56i 



of which have become longer lived through the chlorophyll content of the 

 inner tissue layers. 



In occurrences of this kind, frequent in many years in certain locali- 

 ties, it is noticed that, as a rule, the tip bud, which has already advanced 

 farthest in development, can grow on undisturbed. Then the branches 

 have a whip-like appearance, inasmuch as their tips are richly leaved while 

 the lower internodes remain bare. Another phenomenon with which I 

 became acquainted in older pear shoots consisted in a blackening and dying 

 of the basal parts of the young shoots which otherwise still appeared green 

 and did not dry up until later. 



Potonie has given special study to the phenomena of the restitution of 

 spring shoots lost through frost \ Different varieties of trees behave dif- 

 ferently. In many varieties lateral shoots grow from the still uninjured 

 basal buds of the frozen branch as, for example, in Castanea sativa Mill, 

 and also in varieties of Celtis and Platanus. If the young shoot is entirely 

 destroyed, a new foliage growth takes place in many plants by the forma- 

 tion of "accessory sprouts." Many tree varieties, especially with increas- 

 ing twig nutrition, form not one alone but a succession of buds in the axil 

 of a leaf by the sprouting of the inner bud stem called "loxver buds." 

 These "lower" or "accessory buds" under normal conditions can develop 

 only on strong shoots of some trees (Cercis). In disturbances, however, 

 as for example, severe pruning, grazing and frost, which destroy the shoot 

 produced from the main bud, they also form the compensatory material 

 in other trees, as for example, in Calycanthus fioridus, Cercis Siliquastrum, 

 Gymnocladus, Liriodendron tulipifera and Robinia Pscudacacia, and de- 

 velop as many as four "lower" buds hidden in the base of the petiole. On 

 the other hand, compensation can also be secured from their so-called 

 "fringing buds" formed the year before. These are the buds in the axils 

 of the basal bud bracts which at times succeed in developing regularly as is 

 perceived clearly in many varieties of willow. If the covering formed by 

 the union of the two bracts drops off, an axial bud is found, corresponding 

 to each half bract and this can form a compensatory branch when the 

 main branch is injured. 



In other cases the tree must depend on the dormant buds of the shoots 

 of the previous year for compensation, as may be observed especially with 

 Rhus, Carya glabra Mill, and Juglans nipestris Engelm, while Carya amara 

 Mich, and Pterocarya fraxinifolia Lam. chiefly unfold "lower buds." 

 Conifers generally replace the frozen sprouts by the awakening of buds 

 dormant up to that time, and also by a new formation of bud primordia in 

 otherwise budless leaf axils, especially those of the bracts at the base of the 

 annual growth. 



No special limitation in the kind of compensation in frozen shoots of 

 different varieties of trees can be made, however, since the strength of the 



1 Potonie, tJber den Er.satz oi-rrorener Fruhlingstriebe durch accessorische 

 und andere Sprosse. Sitzungsber. d. bot. Ver. d. Prov. Brandenb. XXII, 1880, p. 81. 



