656 Bailey .— The Evolutionary History of the 
this it might naturally be inferred that injuries to the wood of these 
reduced plants would recall the foliar type of ray. That such is indeed 
the case is shown by Fig. 8, a cross-section of the wood of Alnus mollis , 
cut in the immediate vicinity of a severe injury. The injury has obviously 
produced a large aggregate ray which may be clearly recognized by its 
retarding influence upon the growth of neighbouring elements and the 
distinct sag in the outline of the annual rings. A similar condition is 
illustrated in Fig. 9, a cross-section of the mature root of Alnus mollis . 
The wood at the lower side of the photomicrograph, which was formed 
before the injury occurred, possesses only uniseriate rays, but in the 
upper portion of the figure may be seen numerous aggregate rays which 
originate at the injury. Additional illustrations of the traumatic recurrence 
of large rays have been observed by the writer in Castanea , Castanopsis , 
and Alnus , as well as in woods which have replaced the compound or 
aggregate by the diffuse type of foliar ray. Fig. 10, a cross-section of the 
outer portion of an insect gall in a mature twig of Ostrya virginiana , (Mill) 
Koch., illustrates the recurrence of an aggregate ray in a type of wood that 
possesses normally only small bi- and triseriate and uniseriate rays. 
Contour of Mature Twigs. 
There remains for consideration a striking piece of evidence in regard 
to the reduction of the foliar ray in lower Dicotyledonous plants. A 
characteristic and important feature of the foliar ray is its retarding 
influence upon the growth of neighbouring radii of the stem (see Figs. 2 
and 3). This influence is so strongly developed in many plants that the 
depression in the outline of the annual rings which marked the former 
position of the foliar ray persists for some time after the disappearance 
of the ray. The writer has shown ( 3 ) that the fluted stem of the Blue 
Beech, Carpinus caroliniana , Walt., is produced by congeries of aggregate 
rays whose concentrated retarding influence upon the growth of certain 
radii produces the large grooves in the stem. A careful study of this 
plant shows that the foliar rays of the aggregate type are gradually 
being replaced by the diffuse condition. (In the closely allied genus 
Ostrya the diffuse condition is dominant.) In specimens growing under 
unfavourable conditions the writer has found large stems from which the 
aggregate ray has disappeared almost completely. However, the character- 
istic flutes remained, although less strongly developed. These plants were 
later cut down, and during subsequent growth of the stump (by means of stool 
shoots) congeries of aggregate rays were recalled in the depressed segments 
of the new layers of growth. The stimulation of the injury and surplus supply 
of food substances in the root produced evidently a reversion to conditions 
which existed before the plant suffered vegetative reduction. In PI. LX 1 1 1 , 
Fig. 17, a cross-section of a vigorous mature shoot of Castanea pumila , 
