i yo DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



a four-ribbed shoot, but two of the ribs are quite rudimentary and have 

 the form of tufts or spines over the interval between the cotyledons, so 

 that casual observation might lead one to suppose that the seedling had 

 from the first two ribs ; this is what happens in Phyllocactus Phyllanthus. 

 The same appearances are presented, as I have shown, in species of 

 Epiphyllum and Rhipsalis with flat shoot-axes. In those Cacteae also 

 which do not subsequently flatten their shoot-axes ' Cereus-like ' shoots 

 appear in germination, and this is of the greatest interest from the 

 phylogenetic point of view. Rhipsalis Cassytha and Rh. paradoxa are two 

 species growing upon trees which in the adult state are as unlike as 

 possible. The former has long thin spineless shoots, the latter has limited 

 alternately three-ribbed shoots. The seedlings of both species are alike 

 in their chief features, apart from mere points of size, and exhibit the 

 ribbed spinose shoots which we have called ' Cereus-like' l . 



It would take me too far to speak of other examples of the Cacteae ; 

 many of these will be found in my ' Pflanzenbiologische Schilderungen' 

 to which I have referred. The instructive exhibition by the seedling plants 

 of the Cacteae of the development of different forms of the vegetative body 

 out of a common basal form may perhaps be connected with the fact 

 that in this group we have, as it appears, a phylogenetically recent family 

 in which a sharp separation of the genera has scarcely yet taken place. 



SUMMARY. 



In the development of plants from the germ, be this spore or seed, 

 there appear frequently relationships of configuration which are different 

 from those exhibited by the adult plant, and this is chiefly the case when 

 the seedling is adapted to other conditions than those which surround 

 the subsequent stages of development. The configuration of the primary 

 stages may be different within one genus and may vary also in one species. 



In one series of cases, as is shown by comparison with allied forms, 

 these must be considered as original forms relatively to the adult ones, 

 and as becoming subsequently adapted to changed relationships. Thus 

 the seedling plants of species of elm, beech, and hornbeam, constructed 

 out of sympodially developed dorsiventral shoot-systems, are orthotropous 

 and radial, as is the case in allied forms throughout their whole life. 

 The seedlings of xerophilous plants do not exhibit the characters which 

 are associated with a xerophilous mode of life ; and the same may be 

 said of plants climbing by tendrils and of other forms. 



On the other hand, the juvenile states in many instances are un- 

 doubtedly those which have been changed by adaptation, as in Hedera, 

 Marcgravia, Salvinia, and others, and frequently the seedling exhibits in 



See the figure of the habit in my ' Pflanzenbiologische Schilderungen,' i. 



