THE PHYSIOLOGY OF PLANTS 



§ 22. How is the leaf developed? 



If we examine the extreme tip of the stem, we find that it 

 is composed of undifferentiated cells, fitted very closely together, 

 with their delicate cell-walls, and containing a large amount 

 of protoplasm. 



These isodiametrical cells form the primary meristem. But 

 very close behind the apex the tissues of the stalk begin 

 to become differentiated, and first of all the cells of the outer- 

 most layer elongate and begin to form a tabular row of cells, 

 which becomes the epidermis of the stem. Near the centre 

 of the axis a number of rows of cells become differentiated by 

 rapidly dividing by longitudinal walls, and forming therefore 

 strands of tissue consisting of very narrow cylindrical elements. 

 These strands, which go on dividing, are the rudiments of the 

 vascular bundles, which occur scattered in Monocotyledons, 

 but are arranged in a ring in Dicotyledons, where they will 

 afterwards form the wood cylinder. The remainder of the 

 primary meristem, which does not enter into the formation of 

 epidermis or vascular bundle, remains as a parenchymatous 

 ground-tissue, the central portion being called the pith, the 

 outer portion the cortex. 



In this first period of differentiation of tissues, it will be 

 noticed that at certain points groups of cells which belong to 

 the outer ground-tissue, close below the epidermis, begin to 

 divide. The newly-formed cells elongate at right angles to 

 the axis, and therefore cause small protuberances from the 

 young stem. 



The young epidermal cells which cover in these projections 

 also begin to multiply, and thus continue to form a protective 

 layer to the inner tissues. These begin to become differen- 

 tiated by the appearance of various strands of tissue, which 

 are the rudiments of vascular bundles. 



This complex of cells which bulges out from the axis is the 

 first stage of development of a leaf, which takes its origin 

 from certain divisions of the young cortex, and may therefore 

 rightly be looked upon as an expansion of cortical tissue. 



The young leaf very soon ceases to grow at its apex, and 



