216 Cytology and Genetics of the Germs Fuchsia 



The pollen grains of Fuchsias follow the general Onagraceous type. 

 Their membrane consists of an exospore, a mesospore, and an endospore, 

 and is furnished with two or more interstitial bodies. The Fuchsias 

 have been divided into two groups according to whether their pollen 

 grains possess two or three interstitial bodies. Fuchsia glohosa is a 

 typical example of the group which f»ossesses three interstitial bodies 

 upon each pollen grain, whilst Fuchsia procunibens has pollen grains with 

 only two interstitial bodies. 



Whilst these numbers arc characteristic of the majority of fertile 

 pollen grains in each case, yet they are not constant in those instances 

 in which irregular pollen development takes place. In these cases the 

 number of interstitial bodies appears to vary with the size of the pollen 

 grain, and in some of the smaller grains only a single interstitial body is 

 formed. Fig. 27 shows such a small, supernumerary pollen grain with 

 only a single interstitial body. Although not shown in this figure, these 

 small pollen grains, apart from the number of interstitial bodies they 

 possess, have membranes which are identical in structure and chemical 

 composition with those of the larger grains. 



We have already seen that the distribution of the chromosomes to 

 the pollen grains is an irregular one, and that the small grains receive 

 only a small proportion of their normal number of chromosomes. 



Notwithstanding this all the pollen grains develop walls which are 

 characteristic of the genus both in structure and chemical composition. 

 These facts have an interesting bearing upon the theory of the localisa- 

 tion of generic and specific characters in particular chromosomes, since 

 the chromosomes which any particular pollen grain receives is perfectly 

 haphazard in the present instance. The explanation is probably similar 

 to the one which has been suggested in the case of the development of 

 certain animal eggs in which the cytoplasm becomes set to a definite 

 line of development at an early stage. We may probably assume that 

 the cytoplasm of the pollen mother-cell has already been set, through 

 the influence of the still undivided nucleus, to a definite course of 

 development, and that it already has the mechanism implanted in it for 

 the formation of pollen membranes of a definite structural and chemical 

 constitution. It is a matter of secondary importance for carrying out the 

 work which is allotted to it at an early stage how the chromosomes 

 subsequently become distributed at the meiotic division, or how it 

 becomes divided up at the conclusion of that division. 



From the foregoing account it will be seen that the development of 

 abnormal numbers of pollen grains from the mother-cells depends upon 



