546 



tomical condition, the sterile heads, due to frost, are (hl'ferentiated from the 

 similar, well-known injuries to the heads due to thrips, the suction-points of 

 which remain superficial. At any rate, thrips are found not infrequently on 

 heads injured by frost since these animals seek out weakened organs; but 

 their usual small number and the change in the tissue of the spindle leaves 

 no doubt that the infection is secondary. 



The fact that T have succeeded in producing, hy artificial frost, all the 

 injuries to leaf, stalk and head described here is decisive. All the different 

 forms of shrivelling of the grain could also be produced experimentally. 



The condition of sterile heads tlue to frost occurs only in dilYerent 

 years and not extensively except in definite localities. 



The thought that only certain parts of the stalk are injured by frost, as 

 must be presupposed in the condition of sterile heads, at first seems strange, 

 but one at once becomes more familiar with it if the alTected regions are 

 examined, luther the basal part of the head, which ai)peared last from the 

 sheath, together with the adjoining upi)er pari of stalk, is afTected, or the 

 part of the internode lying directl}' under the node, showing the frost band. 

 Hie parts named, however, are the most tender and susceptible of the 

 whole stalk and we find analogous phenomena also in dicotyledonous plants 

 where the stems of the blossoms and fruit are injured and blackened only 

 directly at the base of the blossom, while the older part remains healthy. 



It could not be determined by observation what atmospheric conditions 

 must exist in order to produce interrupted heads or bands on the stalks, 

 because attention was not called to the phenomenon until some time after 

 the action of the frost. Some of the meteorologists consulted incline to the 

 ojjinion that dew plays a ])art in this. 



Frosty nights in May arc usually windless and the injury to the i)lants 

 results from the cooling down of the organs by their radiation of heat. The 

 ujiper surface of the soil itself cannot be cooled down very greatly in a 

 close standing r}e field since it retains its dail}- warmth for some lime 

 through the mantle formed by the air, found between the stalks, which can 

 be moved with difficulty. The greatest amount of cooling through radiation 

 can take place only in the upper part of the stalks. These, however, are 

 covered by the evening dew. The morning wind rises suddenly with the 

 sunrise and starts a rapid evaporation of the dew. The cold, due to this 

 evaporation, can fall even below the freezing point. The places \\ith a 

 lesser amount of dew, the i)arts which are protected by other stalks lying 

 in front of them, thus remain protected from this cooling down to the freez- 

 ing point. The (listril)ution of the dew on the same part of the plant. 

 howe\er, will differ since the ])laces which, through bending, are inclined 

 more horizontally than others, will retain e\en larger amounts of dew. 

 Among the organs exposed to the freezing temperature, however, only 

 especially tender ones will suflfer. This explains the fact that on a head 

 isolated places alone can be injured. In addition to the fact that the base 

 of the head is proved to be the most injured, the circumstance that the 



