CHAPTER VIII. 



HAIL. 



All injuries from hail form wounds, with a consequent loss of sub- 

 stance ; any chemical action as a result of the cold of the hailstones cannot 

 be demonstrated ; only the mechanical blow which either tears away various 

 parts of the tissue and, by drying, causes them to go to pieces, or slits the 

 leaves and branches in knocking off more or less large pieces. 



The small piece of rye-blade, which is shown here, has been struck by 

 hail at the points g, :: and v, and shows the effects of the blows of the hail 

 stones. In considering such a section after a hail storm which has not been 

 severe enough to knock off the leaves, or heads, or to break the whole stalk, 

 we find, as every one knows, whitish or white spots on the green striped 

 upper surface. The striping is produced by alternate dark green furrows 

 and lighter colored lines. In cross-section, it is seen that these furrows 

 consist of a soft bark parenchyma, containing chlorophyll, while the lighter 

 colored stripes are composed of thick-w^alled fibre-like cells (p). These 

 fibre strands stiffen the blade. The thicker their walls are, the more re- 

 sistant the blade is and the less inclined to fall. In Fig. 91, the green 

 parts are seen to be changed most. The cells at g appear uninjured ; at a 

 only dry cell walls are found, which are connected with one another by a 

 scaffolding-like structure. Toward the centre of the blade, however, there 

 is green living tissue (u). Here, the blow of the hailstone has not destroyed 

 the epidermis (^) at all,, but has bruised the more delicate bark parenchyma 

 underlying it so that part of the cells have died. Only a few pieces of the 

 cell walls of the former juicy bark tissue remain and, at this point the hail- 

 stone has had such force that it has broken the thick-walled, tough epidermis 

 at 0. Air has entered through this opening and this hail spot appears white 

 to the naked eye, Avhile at m a greenish tone may still be noticed. 



Similarly, the loss of tissue will take place in other parenchymatous 

 parts of the plant and the assimilatory activity will fall according to the 

 severity of this loss. Yet, this reduction of the life-activity may become of 

 great influence only if the hail storm sets in at a time when vegetative 

 growth has stopped and the plant has entered upon the reproductive period, 

 when it withdraws the cytoplastic substance from the leaves. 



