171 Platit Cytology. 203 



of the morphological but equally of the physiological specific 

 characters of plants. In my study of the great physiological 

 group which we commonly speak of as " sensitive plants," I 

 have been impressed by the evident specific physiological 

 relation of the tissues to environmental conditions. In cold 

 countries such plants can scarcely be said to exist, except in 

 the person of the Wood Sorrel, in milder regions they begin 

 to attract some attention, in sub-tropical countries they are 

 pretty frequent, and in tropical regions they often give a 

 character to many landscapes. But the increase in num- 

 ber of sensitive species toward the Tropics is correlated with 

 increased sensitivity, so that from the Wood Sorrel to the Sen- 

 sitive Plant we can arrange a graded series which show on 

 stimulation a shorter latent period, increasing capacity for 

 propagation of stimulus, a quickened period of contraction, 

 as well as reduced neutral period, and period of re-expan- 

 sion. 



Bonnier's beautiful investigations on lowland and alpine 

 plants should furnish a model for many similar studies in the 

 future. The modifications he traces in epidermal cuticulariza- 

 tion, in chlorophylloid cells, in tracheidal tissue and other 

 bundle elements, demonstrate the true morphological origins 

 of our Alpine Flora. Such considerations inevitably lead us 

 to inquire — What is a species ? Instead of attempting a direct 

 answer allow me, in a few lines, to speak of what I have 

 ventured to call "taxonomic cytology." 



(f ) Taxonomic Cytology. — Even if one should speak with 

 bated breath of things taxonomic, before a Society for Plant 

 Morphology and Physiology, at the risk of being consid- 

 ered a law-breaker, I will attempt to slip in the greater adjec- 

 tival subject, under cover of the lesser substantive plea. 

 Urgency seems to furnish the apology. During the past 

 dozen years or thereby, as questions of heredity, transmission 

 of character, and validity of species distinction have been 



