294 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



actual quantities necessary to produce the action described in a later 

 section of this paper would be small and in some cases lie beyond 

 detection by ordinary microchemical or cytological methods. 



Some of the earlier results obtained by a study of the growth 

 of opuntias have already been described by the senior author.^ 



The comparison of the action of Opuntia with that of roots and 

 stems of peas, beans, wheat, corn and oats, etc., led to the inference 

 that many of the accepted conclusions concerning growth rested 

 upon data obtained from material representing a specialized or 

 narrow range of physiological action. An inspection of the records 

 of measurement shows that no distinction is usually made as to 

 whether the elongation is due to the action of one embryonic tract 

 as in the case of roots or hypocotyls, or of many as in the case 

 of stems and leaves. It is also to be noted that even in the simpli- 

 fied action of roots the elongation is a dififerent expression from 

 that of such an organ as a sporangiophore. ^Measurements of 

 growth of the tip of a root include the imbibitional swelling of 

 younger cells, the combined swelling and turgidity effects of older 

 protoplasts, with all of the modifications due to salinity, acidity, 

 alkalinity, character of the respiration, permeability of the mem- 

 branes and albumen condition. 



The elongation of a stem may include the total action of several 

 internodes representing various stages of the grand period of 

 growth, while it may be assumed that in some cases the records of 

 leaves represent the variations in length of these organs and of 

 one or more internodes. 



The experimental material used in the investigation described 

 in the present paper included the conventional subjects, Zea and 

 Triticum, which were tested for purposes of orientation. Chief 

 attention however was given to succulents which have long been 

 known to present a type of respiration different from that of the 

 leafy and slender-bodied plants. Futhermore, the massive bodies 

 of the succulents presented characteristic body-temperature condi- 

 tions which could be readily measured. 



The flattened shoots of Opuntia present a single growing region 



5 See MacDougal, " Mechanism and Conditions of Growth," Mem. N. Y. 

 Bot. Garden, 6: 5-26, ipid. 



