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A TEXTBOOK OF THEORETICAL BOTANY 



thorn branches, but in this case the leaves are also reduced to spines and the 

 function of photosynthesis devolves on the branches including the thorns, 

 which are all green. Gorse seedlings raised in a damp atmosphere do not 

 develops thorn or spines, but produce leafy side shoots of reduced growth. 

 A similar change has occurred in the Apple {Pynis mahis) which is thorny 

 in the wild state, but not in cultivation, where the thorns are represented by 

 fruiting spurs. 



The above examples are all straight thorns, but thorns may also be curved 



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Fig. Q14. — Colletia cruciata. Flowering shoot 

 showing decussately arranged thorn-cladodes. 



in the form of a hook, as in the South African tree. Acacia detinens, from 

 which the unwary may find it almost impossible to escape. 



Intermediate structures between thorns and cladodes are known. Such 

 for example, are the remarkable flattened branches of Colletia cruciata 

 (Rhamnaceae) (Fig. 914), which are unlike other cladodes in being flattened 

 in the vertical direction. They, and the secondary cladodes which grow 

 from them, end in thorn-like points. 



3. Winged Stems. These provide another example of the stem taking 

 over the photosynthetic functions and something of the structure of leaves. 

 The wings may be mere ridges, as in the Broom (Cytisus) or narrow frills as in 

 Scrophidaria alata, but they are often so developed that they are many times 



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