Rays of woody dicotyledons may be classified into six salient types: heterogeneous type I; heterogeneous type II; homogeneous type I; homogeneous type II; uniseriate, heterogeneous type III; uniseriate, homogeneous type III. 2. There is a high correlation between vessel type and ray type, the evolutionary sequence being from the heterogeneous type I, the most primitive, through the transitional heterogeneous type II and homogeneous type I, to the highly specialized homogeneous type II rays. 3. The uniseriate, homogeneous type III rays are more highly specialized than the uniseriate, heterogeneous type III rays. 4. The uniseriate ray is a highly specialized structure owing to the elimination of multiseriate rays. 5. Specialization of rays is dependent upon the degree of heterogeneity rather than upon the size of the rays. 6. The so-called compound ray is merely an unusually wide multiseriate ray. Such rays are of common occurrence in both primitive and specialized types of dicotyledons, and may be of either the heterogeneous or the homogeneous type. 7. The so-called aggregate ray is a specialization which occurs sporadically. It is an offshoot from the main line of structural specialization in rays.
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David A. Kribs (1935) studied this question.