INTRODUCTION TO THE PROBLEM 



circumstances in the Flowering Plants and is not uncommon in the Thallophyta. In 

 other groups, if fertilization is prevented, the egg itself invariably dies, but other parts 

 of the gametophyte may, under suitable conditions, proliferate directly into sporophytic 

 tissue. Sometimes, through a genetical aberration, such proliferation may become the 

 rule, and one or both of the normal sex organs may become eliminated from the life 

 history in consequence; several examples of this in ferns will be examined cytologically 

 below. In other cases a genetically normal prothallus can, by prolonged prevention of 

 fertilization through the withholding of free water, be induced to develop apogamously 

 as an exceptional circumstance; some examples of this will also be examined. In both 



a ~ b 



Fig. 8. Some examples of apospory in ferns, a. Apospory induced by malnutrition in an otherwdse 

 normal young plant of Osmunda regalis L. var. cristata Moore; the petiole of a juvenile leaf, shown at 

 the base of the specimen, after forking twice has grown out into a forked prothallus covered with 

 rhizoids. From a photograph supplied by Prof. W. H. Lang, x 8 approximately, b. Apospory 

 in Athyrium Filix-femina (L.) Roth var. clarissima ]one.s. Portions of an adult frond laid on soil and 

 kept moist for several weeks have proliferated into prothalli, some of which are already bearing 

 apogamous young plants. Natural size. 



cases the result is the same, a sporophyte with the gametophytic chromosome number 

 is produced. 



'Apospory' is the aberration on the part of a sporophyte by which proliferation of 

 gametophytic tissue takes place from it without the intervention of normally con- 

 structed spores. As in the case of apogamy, it is sometimes possible to induce apospory 

 in genetically normal plants by appropriate experimental treatment. Cutting off the 

 leaves of juvenile plants and laying them on soil is a well-known method which, if 

 successful, will result in the proliferation of prothalli from the tips of the leaf or from its 

 cut surface. In other cases, owing to genetical peculiarity, the same process can be 

 carried out even with the large leaves of fully mature plants ; examples of this are 

 certain well-known horticultural 'varieties' such as 'Lastrea pseudo-mas' var. cristata- 



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