MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 287 



ness, and the varying amounts of differentiation in the separate in- 

 stances may be infalhbly traced to the action of light upon the plant 

 during the period of definitive formation of the organs in question. 

 Thus for example the exposure of the plumule to illumination in a 

 bursting seed, or of a bulb or tuber to the action of light during the 

 previous vegetative season, will be manifest in results in the way of 

 differentiations of tissue carried to a stage beyond that shown by 

 organs laid down and wholly developed in darkness. This non-dif- 

 ferentiation is accompanied by more or less growth or increase in 

 volume, and perhaps in number, of the primitive tissue-layers, which 

 may or may not carry the various tracts far beyond their normal size, 

 but which may in consequence give the organ of which they are a 

 part an abnormally large or small size, length, thickness, cross-sec- 

 tion or expansion. As a matter of fact a comparison of the normal 

 and etiolated behavior of a plant offers a splendid demonstration of 

 the fact that growth, and development or differentiation, are not only 

 independent but easily separable processes if the action of the factors 

 inducing differentiation is removed or prevented. The lack of dif- 

 ferentiation and the augmentation of the rate and amount of growth 

 is most noticeable in the simple tissue-tracts in the medullary regions, 

 and in the cortex, and furthermore these differences are most highly 

 accentuated in branches and petioles. The distinctive characters of 

 the subdivisions ordinarily present in the above regions including the 

 perimedullary layer, the medullary rays, the inner cortex, the median 

 cortex, and the outer cortex (often collenchymatous in its character 

 in normal plants), are almost entirely lost, and appear only in etio- 

 lated plants that endure extended periods of confinement in the 

 dark, and then in only a slight degree of the normal differentiation. 

 Endodermis and pericycle were entirely lacking from all perfectly 

 and absolutely etiolated aerial members. From an organographic 

 point of view the least reduction was to be found in the, presumably, 

 most primitive organs and parts of organs, such for example as the 

 hypopodia of leaves, and so far as our information goes a parallel 

 reduction is to be found in the separate tissues. The primary xylem 

 elements appeared in much the usual manner, but the structural de- 

 velopment of these elements which is dependent upon changes in the 

 walls including the deposition of quantities of aplastic matter failed 

 in carrying them toward a normal stature and form. The develop- 

 ment of the secondary fibrovascular elements is of course dependent 



