256 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



frost injury of the interior lens stops short of the full length of the 

 lens. The injury is continued by a fairly sharp densewood band. The 

 length of the lens depends, of course, upon the amount of cambium 

 which was active and which had set off xylem cells at the time of im- 

 pact of low temperature. TTP 21-3 is an interesting case. In sections 

 a, 26 cm. from the tip, a sharply bordered interior lens containing frost 

 injury and recovery extends for 90° around the circuit. In sections h, 

 5.5 cm. from the tip, an arc of densewood represents the lens and in- 

 jury of a. The increment for 1941 in TTP 24-io-a contains a con- 

 current interior lens system, each member of which contains a frost 

 injury so intimately connected with the densewood that no doubt can 

 arise as to the cause of the lenses. The 1941 increment of TTP 24- 

 14-a possesses an interior lens caused, very probably, by low temper- 

 ature (pi. 19, fig. 2), Frost-injured cells are accompanied by irreg- 

 ularly shaped cells with thick, heavily lignified walls and both narrow 

 and wide lumens. Under anything except high magnification of 

 stained sections, the interior lens appears to be an integral part of the 

 densewood of the previous growth layer. 



The substance of the work on temperature shock shows rather 

 clearly that the effects are twofold : ( i ) anatomical, chiefly in the pro- 

 duction of frost injury and recovery, with the accompanying paren- 

 chyma cells, resin canals, and compression wood, features commonly, 

 but not universally, restricted to the branches; and (2) physiological, 

 chiefly in the formation of densewood and the deposition of gums, 

 resins, and tannin, effects probably not restricted to any one part of the 

 tree body above ground. 



Postseasonal growth may properly be applied to any growth occur- 

 ring after the grand and unified period of growth which follows 

 winter dormancy. It may be present as a complete, entire growth 

 layer, as a lens or series of concurrent lenses complete or incomplete, 

 as isolated cells, or merely as an extension of the terminal bud. In any 

 case, postseasonal growth indicates that the cambium has been stimu- 

 lated to activity at least a second time within the same general growing 

 season or during the following autumn or winter. 



On November 16, 1939, nine trees were moved from the Experi- 

 ment Substation to the Conservatory where light, water, and heat 

 were supplied. Growth, which had ceased previously for the 1939 sea- 

 son, was reinitiated, and the resultant "Conservatory growth layer" 

 was either lenticular or entire. Some of the sections were cut from 

 branches while growth was in progress. The response to warmth and 

 moisture simulating summer conditions, even in the presence of 

 shorter day-lengths, was decided. It indicates, along with the wealth 



