CHAPTER 12 



INDUCED APOGAMY 



In contrast to the species discussed in the last two chapters, in all of which apogamy was 

 a permanent feature of the life history, fixed and determined by a genetical mechanism 

 carried by the nucleus, it has long been known that normal sexual prothalH in a number 

 of species can be induced to become apogamous as an exceptional and temporary con- 

 dition if normal fertilization is prevented. One method of doing this, first used by Lang 

 in 1898, is of the simplest. The prothalli are grown on soil in covered pots standing in 

 water. The glass cover, which may conveniently be half a Petri dish placed over the top 

 of the pot, prevents the access of free water to the prothalli from above while permitting 

 ample moisture for physiological purposes to reach them from below. Under these con- 

 ditions no drops of free liquid are formed upon the prothalli, and the sex organs, al- 

 though fully developed, cannot open. Young plants cannot therefore be formed by 

 sexual means, though they will readily appear if the lid is removed and surface-water 

 supplied. The unfertilized prothalli will continue to grow and may reach an unusually 

 large size before irregularities of form become apparent, but when these begin apoga- 

 mously developed sporophytes may result by methods which, morphologically, differ in 

 many details from the sequence of events accompanying obligate apogamy, although 

 the end-result is somewhat similar, namely, a vegetative bud with leaves, roots and 

 ultimately a stem which may establish itself in the soil and grow on as an independent 

 individual. Two rather diflferent types of morphology were described by Lang, both of 

 which have been found again by subsequent authors. In many instances a cylindrical 

 process covered at first with archegonia may grow out from the region of the central 

 cushion ; the centre of this process may later become vascular, while leaves, roots and a 

 stem may develop from the tip. In other cases isolated organs from sporangia to leaves 

 and roots could proliferate from one or both surfaces of the prothallus without the inter- 

 vention of any special organ, or could grow out from the apex or margins. The most 

 surprising instance of such isolated organs were two cases (in horticultural strains of 

 Dryopteris and Scolopendrium respectively) in which sporangia, unaccompanied by other 

 sporophytic organs except sometimes protective scales, were borne directly on a 



prothallus. 



A list of the species used by Lang (1898) in the course of two and a half years is given 

 below, together with such of his morphological notes as refer to the physical characters 

 of the apogamous developments. As will be seen, a cylindrical process precedes the 

 formation of apogamous buds in the great majority of cases. It is perhaps to be regretted 

 that such a large number of the strains used were horticultural varieties and not the 

 typical wild species; this may perhaps have been of importance in determining the high 

 incidence of apogamy. Marked differences evidently existed in the ease of induction of 

 sporophytes in different varieties of the same species (cf Scolopendrium, Polystichum 

 angulare and Athyrium niponicum), suggesting that predisposing causes of a genetical kind 



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