210 SCIENCE PROGRESS. 



rather than heavy seeds, and so on. Plants with a good 

 distribution mechanism can, on Wallace's hypothesis, better 

 afford to do without cross-fertilisation. When both occur 

 together, we get cases like the Composite, of widely 

 scattered dominant and aofgressive families. 



Another great factor in a plant's life history is vegetative 

 reproduction. Similar considerations apply to this as to the 

 two great factors already considered. In itself the most 

 certain and economical mode of propagation, it has to be 

 paid for by the loss of the advantages of fertilisation and 

 rapid distribution, and so we do not find many plants pos- 

 sessing only this means of multiplication. 



FORMS OF FLOWERS. 



Since the publication of Darwin's work but little has 

 been done until quite recently (3). Vochting has experi- 

 mented upon the effects of light upon the production of 

 cleistogamic flowers. In species which in nature produce 

 both these and ordinary flowers, a reduction of light to a 

 sufficient extent causes the production of cleistogamic 

 flowers only, and in all cases reduction of light tends to 

 hinder, or even prevent altogether, the full development of 

 the flower. Other workers have shown that probably low 

 temperature, poor soil, etc., may also act as factors in the 

 production of cleistogamy. It would thus appear that 

 cleistogamy is a phenomenon largely dependent upon ex- 

 ternal causes, although in many plants it is sufficiently fixed 

 and well-marked to be termed an adaptation. 



Vochting's experiments also bring out the interesting 

 point that reduced light causes increased vegetative multi- 

 plication, which is in accord with the actual facts observed 

 in northern regions, etc. 



Gynodicecism, with its allied phenomena, has also re- 

 ceived considerable attention. A species is said to be 

 gynodicecious when there occur two separate forms, one 

 bearing female the other hermaphrodite flowers. If both 

 kinds of flowers appear on one plant we speak of gyno- 

 moncecism. If one form bear male, one hermaphrodite 

 flowers, the species is androdicecious. The great number of 



