541 



form yellow, gum-like mass. At other places (r) the parenchyma in the 

 inner part of the sheath is torn or is filled -with peripheral holes, due to the 

 raising of the epidermis. Elongated cells occur near such holes or often 

 instead of them, and point to the fact that, in freezing, the stalk is most 

 often contracted tangentially, thus pulling the epidermis out of shape. 

 Because the epidermis is not so. elastic as the rest of the bark tissue, it 

 remains permanently elongated as the result of this tension. When the 

 frost ceases, it is raised in places (/ and /'), or perforated, and its pressure 

 on the underlying parenchyma decreases, causing the parenchyma cells to 

 elongate into pouches ird). The elongated cells, usually under the outer 

 epidermis (c), but more rarely on the inner side (c'). often possess strongly 

 curved, or stretched walls. 



These conditions are magnified and illustrated in Figs. 112 and 11.3. 

 Here the processes of wall swelling appear to be so great that one is able to 

 distinguish only indistinctly the limits of the individual cells; some cell 

 lumina disappear almost entirely (7'). The loosening of the epidermal 

 pressure, connected in the above case with the phenomena of swelling, has 

 caused the over-elongation of the underlying tissue and the formation of 

 considerable groups of bent, abnormally enlarged cells in some places {rd) 

 and isolated ones {z) in others. 



Finally, the phenomenon of splitting within and around the vascular 

 bundles is most worthy of consideration. In the vascular bundles the si)lit- 

 ting takes place usually in a radial direction (Fig. iii^) ; in fact in such a 

 way that the more tender tissue between the two wide ducts is torn. The 

 part surrounding the vascular bundles can be so greatly torn (r) that the 

 bundle projects into the cavity. This phenomenon gives the impression 

 that the parenchyma had contracted so strongly, as a result of the frost 

 action, that it is torn off from the resisting, firm bundles. In case such 

 differences of tension make themsehes felt less extremely, the parenchyma 

 near the bundles is only greatly strained, causing a subsequent production 

 of enlarged cells with curved walls (^'). 



Injuries to the vascular bundles, for which the vascular elements must 

 certainly compensate by their conductive activity, are of great importance 

 to the life of the plant. This explains the developmental retarding of plants 

 injured by frost. Such plants, even without the cooperation of the para- 

 sitic organisms, which especially seek out weak seed, furnish less straw and 

 especially badly nourished grain. As a rule, however, there is an addi- 

 tional parasitic injury due to rust, black fungi and other leaf and beard 

 inhabitants. The development of the stalk is irregular since all plants in 

 the field never suffer ecjually strongly ; besides the individual differences in 

 power of resistance, the inequality of the soil sometimes favors the frosts 

 and sometimes gives protection from them. The more injured specimens 

 stand under the shade and pressure of vigorously growing ones. A defi- 

 ciency of light and air and the increase of moisture among the oppressed 

 plants favor the infection and extensive distribution of the fungi. 



