BIOLOGY OF HIGHER SEED PLANTS 47 



(2) The pith. Note the irregularly lobed pith in some 

 kinds of wood. The irregularity of the pith is due 

 to the effect of branches and leaves on the form 

 of the pith. 



c. Draw a sector of the section, indicating by shading and 

 by circles the different layers of the bark and wood. 

 The sector should be three or four inches from epi- 

 dermis to pith and two inches wide. Draw each tissue 

 layer accurately as regards its relative width and 

 extent, but do not draw cells. The spring ducts may 

 be indicated by circles and the summer wood by 

 shading. 

 2. Cellular structure of the tissues. (Use prepared slides.) 



a. The cork (brown baric). Study the character of the 



cork cells and compare them with the cork cells 

 studied earlier in the course. Is the epidermis still 

 present or has it been sloughed off after the formation 

 of cork ? 



b. The cortex. Note its limits and the nature of its cells. 



Do its cells contain protoplasm and chloroplasts ? Are 

 there intercellular spaces ? 



c. The phloem. See Fig. 52 of the text. 



(1) Outer phloem layer. Study carefully the cell walls 



and the contents of the strengthening, or skeletal, 

 layer of the phloem (stained red with safranine). 

 Can you find cells with canal-like perforations of 

 the greatly thickened cell walls of these cells ? 



(2) Inner phloem layer. Note the nature of its cells and 



their gradual merging into the cambium (see 

 next topic). 



(3) Cambium. The cambium is composed of three or 



four cell layers adjoining the red stained wood 

 on its inner side and the phloem on its outer 



