24 LEAF ARRANGEMENT 



openings are one of the most interesting features of the leaf. 

 When there is an adequate supply of water the elliptical guard 

 cells are drawn apart and give free exit to vapors and gases. 

 However in cases of drought when a continued loss of water 

 would prove harmful to the plant, or when an interchange of gases 

 is no longer required then the guard cells close and so offer such 

 a barrier against further loss of water that only the severest con- 

 ditions, such as prolonged heat and drought, can overcome. The 

 stomata in the majority of cases range in size from .0002 sq. mm. 

 to .0008 sq. mm. and in numbers from four to seven thousand to 

 a sq. cm. so that these microscopic openings furnish adequate space 

 for the interchange of gases and the transpiration of water. 



14. Noteworthy Features of Leaves. Now that some idea has 

 been gained of the nature and extent of the work performed by 

 the leaves we are prepared to comprehend the meaning of the 

 large extent of the leaf surface and of the arrangement, structure 

 and modification of leaves. The broad blades are devices for 

 gathering or absorbing as much of the sunlight as possible and 

 they increase the surface of the plant many hundreds of times. 

 Contrast the extent of surface of a tree in full foliage with the 

 bare branches of the winter. A mature maple develops annually 

 from one to two thousand yards of leaf surface. 



One of the most noteworthy features about the leaves is their 

 arrangement or " hang " on the branches. In some plants they 

 are arranged in two rows, in other instances three, four, five,, 

 eight or more rows. By fastening a thread to a leaf and wind- 

 ing it about the stem so as to touch the petiole of each succeeding 

 leaf the arrangement of the leaves becomes more obvious. Some 

 plants have their leaves opposite in two rows or ranks, others 

 opposite in four ranks, each succeeding set of leaves being at 

 right angles to the lower set. This latter arrangement is called 

 decussate (Fig. 13). More commonly the leaves are spirally- 

 arranged and the thread passed once, twice, thrice, five, eight, etc.. 

 times around the stem before a leaf is reached that is exactly over 

 the one from which we started (Fig. 14). The number of leaves 

 passed before reaching one that stands over the first leaf indicates 

 the number of rows or ranks of leaves on the stem. This varia- 



