645 



seemed faded, flattened and papery dry. Their extremely scanty cell con- 

 tents formed a colorless ball, lying free in the inner part of the cell and 

 turning yellow with iodine. In the older needles, the cell walls of which 

 had remained perfectly colorless, the abundant cell contents appeared in the 

 form of pale grayish red, or yellowish brown, uniform masses lying against 

 the wall. The appearance resembled that produced under the influence of 

 acid gases. In spruces too the discoloration of the needles, produced by 

 intense summer drought, is very similar to that produced by sulfurous acid. 



A similar dropping of the leaves, due to heat and drought, may also 

 occur not infrequently in other conifers, especially when suddenly left 

 standing alone. My experiments with spruces showed, in regard to the 

 process of dropping needles, that when the rays from a lens were focussed 

 at the base of the needles, these could be loosened at once with a slight pres- 

 sure even if they showed no discoloration. When the needles were injured 

 at points higher up they remained attached. In the burned places the cell 

 contents had contracted into a band-Hke, green to brownish-green mass in 

 the centre of the cell, and even their granular structure could still be per- 

 ceived. The contracted content masses lay usually in the same position in 

 the different cells, i. e., in the direction of the long diameter of the needle. 



Injuries to the bud from sunburn are comparatively rare. This is to be 

 attributed, in part, to the protection of the covering of the buds by a hairy 

 felt, gum, resin, cork layers, or the like, which often are found to be espe- 

 cially effective; in part, also to the abundant cytoplasmic contents of the 

 young tissue which, therefore, are changed with greater difficulty. In the 

 tropics, special protective precautions may often be found. According to 

 Potter^, for example, in Artecarpus, Heptapleurum, Canarium ceylanicum, 

 and others, the stipules of the older leaf organs serve as a protection for 

 the young leaves until they become strong, or the entire old leaf at first 

 forms a protective covering for the young one ( Uvaria purpurea, Gos- 

 sypium, etc.) 



In peach forcing in England, a dropping of the peach buds has been 

 observed. In places, where a damp cloth was stretched over the plants as 

 a protection against the action of the sun, no dropping of the buds was 

 found^. 



Sunburn in Blossoms and Fruits. 



In injuries to blossoms, no absolutely high degree of temperature is 

 necessary; even the usual temperatures can become injurious for shade 

 loving plants in an unfavorable place of growth. The tuberous Begonias 

 form the best known example, the blossom edges of which easily become 

 brown, if the plants cannot benefit from the evaporation from moist soil. 



An unusual excess of heat affects fruit in two ways. On the one 

 hand, it produces premature ripening, i. e., the appearance of the processes 



1 Potter, M. C, Observations on the Protection of Buds in the Tropics. Journ. 

 Linn. Soc. XXVIII, 1891, p. 34,3. 



2 Gardener's Chronicle 1893, XIII, p. 693. 



