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 viOstrya 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. LXIII, 

 Fig. 17, a cross-section of a vigorous mature shoot of Castanea pumila, 



