250 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 



variation from the normal was to be seen in branches of adult trees 

 of F'agus grown in darkness, in which the phellogen, epidermis, 

 collenchyma and cortex were present in an order differing from the 

 ordinary forms only by a lessened thickening of the walls of all 

 these tissues. In Cornus the cortical cells showed abnormally large 

 measurements in all radii with a lessened thickening of the wall, while 

 the epidermal cells began to collapse earlier than in the normal, the 

 underlying tissues also being less thickened. This was accompanied 

 by a precocious development of the phellogen, which however had 

 much the normal structure and arrangement. The most striking 

 departures from the ordinary formation of bark were offered by the 

 various species of Castanea, Hicoj'ia and ^lerciis constituting the 

 third type. The seven species brought under observation agreed in 

 the normal formation of phellogen in the epidermal region. In etio- 

 lated specimens of Castanea and Hicoria the epidermal cells remain 

 intact for some time, and a layer of cortex in the median region in 

 Hicoria^ and about in the locality of the collench3^ma in Castanea 

 begins to collapse (p. 93). Internal to the collapsing layer, and some- 

 times in contact with it in Hicoria^ an abnormal layer of phellogen was 

 formed. The cortical cells of plants of this group showed no increase 

 in size or marked changes of form outside of those consequent upon the 

 above alterations in behavior of the bark-forming tissues. The walls 

 of these elements were less thickened than in the normal, and fewer 

 crystals might be seen than in the normal. The procedure described 

 above has the effect of reducing the external diameter of the portion 

 of the stem in which the action takes place, in consequence of which 

 the stems are smaller at the base than the normal, or than the upper, 

 younger portions of the same stems. In ^icrcus the epidermal and 

 subepidermal regions collapse early in the development of the stem. 

 Ordinarily the epidermis endures for a long time and a phellogen is 

 formed immediately underneath, consisting of eight to ten layers of 

 cells. The cells which usually undergo collenchymatous thicken- 

 ing internal to the phellogen seem to be involved in the collapse, and 

 a median layer of the remaining cortex gives rise to a phellogen con- 

 sisting of several layers of cells. The cortical elements appear larger 

 in all dimensions than in 'the normal. The collapse of the epidermal 

 and outer cortical cells begins with the outermost layer and extends 

 inward slowly, having the final effect of reducing the thickness of the 

 portion of the stem affected even more than is done by the similar 



