1480 A TEXTBOOK OF THEORETICAL BOTANY 



Yet although polyploidy is a concomitant of apomixis it does not 

 necessarily determine it, since many polyploids are sexually fertile. It 

 simply provides the genotypical background on which apomixis may arise. 

 Apomixis signifies a departure from sexual crossing in the direction of 

 vegetative means of dispersal and any hindrance to meiosis, climatic or 

 otherwise, may influence the balance in this direction, turning sexual 

 diploids into polyploids which are at least facultatively apomictic. The 

 polyploid oosphere escapes fertilization by rapid division. The haploid 

 oosphere has a relatively long resting period, which allows a necessary 

 interval of time for the completion of the fertilization processes, but in 

 parthenogenetic apomicts division of the oosphere may begin without any 

 resting stage and developing embryos may be already present at anthesis. 



The subjects of apomixis, polyploidy and vegetative propagation have 

 important bearings on plant distribution and we shall revert to the subject 

 in the next volume. 



Fertilization has many consequences in the flower apart from the pro- 

 duction of an embryo. Ovules grow into seeds, ovaries grow into fruits 

 and vegetative parts are often involved in this development as means of 

 protection or dispersal. Most of these phenomena will be touched upon in 

 the next chapter. There is one phenomenon which is somewhat distinct 

 from these, however, and that is the withering of the perianth. This is 

 sometimes accelerated by pollination (Orchidaceae) and is sometimes 

 independent of it {e.g., Cichoriuni). Though the causation of flower withering 

 has for long excited attention, it cannot be said that any clear explanation 

 is yet forthcoming. It has been established that the fate of the perianth is 

 decided before anthesis, except in flowers like the Orchids where the onset 

 of moribundity dates from pollination. It is not due to starvation, since 

 part of the carbohydrate content of the perianth usually remains unused. 

 Low concentrations of cyanide, anaerobic conditions and low temperatures 

 increase the lifetime of the perianth if applied immediately the flower opens. 

 At later times their efl'ect is quantitatively lessened. Oxidative processes, 

 involving protein breakdown, are apparently inaugurated before anthesis 

 and can be retarded but not reversed. 



There is a marked export of Nitrogen, of carbohydrates and of minerals, 

 including Phosphorus, Potassium and Magnesium, before withering. The 

 CN-sensitive oxidation removes something that the flower has only in a 

 limited amount and which is non-importable. The osmotic potential of 

 the cells decreases markedly and at the onset of withering the petal cells are 

 plasmolysed. 



The protein breakdown is only the first link in the chain and it may be 

 that there is a poisoning efl'ect by some protein derivative shortly before 

 final withering. On the other hand some flowers {e.g., Rosa) abstrict their 

 petals by the formation of an abscission layer before any external change 

 is apparent. 



