January, 1911. 



KNOWLEDGE. 



a layer of cork alread\' extended through the cellular 

 tissue at the base of the petiole in September. 

 Immediately above this a layer of cells had become 

 lirown (suberised) : and, separated from this b\- two 

 or more rows of the ordinary colourless poKliedral 

 cells of the leaf-stalk, what he termed the separatint,', 

 or "absciss," la\'er orit,'inated. This onI\- formed 

 between the 4th and the 15th of October, extending 

 across the stalk from the inner or a.xillary surface, 

 and contained in its cells protoplasm and starch- 

 grains. It is, in fact, what we now term " secondar\- 

 meristem." \'on Mohl only recognised two layers of 

 cells in the absciss-laver, which he believed to split 

 apart, while he thought that tlie tihro-vascular bundles 

 were broken mechanicall}' b}' the weight of the blade 

 and the strain of wind and rain. He perceived, how- 

 ever, that the fall of the leaflets between the lOth and 

 20th of October, and the subsequent fall of the 

 petioles was independent of the cork-la_\er fornicd at 

 least a month before. This cork layer, in fact, is 

 not formed in advance in those ferns which are 

 deciduous, in beech, elm or most oaks, \'on Mohl 

 also noticed that when lea\'es fell suddenly, after an 

 autumn frost, a thin la\-er of ice had formed in the 

 delicate sappy cells of the absciss-laj'er, torn cell- 

 walls evidencing the violence of the disruption. 

 In 1863, Julius Sachs traced the gradual removal 



of the contents of the leaf-cells. The protoplasm 

 and nuclei are dissolved, the chlorophyll-granules 

 become disintegrated, the starch disappears, leaving 



r .'^sT 



h.URE 2. 



A beech wliieh has lust the leaves fnjin the ends of 

 the shoots. 



iMCUKIi J. 



The teniiiiial leaves still reiiiaiiiin.i; mi hiuleii tree. 



only the few \'ellow granules, or the reddened cell- 

 sap, w hich produce our autumn tints ; while starch, 

 potash and phosphoric acid travel down the leaf- 

 stalks to be stored up in the twigs, and only the 

 waste or end-products of metabolism, calcium-oxalate 

 cr\'stals, resins and alkaloids remain to be thrown off 

 with the falling leaves. 



In 1882, M.M. Guignard and Van Tieghem re- 

 turned to the studv of Gyiiinoclacfiis; but began their 

 investigation in the middle of June, They found 

 that no cork is formed at the base of the leaflets. It 

 is not worth while to heal the wound on the leaf- 

 stalk which is itself to fall in a day or so. The 

 suberised la^-er was formed at the base of the 

 main petiole b\' the middle of June : then a 

 la\'er of meristem, the " phellogen " or cork- 

 cambium, originates below it and the absciss- 

 laver above it, before the end of June, This layer 

 spreads inwards from the epidermis through the 

 cellular tissue of the bast and wood-bundles. It 

 consists not of two, but of three, layers of cells of 

 which the middle row is absorbed. The two remain- 

 ing rows, still living and turgid, swell outwards w ith 

 roun<led surfaces, and so create a strain which snaps 

 the fibres and vessels. These observers also induced 

 leaf-fall artificially at midsummer, by placing a cut- 

 branch in a box filled with moist air, and the_\- found 

 that after the fall of the leaf the cellular tissue of 

 the vascular bundles whose ends are exposed on the 

 leaf-scar becomes " merismatic," i.e., undergoes cell- 

 division, forms cork, and penetrates and fills up the 

 ends of the vessels. 



It is well to bear in mind that prolonged drought 



