FIBRO VASCULAR TISSUES: RAYS 83 



in the region of the aggregate ray which results in their being 

 locally depressed. 



To the right is to be seen another leaf with its corresponding 

 gap and ray. In this case the ray structure where it is still near 

 the leaf trace (solid black) is the same as in that just described, 

 namely, aggregate. Farther out, however, the components of the 

 aggregate ray, instead of maintaining their original relations to one 

 another, begin to diverge in the tangential plane. At the same time 

 vessels which are conspicuous by their absence while the ray is in 

 the aggregate condition begin to appear in the widening strands of 

 wood which separate the diverging rays. This process continues 

 and becomes more and more marked in successive outer annual 

 rings. The final result is that what was once a congery or aggre- 

 gation of .rays separated from one another by purely fibrous strands 

 becomes a more and more diffuse cluster of rays separated by ever- 

 widening vascularized intervals of wood. In this condition the 

 original aggregation of rays not only becomes diffuse in a fanlike 

 fashion in the outer region of the woody cylinder, but the individual 

 rays subdivide, thus accentuating the condition of diffusion. The 

 phenomenon of the subdivision of the rays for the sake of simplicity 

 is omitted in the diagrammatic representation. It is clear that the 

 appearance of the conditions depicted in the ray to the right of the 

 diagram (at b) in the case 6f all the foliar rays of a stem would 

 result in a diffusion of rays of a medium breadth throughout the 

 older wood in other words, to the condition shown in Fig. 60 for 

 the adult wood of Casuarina equisetifolia or an allied species. 



Turning now to the left of the diagram (at c\ we observe a 

 foliar or leaf ray of still another type. Here, as in the diffuse con- 

 dition of the foliar ray represented in b, the original state is that of 

 aggregation with the exclusion of vessels, a situation which is per- 

 manent in the type diagrammed at a. In the later annual rings in 

 this type the aggregation becomes a homogeneous mass of paren- 

 chyma by the disappearance of the fibrous strands which separate 

 the components of the aggregation or congery from one another. 

 Where the clusters become fused into large homogeneous bands of 

 storage tissue by the parenchymatous transformation of the original 

 separating fibers of the aggregation, the result is the compound ray 



