198 The Phenomena of Morphogenesis 



connected in an interwoven pattern. These small elongate cells expand 

 with the growth of their parenchymatous neighbors and develop into 

 long sclerenchymatous cells aggregated into strands a fraction of a milli- 

 meter wide which are organized into the complex fibrous "sponge." This 

 sponge is not a random mass of fibers but has an organization of its own, 

 for the outer members of it are arranged in rows transverse to the axis 

 of the fruit and most of the inner ones extend lengthwise. They are 

 united into a continuous system. This system seems to be the expression 

 of a histological pattern superposed upon the fundamentally different 

 system of regularly arranged parenchyma cells of the early ovary. How 

 the course of its interconnected but continuous strands is established is 

 a baffling problem. In somewhat the same way as these sclerenchym- 

 atous strands develop, the young bundle initials of the veins arise in 



Fig. 8-10. Young ovary of Luff a. Successive early stages in the origin of a fiber strand 

 differentiating in ground parenchyma. ( From Sinnott and Bloch. ) 



the mesophyll of a developing leaf blade, as described by Meeuse ( 1938; 

 Fig. 8-11). 



Cambium. A familiar example of a complex pattern of differential de- 

 velopment is that of the vascular cambium and its products (p. 84). The 

 typical cambium consists of a continuous tangential layer of elongate 

 initials in which most of the divisions are in the tangential plane. The 

 cells cut off on the inside develop into tracheids, fibers, vessels, paren- 

 chyma, and ray cells of the xylem, and those on the outside into sieve 

 tubes, companion cells, fibers, and other phloem cells. There are pro- 

 found differences between a huge vessel element in oak wood and a 

 small parenchyma cell beside it but both come from similar cambium 

 cells. 



There are a number of morphogenetic problems presented by a study 

 of this development of secondary vascular tissues. 



