MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 249 



in the cells during a much longer period than in the normal plant; 

 consequently it can respond to stresses and other factors, which may 

 cause it to undergo increase in size, alterations in form, or multiplica- 

 tion of the cells by division. Thus the epidermis is compelled to 

 make adiustment to the augmented diameter which some stems attain 

 by increase in bulk of the cortical and medullary tracts. 



Collenchymatous Layers in Etiolated Stems. — Collenchyma 

 shares with other mechanical tissues in a general reduction which, 

 in some instances, amounts to a total disappearance from etiolated 

 stems. The collenchymatous layer ordinarily underlying the epi- 

 dermis failed to appear in etiolated stems of Galium and Meni- 

 spermani, and consisted of a tract of tissue with cells reduced in 

 number, or thickening of the walls in Aster, Sati7'urus and Viola. 

 Phytolacca offers the single exception of the formation of this tissue 

 in a manner approaching the normal in etiolated stems. The sub- 

 epidermal layers generally shared the enlargement of lumen and 

 lessened thickness of wall that was to be observed in the cortical 

 tissues of the greater number of species examined. It seems justifi- 

 able to conclude therefore, that collenchymatous tissue is not affected 

 in its development by light and darkness directly, but that the differ- 

 entiation of this tissue is due entirely to secondary causes dependent 

 upon the phenomena included in the growth and development of the 

 tissues interior to it. 



Formation of Periderm on Woody Etiolated Stems. — The epider- 

 mal and cortical tissues of the trees examined in my experiments offer 

 three types of reaction. Branches of Poptilus grown in darkness 

 were characterized by the failure of the tissues to develop beyond 

 an extremely simple stage and the cortex was composed of elements 

 which were larger, and with thinner walls than in the normal, while 

 they were variously distorted from the somewhat regular form of the 

 normal elements. The differentiation of the cortex into an outer 

 and inner region was wholly lacking. No activity of any kind was 

 to be found that presaged the formation of bark. In a second type 

 the behavior of the tissues involved only comparatively unimpor- 

 tant departures from the normal. This procedure was seen in 

 Acer, although differences due to lack of thickening in the walls 

 of the epidermis, collenchyma, and cortex were to be seen. The 

 elements of the latter were compressed radially. A similar state of 

 affairs was to be seen in seedlings of AEsculus. Perhaps the least 



