288 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 



on embryonic layers or generative tissues, and as these were not 

 formed in all species the stele often showed great variation from 

 the normal. The incomplete deposition of aplastic material in the 

 membranes affects the mechanical qualities of the organs most seri- 

 ously, and allows them to remain a much longer period in an exten- 

 sible condition, and capable of abnormally great growth-extension, 

 although as may be seen, this excessive growth does not always fol- 

 low. The diminished deposition of aplastic matter in the cell-walls 

 affects the epidermal layers even more seriously than the components 

 of the stele, and as an accompaniment of this increased extensibility 

 and prolonged capacity for division the epidermis exhibits a great 

 variability of behavior in the different species. It is to be borne in 

 mind however, that the failure to deposit material in the membranes 

 may not be attributed to any lack of formative matter, since this is 

 often present in comparatively enormous quantities in completely 

 etiolated plants. 



It is to be seen therefore that the phenomena of etiolation rest 

 upon, and consist in the behavior of the plant consequent upon the 

 absence of the morphogenetic influence of light. Some species 

 show an adaptation to this absence of light, or to the positive influence 

 of darkness, by which the shoots or petioles are elongated in such 

 manner as to constitute an effort to escape from darkness, or to attain 

 illumination. 



The Stimulative Influence of Light. — It has been so thoroughly 

 demonstrated by the researches of the last half century that the dor- 

 siventrality of many of the lower forms, and the development of 

 branches and other members is dependent upon illumination that no 

 further discussion of the subject is necessary in this paper. Simi- 

 larlv temperatures may affect some of the morphological develop- 

 mental processes. Every species appears to be attuned to a certain 

 range of intensity of the various environmental factors, and to be 

 capable of carrying out its chief morphologic and physiologic activ- 

 ities only within this range. Failure to obtain the customary expo- 

 sures means a loss of the stimulating effect of the agent in question, 

 and the activities ordinarily set up by the stimulating action of the 

 missing force do not appear. Thus a low intensity of radiant energy 

 in the form of heat may suffice to cause the plant to grow for a season 

 or even for a term of years, but the stimulating effect of a higher in- 

 tensity not being furnished, the plant does not develop floral organs, 



