58 



SCIENCE 



[N. S. Vol. XXXI. No. 785 



growing points of tlie shoots and becomes 

 less and less when the more stable and 

 permanent tissues like leaf parenchyma are 

 attacked. The same fungus which causes 

 real hyperplasia in young tissue produces 

 but a hypertrophic enlargement in the 

 adult parenchyma. 



Such being the case, one might be war- 

 ranted in reasoning by analogy that the 

 still more plastic cells of the gametophyte 

 would be even more profoundly influenced 

 by stimulation and such indeed appears 

 from MacDougal's experiments to be the 

 case. It is also not unreasonable to sup- 

 pose that the inciting cause of the healing 

 of wounds, of stimulus to growth after 

 injury, and even of regeneration phenom- 

 ena themselves, hai'ks back to a question of 

 chemical stimulation. In the more mas- 

 sive tissues, at any rate, wounding results in 

 the exposure of interior cells directly to the 

 action of the oxygen of the air, and is ac- 

 companied by increased metabolic activity. 

 The more rapid growth of injured parts, 

 the awakening of doi-mant buds, may well 

 be influenced or initiated, though probably 

 not eventually controlled, by chemical stim- 

 ulation arising from this or similar causes. 



It is not to be supposed in any case that 

 the chemical substances in question them- 

 selves constitute— by any direct union with 

 the protoplasm— the modifications which 

 ensue. It is only possible here to touch 

 thus briefly upon the morphological re- 

 sponses induced by chemical stimulation, 

 for the field is an enormous one. In some 

 of its aspects, the study of the immediate 

 effect of environment upon the external 

 and internal form of a plant comes under 

 this head. There is without question a 

 large and inviting field for investigations 

 into the nature of the changes in structure 

 which are correlated with chemical stimu- 

 lation. 



It is necessary also to pass over without 



further comment the directive effects of 

 chemical stimulation upon growth and 

 movement, concerning which there are 

 many investigations as to the expression of 

 the reaction, but very little information as 

 to the intimate causes of it. 



After this brief consideration of the 

 changes in the actual amount of elaborated 

 substance, of stature and of structure 

 which commonly attend chemical stimulus 

 and which are the outward signs of its 

 workings, we may next turn to the more 

 fundamental question of what we know of 

 the influence of this excitation on the 

 physiological activities of the plant. 



One fact which is clearly marked in the 

 case of certain fungi that have been in- 

 vestigated is that the protoplasm, when 

 stimulated, works more economically in re- 

 spect to the carbohydrate food material 

 supplied than when unstimulated. The 

 latter produces a larger crop, as estimated 

 from the dry weight, from a given amount 

 of sugar than the normal culture does: 

 there is less waste. Were the metabolic ac- 

 tivity of protoplasm to be interpreted 

 simply in terms of economy of action, one 

 might be tempted to speak of such a con- 

 dition as more nearly approximating a 

 perfect or so-called normal; but when we 

 reflect that we know so little of the chem- 

 ical action and interaction of living proto- 

 plasm, it would be unwarranted to assume 

 that mere economy of consumption of one 

 form of food material would tell the whole 

 story. The plant is attuned to an average 

 condition and its attunement to that con- 

 dition constitutes the nearest approach to 

 what we may call normal. The increased 

 availability of the sugar under chemical 

 stimulation may be regarded as an un- 

 toward, fortuitous condition which, while 

 it may be optimal for the processes in- 

 volved in building up vegetative hyphse, is 

 not optimal for the development of the 



