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A TEXTBOOK OF THEORETICAL BOTANY 



lanceolate, alternate and petiolate. The juvenile leaves, on the other hand, 

 are variously shaped ; frequently obtuse, sessile, or even connate in pairs, 





Fig. 951. — Eucalyptus sp. Young trees showing juvenile 

 foliage below and mature foliage above. 



and standing out stiffly from the branches (Fig. 952). The difference is so 

 marked that one could easily believe that the mature shoots belong to some 

 other species grafted on the juvenile tree. Almost equally marked, though 

 less spectacular changes of leaf shape may be seen in many herbaceous plants 

 such as Campanula rotundifolia (the Harebell), Maha moschata (Fig. 953), 

 and in several species of Scahiosa, in which the juvenile leaves are entire and 

 the mature leaves are deeply divided. In many plants, indeed perhaps in 

 most, there is some degree of change in leaf form, from node to node. The 

 rate of this change, which is genetically controlled, has been used as an 

 index of the physiological maturity of the shoot. 



Certain species of Acacia develop only phyllodes {i.e., flattened petioles) 

 in the mature state but produce the typical multipinnate leaves of the genus 

 on their seedlings. The change over takes place through intermediate stages, 

 marked by progressive broadening of the petioles and the reduction and final 



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