194 THE BIOLOGY OF FLOWERING PLANTS 



The occurrence of chlorophyll in the stem is widespread 

 in desert plants with small and fugacious leaves, or with 

 none. Its distribution has been studied by Cannon (1908) 

 for desert plants of Arizona. Its most important position 

 is in a band in the parenchyma of the cortex, which may 

 persist through the life of the shoot or may be cut oif by 

 cork formation. It sometimes occurs in the epiderm, and 

 sometimes even in the parenchyma of the wood. It pene- 

 trates to a depth much greater than is customary in leaves. 

 The smaller the leaf surface the more important the 

 chlorenchyma of the stem becomes. 



As assimilating organs the stems suffer from their small 

 surface, even though this be expanded by ridges or wings, 

 but as regards reduction of transpiration this is important. 

 They avoid direct insolation even more effectively than 

 leaves in the profile position, as may be seen admirably in 

 switch plants like the broom. At the same time they are 

 illuminated from all sides, and this is advantageous for 

 assimilation. 



Phyllodes and Cladodes.— A further stage in the trans- 

 ference of the leaf functions occurs when the stem or the 

 leaf-stalk assumes leaf form and structure. Leaf-like stems 

 are termed cladodes, leaf-like petioles phyllodes. Of native 

 plants the butcher's broom, Ruscus aculeatus, has cladodes 

 (Fig. 21). The leathery shoots are quite like the sharp- 

 pointed upper leaves of the holly ; the position of the flowers 

 in the middle of the upper surface indicates their morpho- 

 logical nature, as does the fact that they spring from the 

 axils of scale leaves. The species of asparagus frequently 

 grown in greenhouses as " ferns " for decorative purposes, 

 seem to have needle-like leaves, but examination shows 

 that these are shoots coming from the axils of scale leaves. 

 Phyllodes are found in many leguminous plants. Their 

 nature may be ascertained by comparison with related 

 species. In many cases, too, the primary leaves have the 

 normal form and give place, often through a series of 

 transitions, to the mature phyllodes. This is well seen in 

 Acacia nereifolia (Fig. 22). We may here mention the theory. 



