539 



of abnormal tension conditions. To these should be ascribed also the ver\- 

 conspicuous phenomenon of the production of holes (/) regularly arranged. 

 The holes are produced by the blister-hke upraising of the epidermis from 

 the real leaf flesh, usually on the upper side, between the rows of stomata 

 (sp). The under, or outer side, of the leaf shows only scattered holes of 

 small extent. The tangential elongation of some of the epidermal cells, 

 noticeable at times ( cp and es) offers an important proof of the production 

 of these holes. The epidermal arch has become larger than it was before 

 the action of the frost ; this elongation is the result of the stretching of single 

 cells. Besides this upraising of the leaf, a radial splitting in the vascular 

 bundle indicated at /' is very characteristic of frost injury; this becomes of 

 especial importance in the axillary body. 



To distinguish the formation of holes, due to the action of frost, from 

 the tearing of the tissue, due to senility, we give in Fig. no the cross-section 

 of the first sheath-like leaf of a rye plant, the inner tissue of which, in the 



Natural cavities in the sheath-lilve rye leaf, formed during- growth. 



course of normal development, splits at death. The holes (h), produced 

 thereby, are ahvays tangential. 



Internal Injuries in the Grain Stalk. 



More important, however, than the leaf injuries is the eft'ect of frost in 

 the stalk itself. A\'e usually have no suspicion of this, since, to the naked 

 eye, no change is noticeable in the plant. Fig. 1 1 1 illustrates a lower node 

 of rye injured by frost. 



The tissue of the stalk (//) is firmly enclosed by the sheath (Sch), the 

 outer epidermis of which is indicated by e, the inner by e' while e" indicates 

 the upper epidermal cells of the stalk. The browning of the ducts in the 

 different bundles, which occurs in all frost injuries, is indicated at u and ?/' 

 wdiere the narrower spiral ducts between the wide annular ducts seem the 

 most injured. At br are found aggregations of brown parenchyma cells 

 in the sheath; at br' the same in the stalk itself. At v and v' are shown 

 brown groups of cells in the sheath and in the stalk, the walls of which are 

 very strongly szvoUcn up so that the whole cell seems converted into a uni- 



