60 STIPULES AND TENDRILS. 



long. In the Palm, Palmetto, and other tropical plants, it is 

 much longer, and assumes more the appearance of a branch 

 than a petiole. The lamina varies, also, from the size of the 

 minute scale-like leaves of the Moss to the prodigious dimen- 

 sions of some tropical leaves. These organs, in general, bear 

 no proportion in size to the plants on which they are found. 

 On some species of the Oak, of the most sturdy kind, the leaves 

 are small, but feebly corresponding with the gigantic tree itself, 

 while the leaves of the Rheum rhaponticum (Pie-plant) are ex- 

 ceedingly large. The plant itself, divested of its leaves, is com- 

 paratively insignificant in size. 



100. The duration of leaves is various. Some fall almost 

 as soon as developed, and are then said to be caducous ; 

 others remain till the end of the summer, and fall with 

 the cessation of vegetation, when they are called deciduous ; 

 while others remain during the winter, and are denominated 

 persistent, producing the various evergreens of our forest. 

 Various hypotheses have been formed to account for the fall 

 of the leaf, but the most satisfactory one, to our mind, is that 

 given by Professor Lindley, which is, that while the stem and 

 leaf are both increasing in size, there is an exact adaptation of 

 the base of the leaf to the stem, and no interruption takes 

 place ; but when the leaf becomes perfectly developed, and is 

 susceptible of no further increase, the stem continues to enlarge 

 by the deposition of new matter from the leaves above, which 

 breaks the joining vessels, and the leaf of course falls. The 

 breaking of the vessels may be easily observed in the leaves of 

 the Magnolia heterophylla ; hence the reason for the fall of 

 leaves on the lower part of a stem first. It would seem from 

 the above, that the duration of leaves depends upon the time 

 that they are capable of adapting themselves to the stem on 

 which they grow. Some can exist only for a few days, others 

 through the summer, while the evergreens are so constructed 

 as not to be dislodged but by the growth of the wood of the 

 succeeding year. 



Section 7. — Stipules and Tendrils. 



101. At the base of many leaves we meet with two leaf-like 

 organs, separated by the substance of the base of the petiole, 



Palm ? Palmetto ? How does the lamina vary ? Are the leaves propor- 

 tional to the size of the plant on which they are found? Illustrate it by 

 examples. — 100. What is said of the duration of leaves? When cadu- 

 cous f Deciduous? Persistent? What causes the fall of the leaf? 



