280 BOTANY part i 



These formative processes of growth in the ovules, ovary, and ultiniatelj' also in 

 other parts of the flower, are to he regarded as phenomena of correlation. When 

 fertilisation does not take place, all those changes which lead to the development of 

 a ripe fruit from the flower do not nsnally occur. Instead another correlative 

 influence arises which leads to the casting off of the now useless organ as a whole. 

 Some few plants, especially such as have been long cultivated, are to some extent 

 an exception to this. In nearl}' all varieties of the Banana, in the seedless 

 Orange, and in the sultana raisin, no embryo is formed, but in spite of this the fruits 

 develop. The stimulus to this development can proceed either from the mere 

 pollination of the stigma or from the fertilisation of the ovules, which then sooner 

 or later cease to develop without arresting the development of the fruit. In some 

 cases, however, " barren " fruits develop wholly without the stimulus of pollination 

 (parthenocarpic fruits of the Fig, Cucumber, and certain species of Apple and 

 Pear ; also tlie seeds of Cycads) (**'*). 



Influences which aff'ect parts at a distance also proceed from the pollen-grains and 

 pollen-tubes on the stigma. Thus after the stigma of an orchid is pollinated the 

 stigma and the gynostemium swell, and the perianth is promptly arrested in its 

 growth and withers. As Fittikg showed, this influence jjroceeds from soluble 

 organic substances which withstand heating, and can be readily separated from the 

 mass of ungerminated pollen. 



Whether a simple spore or a complex embryo is tlie result of 

 fertilisation it is always distinguished from the cells which gave 

 rise to it by exhibiting nuclei which contain the diploid number of 

 chromosomes (p. 94). On this account a reduction division which 

 restores the normal number of chromosomes is sooner or later the 

 necessary sequel to fertilisation. 



C. The Conditions of Eeproduction (^^) 



It has been already pointed out that the most simple form of 

 reproduction, such as the division into two of an algal cell or the 

 multiplication of a branched plant by dying off of the older portions 

 connecting the bt^anches, is nothing more than a process of growth. 

 The formation of complicated reproductive organs on the other hand is 

 a process which is fundamentally different from purely vegetative 

 growth. Thus the question arises, under what conditions does 

 vesretative growth and under what conditions the formation of 

 reproductive organs respectively take place? Since these problems 

 have as yet been relatively little studied, it is not easy to give a 

 general answer to this question. We must, therefore, confine our- 

 selves to making clear some of the essential facts by means of suitable 

 examples. In other words, we must show that the regular succession 

 of stages met with in nature, and constituting the characteristic 

 course of development of a species concluding with reproduction, 

 can be deliberately modified. The typical development is only one 

 of many possibilities. 



The fungi belonging to the genus Saprolegnia have a non-septate branched 

 mycelium without chlorophyll. They occur commonly in nature on dead insects 



