160 D. ROLLER 



program of survival, namely formation of viable seed. The 

 question under discussion here is if and to what extent regulation 

 of germination performs a similar function in another part of the 

 program of survival of the species, namely establishment of the 

 seedlings in a suitable environment at a suitable time. 



Several germination regulating mechanisms have been identi- 

 fied in seeds, for example control of water entry into seed, 

 control of the gas exchange between the environment and the 

 embryo, germination inhibitors, control by specific temperature 

 response, control by light response. Some of these mechanisms 

 may indeed be visualized as bearing ecological significance. 

 Thus, the control of water entry into the seed, which is achieved 

 by the presence of a water-impermeable layer surrounding the 

 embryo-' ^' ^, may operate in two ways. The impermeability 

 may disappear from a fraction of the seed crop at a time over a 

 long period^' ^, or it may disappear simultaneously from almost 

 the entire population through the action of some environmental 

 factor (frost^o, heat^i, microbial activity in the soil^- or in the 

 stomach of ruminants^^' i"^, or at specific relative humidities^^ 

 or by action of fire^^). In the former case, the repeated 

 attempts at establishment compensate for the haphazardness in 

 choice of environment. In the latter case, specific environments 

 would be favoured. 



Regulation by germination inhibitors has some clear ecological 

 implications, in that certain environments are excluded and 

 others are favoured^'^. Thus, inhibitors in pulp and juice of 

 fleshy fruits exclude precocious germination inside the fruit^^; 

 water-soluble inhibitors in dispersal units of desert plants may 

 act as rain-gauges in preventing germination until a certain 

 critical amount of rain has fallen^^"'-^; inhibitors may also aid in 

 preventing over-population by seedlings (whether of the same or 

 other species)-^. 



Control by specific temperature also has some obvious 

 ecological implications. The simplest of these are the require- 

 ments for specific ranges of more or less constant temperaf 

 tures^s, 26_ ^ more sophisticated control is found in seeds o- 



