PINUS. LEAF. 149 



If these sections be treated with dilute potash, and 

 mounted in glycerine, their structure may be more 

 easily made out. 



C. In sections passing through the phloem will be 

 seen 



i. The Medullary rays as before, but their form is 

 more convex: all the tissues between the medullary 

 rays are derived from cambium-cells of the form above 

 observed. These are 



ii. Sieve-tubes, which retain the form of the cam- 

 bium-cells : the cellulose walls seen in surface view are 

 smooth : those cut longitudinally appear of wavy out- 

 line (sieves). The structure of the latter is well seen 

 after treatment with Schulze's solution for twenty-four 

 hours. Contents transparent protoplasm. 



iii. Bast-parenchyma, derived from cambium-cells 

 by their division by transverse walls. 



iv. Some few cells, especially towards the periphery, 

 containing crystals which give the reactions of 

 calcium oxalate. 



THE LEAF, 



Cut transverse sections of a foliage leaf of Pinus 

 syhestris, taken from a stem of the current year. It 

 may be found convenient to embed in paraffin, or 

 to hold the leaf between pieces of pith, or carrot. 

 Mount as before, and examine with a low power. Note 

 the form of the section ; the flat side is the upper, the 

 convex side the lower. Observe successively the 

 following tissues : 



A. A single layer of Epidermal cells with very 

 thick walls. 



B. A narrow band of thick-walled Hypoderma. 



