314 BOTANY PART i 



and from his own experiments that nitrogenous food ,led to a diminution arid 

 phosphorus to an increase in the development of flowers. 



If after the formation of flowers has commenced the conditions for vegetative 

 growth are re-established, a shoot already predisposed to flower- formation may 

 again become vegetative. Thus when Mimulus Tilingii is brought into conditions 

 of poor illumination the flower-buds already laid down remain undeveloped and 

 resting buds in the axils of bracts develop into leafy shoots. The whole appearance 

 of the plant is thus greatly altered. 



Determination of Sex ( 75 ). Most flowers are hermaphrodite and produce both 

 male and female sexual cells. In other cases unisexual flowers are produced either 

 only or in addition to the hermaphrodite flowers. The fact that the female 

 flowers are developed as a rule in different situations from the male flowers indicates 

 that each of the two forms has its special conditions of development ; what these 

 conditions are is, however, unknown. 



The determination of sex thus becomes a problem of developmental physiology 

 especially when dioecious plants (i.e. those which have male and female individuals) 

 are concerned. Generally the two forms occur in about equal proportions in 

 nature, and this relation cannot be altered experimentally. It is also not possible 

 by means of external influences of any sort to cause a seed to develop into the one 

 or other sexual form. The sex is already determined in the seed as a result of 

 internal causes which will not be further considered here ; these have already 

 acted in the sexual cells or at fertilisation. 



Fertilisation. The product of fusion of the egg and sperm-cell 

 surrounds itself, as a rule, with a cell wall. In the lower plants an 

 oospore or zygospore is thus formed which germinates, usually after first 

 undergoing a period of rest. In the higher plants growth and cell 

 division take place forthwith ; an embryo is produced which in Bryo- 

 phyta and Pteridophyta continues its further development, while in 

 the Phanerogams it soon enters on a period of rest. Before this, how- 

 ever, a number of stimuli have proceeded from the development of 

 the embryo ; these are especially complex in the Angiosperms. The 

 ovule in which the embryo is enclosed commences to grow ; it enlarges 

 and assumes a characteristic structure. It has developed into the seed, 

 and this as a rule is liberated from the ovary and, after a resting 

 period, germinates. The ovary also grows actively after fertilisation 

 and develops into the fruit. The variety in fruits cannot be entered 

 upon in this place. (Cf. Special Part.) 



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

 other parts of the flower, are to be 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 usually 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 nearly 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 



