SPINES: SWITCH PLANTS 193 



and old worlds are named by Schimper, thorn forest and 

 thorn scrub. 



It can be shown experimentally that spine formation is 

 favoured by high light intensity and low humidity ; whin 

 seedlings growing in a saturated atmosphere form broader, 

 soft leaves, and long, " drawn " shoots. Except in those 

 cases where, as in the whin, the spine is also a reduced 

 leaf, the relation to these conditions is not clear. Spines 

 may be a protection against grazing animals, but this does 

 not help us to understand the causal relation of drought to 

 their formation. At this stage of leaf reduction we meet 

 with cases of stems assuming important light-absorbing 

 functions. Most young stems, before cork formation, have 

 chlorenchyma (tissue with chlorophyll) and can thus assimi- 

 late. In mesophytic plants the stem surface is always 

 negligible in proportion to the leaf surface, but in the whin 

 it is greater, and the stem is the more important assimilating 

 organ. 



The climax of this tendency is reached when the leaf 

 disappears altogether and the work of assimilation is entirely 

 taken over by the branches. The broom is a case in 

 which this is approached. It bears small trifoliate leaves, 

 which in dry stations may fall early. They never persist 

 through the winter. The characteristic switch shoots, 

 bright green and prominently ribbed, are the chief assimi- 

 lating organs. In Spartiiim junceum, of the Mediter- 

 ranean region, the leaves are still fewer and more fugacious ; 

 the rush-like stems are the assimilating organs. In Cas- 

 suarina the leaves are fused throughout their length with 

 the axis, only the extreme tip being free ; the appearance 

 is that of a typical switch plant. Ephedra is an example 

 from the Gymnosperms ; here the opposite leaves are 

 reduced to mere scales. That the substitution of the stem 

 for the leaf as an assimilating organ is effective in reducing 

 transpiration has been shown by Bergen (1903) for Spartiuni 

 junceum growing at Naples. In the season when the plant 

 bears leaves the transpiration from these is 2*6 times that 

 from the stem if equal areas are compared. 



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