624 



granular condition, retains the air. and therefore appears black. The small 

 granular papillae still remained when the leaf was destroyed by sulfuric 

 acid; in which treatment the leaf curled up like a worm and the epidermis 

 of the upper side puffed out in places. 



This result agrees with discoveries which had been observed earlier in 

 beech trees after natural late frosts, and which we could prove also on oaks 

 in the open. In the production of such scarcely perceptible rupturing of 

 the cuticle, some special conditions must also have co-operated which were 

 present accidentally in the experiments but do not seem to be always effec- 

 tive in other experiments or in nature, for, soon after late frost, such injured 

 oak leaves could be found in some localities but not in others. Probably 3 

 definite condition of turgor in the leaf is connected with it and this will be 

 dependent again on the constitution of the cell contents at any given time. 



A conception of the fine differences, which are decisive in frost inju- 

 ries, is obtained from the observation that dead particles of tissue, injured 

 by frost, may be found at times in the centre of the mesophyll of the leaf, 

 which apparently is but little, if any injured. The fact that, in experi- 

 ments, these cuticular breaks appear only on the under sides of the leaves 

 may be traced perhaps to a constitution different from that of the upper 

 .cuticular covering, for it is found that in the action of sulfuric acid, the 

 upper covering turned a bright lemon yellow, which color shade was 

 scarcely perceptible in the cuticle of the under side. 



I would like to lay especial value upon the discovery that, under certain 

 circumstances, a rupturing of the cuticular glaze can be produced by light 

 frost. In other breaks in the cuticle (in pomes) fungus spores were found 

 lying in the line of the break and it may, therefore, not be out of place to 

 assume that, in these protected places, such fungus spores have the best 

 opportunity to germinate and to sink their germinating tubes into the organs. 

 In this zvay might, therefore, he explained the attacks upon apparently per- 

 fectly healthy leaves and fruit by fungus infection after a light spring frost. 

 Voglino's^ reports might be referred to here. In 1903, after some frost in 

 April, he found that the fungous parasites had an especially large distribu- 

 tion in plants injured by frost. 



Thus is explained also the phenomenon of the so-called rust etchings 

 in connected rings and irregular surfaces on our fruit. They are cork 

 formations which have set in, in the cuticular tears, as a result of the 

 processes of healing, while the normal cork etchings on the fruit usually 

 begin at the stomata, or rather, the lenticels. 



Protective Measures Against Frost. 



(a) Snow Covering. 



The process, universally used for protecting plants against frost, con- 

 sists in surrounding them v.ith substances which are poor conductors of 



1 Voglino, P., L'azione del freddo suUe piante coltivate, specialmente in rela- 

 zione col parassitlsmo dei funghi. Attl accad. di Torino XLVI. 



