154 TYrE STUDIES 



2. The layer of rigid tissue (sclerencliyma) beneath the epidermis. 



3. The broad layer of green tissue, mesophyll, whose cells have in- 

 folded walls. 



4. Resin ducts in the green tissue. 



5. A central area containing in most species of pine two fibro-vascular 

 bundles and bounded by a bundle sheath. The fibro-vascular bundles 

 lie in a special region of so-called transfusion tissue, composed in 

 part of empty pitted cells and in part of cells containing protoplasm. 

 Each bundle consists of a region of wood (xylem) and bast (phloem) 

 and contains rather ill-defined medullary rays. 



6. The sections of the stomata show epidermal cells on either side of 

 the groove and below them two small guard cells containing chloro- 

 phyll. Each stoma opens into a chamber within the green tissue. 



Show the position of the tissues of the needle in an outline drawing 

 and then treat the details in separate figures. 



E. The stmiiinate cones. These are short-lived structures de- 

 veloped in the late spring with the appearance of the new 

 growth from the terminal buds. They are variously clustered 

 in different species of pine. Draw a habit sketch of a group, 

 showing the arrangement of the cones on the new growth, 

 with its developing needles. 



1. Observe the position of the staminate cone in the axil of 

 a pointed scale leaf. Draw in side view to show the some- 

 what spiral arrangement of the closely crowded stamens 

 (microsporophylls). 



2. Split the cone lengthwise and diagram the attachment of 

 the stamens along its axis. What is the morphology of 

 the staminate cone as indicated by its position on the stem 

 and the nature of the stamens (see F below) ? 



F. The stamen and pollen. Remove a stamen from a cone 

 which has not yet shed its pollen. 



1. Observe its form and structure, — a short stalk, broaden- 

 ing beyond, on the lower face of which are borne long pollen 

 sacs (microsporangia). The tip of the stamen is turned 

 upwards and fits over the pollen sacs of the stamen above. 

 Draw under a hand lens the stamen in end and side views 

 to illustrate these points. 



