ROLE OF THE MICROSCOPE 179 



Anielancher, etc. The distinction breaks down, however, when ap- 

 plied to a number of Rosacete that grow outside the north temperate 

 zone. Pygeum, Chrysobalanus, Parinarium, and a number of other 

 tropical or subtropical representatives of the Rosaceae possess Sanio's 

 metatracheal type of parenchyma distribution. The jacketing of 

 vessels by parenchyma occurs in all of these genera (fig. 2). The 

 parenchyma is, therefore, vasicentric, as well as tangentially banded, 

 and is not entirely di'ffused among the fiber-tracheids as it is in the 

 north temperate genera previously referred to. 



Equally significant is the distribution of parenchyma in the rosa- 

 ceous genus Prunus. In the northern representatives of this genus 

 the parenchyma is usually scanty or nearly absent in the older wood 

 of the stem (fig. 10). This is not the case, however, in many of the 

 southern representatives of the genus. The evergreen, coreaceous, or 

 schlerophyllous species of the section Laurocrasus, e. g., Prunus ilici- 

 folia Walp., P. sphaerocarpa Swartz, P. integrifolia Sarg., P. lauro- 

 ccrasus h., P. javanica Miq., and P. acuminata Hook., are extremely 

 variable, as far as the distribution of parenchyma h concerned. In 

 certain specimens, the parenchyma is almost entirely aggregated about 

 the vessels (fig. 12), and at the end of the seasonal growth rings (fig. 

 14). In other specimens of the wood, more or less diffuse parenchyma 

 is also present (fig. 13). 



Similar variation in the distribution of wood-parenchyma have 

 been observed in other families of the Dicotyledons. Not only may 

 the distribution of parenchyma be subject, in many cases, to marked 

 variations in large groups of plants, such as orders, families and genera, 

 but fluctuations may occur in a species or variety, and in different 

 parts of a single tree. Such a variation is illustrated in figures 7 and 

 8. Figure 7 is a photomicrograph of a traverse section of the wood 

 of the mesquite, Prosopis juliflora DC. The parenchyma is strikingly 

 metatracheal in its distribution. Figure 8 illustrates the structure of 

 the wood of another specimen of this species. The parenchyma is 

 more nearly paratracheal than metatracheal in its distribution. The 

 first specimen was taken from a tree grown in Arizona, the second 

 from a plant grown in Hawaii. 



From this it is evident that the distribution of wood-parenchyma 

 may be extremely constant, or may fluctuate considerably, in any par- 

 ticular group of plants. At the present time there is no a priori 

 method of estimating the constancy of this character in a selected 



