70 MASSACHUSETTS HORTICULTURAL SOCIETY. 



flowers were in every way normal. Two branches which were of 

 equal length when the experiment began, were thirteen and 

 eighteen inches tall at the expiration of six weeks, the shorter and 

 earlier portion being that which was fully exposed. It was 

 therefore evident that while the naked arc is fatal to heliotrope at 

 a distance of three feet, it is not injurious and it accelerates 

 maturity at six feet. Coleus plants have proved to be most 

 interesting subjects for experiment. The purple color of some 

 leaves entirely bleached out in a few hours if exposed within a 

 few feet of the naked arc, while those portions of the same 

 leaves behind a pane of glass remain uninjured ; and the line of 

 demarcation between the two portions will often be as marked as 

 if it were the impression upon a photographic plate. 



It now remains to determine the nature of this injury. If a 

 cross-section is made of an injured leaf of heliotrope, it will be 

 found that the epidermal cells have collapsed, — the walls have 

 fallen together ; and as these dead walls become opaque, they 

 ol:)scure or cover up the bright chlorophyll beneath and make the 

 leaf brown. The chlorophyll bodies themselves may be disorgan- 

 ized, although this result follows only as a secondary phenomenon. 

 A careful study of many plants treated to the electric light, has 

 enabled my colleague, Professor Rowlee, to ascertain the physio- 

 logical effect of the injurious light. The ultra-violet rays produce 

 great activity in the protoplasmic contents of the cells, particularly 

 of the palisade tissue, or hasten the physiological processes. 

 This activity calls at once for large supplies of water, and it is 

 drawn first from the overlying epidermal cells ; and these cells, 

 being emptied of their contents, collapse like an empty grain-bag. 

 The loose mesophyll or underlying portion of the leaf is abundantly 

 supplied with moist air in the large intercellular spaces, and it is 

 also in direct communication with the moisture of the atmosphere 

 through the stomata ; so the lower tissue is much less injured by 

 the sudden withdrawal of water than the upper layer. In other 

 words, vital aeti\ity is hastened so much by the naked light that 

 the plant cannot supply materials quickly enough, and it is forced 

 to death; but by removing the plant to greater distances from the 

 lamp, a point will ])e found where water can be supplied with 

 sutlicient rapidity to meet tlie demands of the quickened activities, 

 and the plant will grow more rapidly or at least mature earlier 

 tlian in normal conditions. 



