430 THE ANATOMY OF WOODY PLANTS 



is the consequence of the separation of the original components of 

 the congeries or aggregation of rays. 



The diffuse type of radial organization appears to have been 

 more in accordance with the needs of arboreal dicotyledons in 

 later geological times in which climatic refrigeration, annual or 

 secular, has become more and more marked. It is practically 

 of universal occurrence in the forest trees of the present epoch. 

 The compound type of ray is limited to a very few species of trees, 

 of which the oak is a notable example. 



Although large rays of the compound type are usually entirely 

 absent in arboreal forms, they are extremely common in herbaceous 

 stems belonging to diverse and not nearly allied families of dicoty- 

 ledons. The accumulation of large quantities of storage tissues 

 about the foliar traces, a disadvantageous condition in deciduous 

 trees, is very frequent in typical herbs and vines of herbaceous 

 texture. There is an interesting correlation between climatic 

 conditions and the presence of herbaceous types which is not 

 only geographical, but likewise geological. Taking the geographical 

 or climatological conditions first, we know that it is a notable fact 

 that arboreal forms are much more prevalent within the tropics 

 than they are in temperate regions. This statement naturally 

 applies to the higher rather than to the lower families of the 

 dicotyledons. In the true amentiferous types, such as the Betu- 

 laceae, Fagaceae, Juglandaceae, etc., there are no herbaceous 

 representatives at all, although these families are characteristically 

 distributed in temperate climates. The absence of herbaceous 

 forms in this instance presents an interesting resemblance to the 

 conditions found among the Coniferales, which are also without 

 species of herbaceous texture, even in the extreme polar limits 

 of their range. The facts in the two parallel cases seem to have 

 a common and significant explanation, which is that both are 

 relatively primitive groups. It is in the higher orders, such as 

 the Compositae, Solanaceae, Leguminosae, etc., that the principle 

 of the occurrence of herbaceous types in temperate, and of arboreal 

 forms in tropical, climates is well illustrated. There are, for 

 example, very few leguminous trees in north-temperate regions, 

 while herbaceous forms are extremely common. Contrariwise, 



