1002.] NATURAL SCIEXCES OF PHILADELPHIA. 469 



and the soil becomes drier and warmer, the xylem elements bee 

 thicker walled and the Inmen smaller, forming the so-called autumn 

 xylem. Where the spring growth is delayed until growth fairly 

 commences and where the soil and ground water temperature remain 

 almost constant, the tracheids approach a uniform thickness and 

 size. The annual rings are consequently not well characterized, 

 and it requires in many cases a microscopic examination to deter- 

 mine the limits of the rings of wood annually laid down. We have 

 in the white cedar, Cupressus thyoides, of the New Jersey bogs an 

 exemplification of this character of ill-defined annual rings due to 

 the influence of the uniform condition of growth. The question 

 may be asked at this point. Why this digression ? The answer is, 

 the whole question of growth has a very nnportant bearing on the 

 entrance, growth and spread of the fungi which cause the disease 

 conditions about to be described. 



It may be well here to preface the discussion of the diseases pro- 

 duced by the two species of Gymnosporangia by referring to a case 

 described by Ward^* which is apropos. The larch disease is due 

 to the ravasres of a fungus, Dasyscyplia Wlllhommii, the hyphfe of 

 which obtain access by wounds to the sieve tubes and Ihe cambium 

 of the stem, finally producing a cankerous malformation. The 

 larch fungus is to be found on trees in their alpine home, but there 

 it does very little damage and never becomes epidemic except in 

 sheltered regions near lakes and in other damp situations. " How 

 then are we to explain the extensive ravages of the Larch disease over 

 the whole of Europe during the latter half of this century ?' ' Ward 

 asks. '' In its mountain home the Larch loses its leaves in Septem- 

 ber and remains quiescent through the intensely cold winter until 

 May. Then come the short spring and rapid passage to sunnner, 

 and the Larch buds open with remarkalile celerity when they do 

 begin — i.e., when the roots are thoroughly awakened to activity. 

 Hence the tender period of young foliage is reduced to a minimum, 

 and any agencies which can only injure the young leaves and the 

 shoots in the tender stage must do their work in a few days, or the 

 opportunity is gone and the tree passes forthwith into its summer 

 state. In the plains, on the contrary, the Larch begins to open 

 at varying dates from March to ^lay, and during the tardy spring 

 encounters all kinds of vicissitudes in the way of frosts and cold 

 1^ Ward, Diseases in Plants, 1901, p. 152. 



