PERIODICITY IN DEVELOPMENT. II 365 



cuttings from younger plants produceid vegetative shoots. Such facts as these 

 Sachs attempted to explain by his hypothesis of specific constructive materials 

 (p. 349). He assumed that special flower-forming materials occurred in the 

 plant about to flower, and hence the rapid formation of flowers on cuttings 

 taken from it. Quite apart from the general criticisms we have brought 

 against Sachs's hypothesis, there are by no means unimportant objections to 

 this special apphcation of it. Winkler (1903) found the shoots which arose 

 from the leaf cuttings of Torenia very liable to form flowers, and this tendency 

 was held in common by all leaves, and even by cotyledons. On the assumption 

 that the materials for the manufacture of flowers postulated by Sachs must 

 be especially abundant in the flowers themselves I prepared flower-cuttings of 

 many species of Achimenes ; though these produced buds in quantities none 

 of them formed flowers. The conditions necessary for the production of 

 flowers from leaf-cuttings must obviously be studied on broader lines ; still 

 these facts may be adduced as interesting evidence that internal factors play 

 a very important part in the formation of flowers. 



In general, however, our information as to the factors which are essential to 

 the formation of flowers is very imperfect ; the facts which have been brought 

 forward prove, however, that the periodicity in blooming usually observed is not 

 fixed once and for all but may be experimentally influenced. On the other hand, 

 the alternation of generations in the Phanerogams is in so far fixed that the 

 formation of spores is followed necessarily by the formation of sexual organs. 



Plants which, in addition to seeds, also form accessory organs of reproduc- 

 tion, such as tubers, bulbs, &c., require special mention, since it can hardly be 

 doubted that the conditions which are necessary for the formation of flowers 

 or seeds must differ from those which govern the formation of propagative 

 buds. On this point, however, little is known ; so far only correlations 

 between them have been established. For example, it not infrequently 

 happens that flowers and propagative-buds are produced concurrently, but that 

 the setting of seed takes place only if the formation of propagative-buds is 

 prevented (compare Lindemuth, 1896). The cases where propagative-buds 

 arise in place of flowers are of especial interest, e.g. Poa hulbosa and many others, 

 for in these cases we do know that special but as- yet unknown external factors 

 accelerate or retard the formation of such flowers or buds (Hunger, 1887). 

 [Looking at the general results we have arrived at in this lecture we must admit 

 that the normal path of development, both in higher and in lower plants, is only a 

 special type of many developmental possibilities. That this normal sequence and 

 not some other is usually met with must be attributed to a normal sequence 

 in the external factors. It is to Klebs especially that we owe this valuable 

 conception with regard to the factors governing developmental processes.] 



A few words may be said in conclusion as to the biological significance of 

 reproduction. In the lower organisms this is especially obvious, for the impor- 

 tance to the organism of swarmspores and of conidia and resting spores capable 

 of distributing the plant in the air requires no explanation. The value of the 

 formation of fruit and seed in the higher plants is equally intelligible. It is 

 immaterial whether the organism forms these organs once and then dies, or 

 whether it continues forming them for several years, their production tends 

 not only to the persistence but also to the increase of the species. Every 

 organism does its best to distribute itself and to seek pastures new ; this 

 characteristic is so marked in living things that one gets the impression that 

 their entire existence is spent in producing their successors. We use the 

 term * impression ' purposely, because attempts to answer such questions tend 

 to teleology ; they lead us at once to the ultimate problem of all, viz. what! is 

 the final aim and object of the existence of the organism ? and to that question 

 no answer can be given. 



Although we may understand in general the biological reasons for the 



