252 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 



characteristic of this plant, when grown in darkness. The differen- 

 tiation of cambium is of course closely connected with the general 

 advance of the stele from the embryonic stage, but it is diflRcult to 

 connect its development with that of any single tissue in the species 

 used in the tests. 



Effect of Etiolation on the Stele. — The walls of the elements 

 of the xylem were less thickened in the etiolated than in the normal 

 stem. In some instances the lumina of the vascular elements were 

 smaller than the normal, in others no noticeable difference from the 

 normal size was to be made out, and in a few, of which ^lerciis is 

 an example, the lumina of the vessels were greater than the normal 

 in cross-section. The inter-fascicular parenchyma, and the thin- 

 walled cells throughout the xylem w^ere smaller in etiolated stems 

 than in the normal, while the pith or medulla attained at least nor- 

 mal bulk and were generally greater than in normal stems. The 

 increase of this tissue has led many investigators to attribute the 

 altered dimensions of etiolated stems to be due to its direct action as 

 a cause. The central parenchymatous mass of a stem however, is 

 seen to continue growth and perhaps multiplication of the elements 

 during the embryonic stage, and if this is lengthened by any cause 

 such as etiolation, the natural increase of the parenchyma would 

 lead to the increase of the bulk of the stem by its unchecked action. 

 The polystelic stems examined increased notably in diameter as a 

 result of etiolation, the added bulk being largely made up of paren- 

 chymatous elements, and the central lysigenous splitting did not 

 always occur, as was found in Asparagus. 



A consideration of the reactions of the various tissues of the stem 

 in darkness fails to detect a single universal and invariable reaction 

 on the part of any tissue, except perhaps in the lessened thick- 

 ening of the cell-walls that is found in all of the external regions of 

 the stem. This lack of deposition of aplastic matter is very clearly 

 a matter of continued growth and development of the etiolated axis, 

 and is not a direct response to darkness or the absence of light. The 

 behavior of the etiolated stem is, therefore, not reducible to its first 

 cause by an analysis of the activity of its constituent tissues, although 

 an examination of the components of the stem indicate the gen- 

 eral nature of the etiolative reaction. The continued simple growth 

 made by the plant, unaccompanied by the customary morphological 

 development and differentiation of the tissues, may be taken to ac- 



