128 THE LEAF 



is nearest tlie abaxial surface of the leaf and at most is formed 

 of a few small sieve-tubes and companion cells; in verj- small 

 bundles, the phloem may be represented by parenchyma cells. 

 The structure of a large vascular bundle in the leaf is quite simi- 

 lar to the anatomy of a stem bundle (ef. Exercise XII). Notice 

 particularly the clear distinction between the sieve-tubes and 

 companion cells of the phloem and tlie presence of an air lacuna 

 at the adaxial edge of the xylem. Fibers are present on both 

 edges of the bundle and may even partially surround it ; notice 

 the thick-walled character of the epidermal cells directly adjacent 

 to the fibers. This arrangement of fillers on either side of the 

 vascular bundle is quite characteristic of grass leaves and is re- 

 garded as a very efficient "plan" for securing mechanical 

 strength in the leaf. 



3. The bud scede. In general, bud scales are distinguished 

 anatomically from foliage leaves by (1) their greatly reduced 

 vascular system, which may consist of a series of parallel or 

 dichotomizing veins, and (2) by a simple type of undifferentiated 

 mesophyll. The outer bud scales of certain trees may produce a 

 well-developed peridenn beneath the outer epidermis (e.g., 

 Aesculus). Mechanical cells, such as fd)crs and sclereides, are 

 often prominent for example in Camellia, Fagus, Querrus, and 

 Populus. (For further details, consult Foster, 1928, i)p. 137- 

 146.) Study prepared trans-sections of the bud scales of several 

 of the forms listed above. 



4. The leaves of giimnO'Sperms. Examine stained trans-sec- 

 1 ions of the needles of riinis noting especially the sunken stomata, 

 tile invaginatcd wails of the mesophyll cells nnd the vascular 

 bundle (in some s])ecies two bundles are present) embedded in 

 transfusion tissue. (Tf. Fames and MacDaniels, pp. 307-308.) 

 Foi- comparison, study trans-sections of the fan-shaped lamina of 

 the foliage leaf of Ginkgo bdoba. (A discussi(m of the histology 

 of this leaf is given by (liamberlain, 1935, pp. 191-193, and Fig. 

 210, p. 190.) 



III. Suggested Drawings and Notes. — 



1. Prepare a diagrannnatic drawing of the trans-section of 

 the lilac leaf, indicatinii- the position and extent of all important 



