58 PLANT PHYSIOLOGY AND ECOLOGY 



shown by the two surfaces. The paHsade tissue consists of rec- 

 tangular cells elongated at right angles to the surface, and packed 

 so closely in rows that the intercellular air-passages are scarcely 

 visible. The sponge cells, though usually irregular in outline, 

 are more or less elongated in the direction of the surface. They 

 are loosely connected, and their irregular forms permit the pres- 

 ence of numerous large air-spaces. The relative amounts of 

 palisade and sponge tissue in the leaf are determined by water 

 and light, and a further discussion of this matter will be found 

 under Adaptation. 



74. The reduced bundles. The repeated division of the fibro- 

 vascular bundles that enter the leaf is adapted to meet the in- 

 creased need for support caused by its form. At the same time 

 it serves to carry water to all parts of the leaf, which is the organ 

 that needs water most. A close examination of a reduced bundle 

 in section shows its intimate relation to the cells of the chlorenchym. 

 The supportive elements of the bundle are greatly reduced. In 

 many instances spiral vessels alone remain, thereby greatly facili- 

 tating the passage of water from the bundles into the cells of the 

 chlorenchym. 



Experiment 15, Structure of a leaf. Cut thin cross-sections of a 

 sunflower leaf, preferably by the paraffin method. Under the high power 

 draw in full detail a segment across the leaf at a point where a small 

 bundle occurs. Pay especial attention to the air-spaces and the number 

 and position of chloroplasts in both palisade and sponge tissue. 



Strip a bit of epidermis from each surface. Count the stomata in 

 two or three fields, and compute the number for a square centimeter of 

 each surface. Estimate the surface of a leaf by first weighing the 

 entire blade, and then two or three pieces of it a centimeter square 

 taken from different portions, and dividing the first weight by the 

 average weight of a piece. The quotient is the surface in square centi- 

 meters. Estimate the number of stomata on each surface of the entire 

 leaf. 



75. Diffusion in the leaf. The water absorbed by the roots is 

 carried throughout the leaf by the reduced bundles. The water 

 passes from the vessels into the cells of the chlorenchym by rea- 

 son of the osmotic pressure of the latter reinforced by the attrac- 

 tion of the protoplasmic membrane. The latter effect is due to 

 the fact that the vessel has no protoplasm to counteract this pull. 

 The water passes from cell to cell by diffusion, exactly as in the 



