PERIODICITY IN DEVELOPMENT. I 



349 



in the normal course of development have formed bud-scales, develop into foliage- 

 leaves. The lowest of these are stimulated to become foliage-leaves although they 

 have already advanced to a greater or less extent towards the scale-leaf forma- 

 tion, and all intermediate conditions occur between foliage and scale-leaves. 

 Further up the shoot quite normal foliage leaves are developed. As a result 

 of this experiment we may conclude that as a general rule the scale-leaves 

 develop under the influence of the foliage-leaves, that a certain number of foliage- 

 leaves have their activity diverted to the formation of scale-leaves. The same 

 is true of the reproductive-leaves. The simplest form occurs in certain ferns, e. g. 

 Blechnum and Stnithiopteris. The reproductive-leaves are in this case foliage- 

 leaves which, owing to the formation of reproductive organs (sporangia), have 

 taken on another function and ap- 

 pearance. While in many ferns 

 these sporangia arise on ordinary 

 foliage-leaves, which retain their 

 assimilatory function,in those men- 

 tioned above a division of labour 

 takes place, some leaves devoting 

 themselves only to the production 

 of sporangia and having their as- 

 similatory parenchyma reduced. 

 GoEBEL has shown that if the 

 foliage-leaves of Stnithiopteris be 

 cut off the reproductive-leaves 

 which appear later (sporophylla) 

 may change into ordinary leaves 

 bearing no sporangia. 



The sporophylla of the ferns 

 are represented in the higher plant 

 by the stamens and carpels, which 

 are distinguished also by bearing 

 sporangia. Further, we can deter- 

 mine here also tracts as forming 

 transitions between foliage and the 

 essential floral organs. Numerous 

 observations and experiments 

 have been made which show that 

 reproductive-leaves may become 

 altered into foliage-leaves owing 

 to unknown causes, or, in indi- 

 vidual cases, owing to the influence 

 of insects, not as yet investigated 

 (Peyritsch, 1882), even although 

 it is quite obvious, both by their 

 position and structure, that they are intended for floral organs. We must there- 

 fore conclude that in the normal plant development there are internal but by no 

 means unalterable causes which induce a periodic alternation or 'metamor- 

 phosis ' of leaves. 



These factors are, in detail, quite unknown to us. One hypothesis by 

 way of explanation has been advanced by Sachs (1880-1). According to this 

 author there are in the leaf, in addition to the products of assimilation pre- 

 viously spoken of, specific constructive materials which pass away from the 

 leaf in all directions and which collect in certain quantities where a definite 

 organ has to be developed. Thus the flower would be formed out of flower- 

 building material, roots out of root-building substance and so on. 



Fig. 104. Acer platanoides. /, foliage leaf, reduced. //, 

 scale-leaf. ///, young .scale-leaf (magn.). /F, young foliage- 

 leaf (magn. and schem.). G, leaf-base ; .S", stalk ; 2,, blade. 

 From GoEBEL, Organographie. 



