68 THE STRUCTURE OF ECONOMIC PLANTS 



might originate from the cambium without the formation of a 

 layer of phloem initials. 



In secondary thickening, the dissected siphonostele may become 

 a solid cylinder through the development in the ray tissue of an 

 interfascicular cambium which is continuous with the fascicular 

 cambium. This does not involve any movement or invasion of cells 

 into the ray region, but is usually accomplished by a progressive 

 activation of the parenchymatous cells immediately adjacent to the 

 cambium in a zone which may be regarded as potentially pro- 

 cambial. This may proceed until there is a cambium completely 

 across the rays. In other cases, the interfascicular cambium may 

 occur first at a median point in the ray where a small bundle of 

 secondary tissue is formed; and, later, by a lateral activation from 

 this point, as well as from the fascicular cambium of the primary 

 bundles, the cambial cylinder becomes complete. 



There are varying degrees of development of interfascicular 

 cambium, ranging from cases where none is formed and the vascular 

 tissue remains as a dissected siphonostele at maturity, to those 

 where the cambial cylinder becomes continuous and a solid zone of 

 secondary vascular tissue is formed. The secondary tissues derived 

 from the interfascicular cambium are not always vascular; and, 

 especially in semi-herbaceous or semi-woody stems, the major 

 portion of such tissue may consist of non-vascular conjunctive or 

 connective tissue which by subsequent lignification affords added 

 mechanical support. 



In most monocotyledons, there is no secondary thickening, and 

 enlargement of the axis is limited to the capacity of the primary 

 cells to grow in size, although there may be a limited amount of 

 division in the cells of the medullary and cortical parenchyma. In 

 many instances, the tissue of the provascular strand is completely 

 differentiated into primary xylem and phloem elements and no meri- 

 stematic cells remain to form a fascicular cambium. This results 

 in a bundle of the closed type. Arber (i, x, 3, 4), Dauphine (11), 

 and Gatin (2.0) have reported intrafascicular cambium in monocot- 

 yledons, the first named reporting on 2.i families in which some 

 member has a cambium. There is no extensive development of 

 secondary tissues, and the interpretation of the meristematic 

 tissue of the bundle as a cambium in many of these cases is con- 

 troversial. In some of the woody monocotyledons, such as Yucca 

 and Dracaena, a type of anomalous secondary thickening occurs in 



