes a cell of the epidermis. 

 The second (by later divisions) produces the cortex. Descendants of the 

 third and fourth form the innermost tissues. Thus the progress of dif- 

 ferentiation can usually be followed in various lineages of cells. Specific 



