iQ2o] SHU LL— SEEDS 387 



kinds acting exactly alike. It would not be possible to accept 

 without modification any theory which assumes that differential 

 septa are alike in behavior. I do not mean to say that water is 

 not simplified in structure as it is warmed, nor that such a change 

 would not increase the rate of absorption, but it seems entirely 

 possible to account for the high temperature coefficients found in 

 absorption phenomena without the necessity of assuming such 

 a change, or making it the sole change involved in the process. 

 The substances of which the seeds are composed, membranes, 

 embryo, and storage products, are all largely colloidal. These 

 colloidal materials undoubtedly are modified in state of aggregation 

 by being subjected during wetting to low or high temperatures. 

 Higher temperatures usually increase dispersion and increase the 



- 



water-holding capacity of organic colloids, and lower temperatures 

 reverse the process. It does not seem possible that such changes 

 could be absent during absorption, and they must go far to explain 

 the differences in intake rates and the values of Q I0 , which stand 

 between those found for purely physical and purely chemical 

 processes. Absorption is a complex process, probably involving 

 both physical and chemical factors, and the values of Q I0 may be 

 considered the resultant of the effects of temperature on both 

 classes of factors. The fact that we get about the same value for 

 Qio in absorption without a semipermeable coat as with such a 

 coat indicates that the membrane is not necessarily the rate deter- 

 mining factor. 



Denny (2) has shown that membranes differ greatly in their 

 power to transmit water. If the seed coat transmits water more 

 slowly than seed substance can absorb it, the transmission rate is 

 a limiting factor on the absorption rate. If the transmission power 

 of the coat exceeds the absorption power of the seed substance, 

 however, the latter determines the rate. Again, if seed coat, 

 embryo, and endosperm form a very non-homogeneous structure, 

 the absorption rate may be dominated first by one of the structures, 

 and later by the others in succession, giving peculiar absorption 

 curves, difficult to analyze mathematically. 



It was noted that Xanthium seeds showed a very rapid initial 

 intake during a minute or less, after which the rate broke sharply to a 



