The Growth and Fall of Leaves 99 



Growth of leaves of seed plants. The flowering plants have 

 either parallel-veined leaves or net-veined leaves, and the place 

 of growth in these two types of leaves is different. In parallel- 

 veined leaves, like those of the members of the grass family, the 

 growth continues longest at the base. If you have pulled a grow- 

 ing leaf from any of the taller grasses like wheat or timothy, you 

 will recall that it broke near the base, and if you put the broken 

 end in your mouth that it had a sweet taste. 



The breaking near the base, and the sugar there, indicate that 

 the final growing region of the grass leaf is at the base. A more 

 exact determination of the growing region may be made by mark- 

 ing a young grass leaf into equal spaces with India ink. This 

 will show that as the leaf develops, it is continually pushed up- 

 ward and outward from the node where it is attached. This 

 mode of growth is characteristic not only in grasses but in many 

 other plants having parallel-veined leaves (monocots, page 130). 

 It is a great advantage in maintaining pastures, but on the other 

 hand we are obliged to mow our lawns more frequently because 

 of it. 



In plants with net-veined leaves (dicots, page 131) development 

 is different from that in either ferns or grasses. The growth 

 of a young leaf of this type — for example, a geranium or 

 nasturtium leaf — may be studied by marking it off into equal 

 squares by means of two series of parallel lines at right 

 angles to each other. After several days it will be seen that 

 the only change has been an increase in size of the squares. 

 The lines in each direction are still roughly parallel. This in- 

 dicates that all parts of the blade, are growing equally. Note 

 that all parts of the blade seem equally firm, which indicates 

 that they are all of the same age. All parts of needle leaves of 

 pine develop at the same time. 



These facts regarding the growth of leaves may be summarized 

 in a somewhat different way. In the ferns the last part of the 

 leaf to mature is the apex. In parallel-veined leaves a region 



