PERIODICITY IN DEVELOPMENT. II 



363 



mine whether such a plant has been in existence for years or for centuries. 

 Many of these rhizomic plants have lost their power of forming seed, and their 

 continued existence depends entirely on the growth of the rhizome. Acorus 

 calamus is a case in point. This plant has been known as a native of Europe 

 for at least 400 years, and there can be no doubt that during the whole of that 

 time it maintained itself in existence and increased by vegetative methods only. 

 [KLEBS has given some admirable examples (1903-4) of continued vegetative 

 growth experimentally induced. He was successful in causing Glechoma hede- 

 racea to remain growing in the form of creeping shoots for years.] 



The examples quoted make it clear that the formation of flowers and 

 reproduction by means of sporangia is not generally necessary for the tribal 

 continuance of the plant, and that the plant, whether it be a flowering plant or a 

 Saprolegnia, can continue 

 in existence by vegeta- 

 tive means only. Inquiry 

 into the factors which 

 bring about flower forma- 

 tion becomes thus all the 

 more necessary. 



When we examine 

 flowering plants under 

 natural conditions we find 

 that the flowers begin to 

 appear when the plant 

 is ' ripe for flowering ' 

 just as sexual - cells ap- 

 pear in the animal when 

 it reaches a certain age. 

 But although flower for- 

 mation, generally speak- 

 ing, takes place at a cer- 

 tain age, which differs 

 with each species, still, 

 exceptions are known, as, 

 for example, the oak, 

 which normally is ' ripe ' 

 in its sixtieth to eightieth 

 year, but which occasion- 

 ally produces flowers in 

 its first year and then 

 dies down (WIESNER, 

 1902, p. 75. Compare Fig. 93). YACHTING (1893) has demonstrated in a very 

 striking manner that flower formation is influenced by external factors. The 

 plant he experimented upon was Mimulus tilingii, which left off forming 

 flowers in light of low intensity but still sufficiently strong to permit of vigorous 

 vegetative growth. The effect of the diminution in light makes itself felt even 

 when the inflorescence has been formed ; it arrests the further development of 

 the flower already laid down in the axils of the bracts (Fig. 112), and induces 

 a general evolution of resting axillary buds into vegetative shoots. 



This illustration of the relationship between foliage and flower formation 

 recalls vividly that between growth and reproduction in Fungi. We have 

 established in relation to these forms that the limits of the general vital condi- 

 tions for reproduction are more restricted, and VOCHTING has shown in Mimulus 

 that the minimum of illumination for flower formation lies higher than for that 

 of vegetative organs. We are, however, as yet far from having reached a solu- 



Fig. 112. Mimulus tilingii^ after VOCHTING (1893, PI. n). /, Top of a 

 normal plant. 77, Top of a plant cultivated in diminished light after 

 development of inflorescence. The inflorescences (a, a) are stunted; 

 vegetative shoots have developed everywhere abundantly. 



