CHAPTER II. THE STEM. \f 



once the functions of leaf and stem, though true leaves are also 

 present. The stems of Cactuses also frequently assume leaf-like 

 forms and perform the functions of leaves, the leaves themselves 

 being present in the form of spines. In Duckweed 

 the functions of leaf and stem appear never to 

 have been differentiated. An organ so constituted, 

 and which is, properly speaking, neither leaf nor 

 stem, is termed a thallus or frond, Fig. 28. It is a 

 rare thing among flowering plants to find leaf and 

 stem thus blended or undifferentiated, but it is 

 very common among flowerless plants. 



Fig. 28. — The common Duckweed, the upper disc-like portion or frond bearing marginal 

 flowers, and sending down a root with a prominent root-cap, from the under surface. 



Practical Exercises. 



1. Gather twigs of six or more different trees, such as occur in your neigh- 

 borhood, for instance, the Sugar Maple, the American Elm the Bass-wood, the 

 Locust, the Cotton-wood and the HorsechestnuL It will be better for the pur- 

 pose of this exercise if they be gathered in the late autumn or early spring. 

 Observe the leaf-scars and their arrangement on each twig. Observe the termi- 

 nal and lateral buds of each, and note in each instance which are the stronger 

 or better developed ; note the positions of the lateral buds relative to the leaf- 

 scars in each case ; note which of the trees bear scaly and which naked buds ; 

 selecting the twigs that have the largest buds, dissect carefully the terminal 

 buds of each, observing, by means of the magnifying glass, the position, 

 structure and arrangement of the bud-scales and of the true leaves which they 

 enclose ; note how the scales are adapted in each case to the purpose of protec- 

 tion, and, lastly, observe the area on the twig ringed by the scars of the bud- 

 scales of the previous year, and answer, if you can, the question why this part of 

 the twig did not elongate the same as that portion of it which bore the true leaves. 



2. Cut off most of the blade of a leaf of the common Begonia, leaving only 

 the basal portion and the petiole, and plant it in damp sand, keeping it moist 

 and at a temperature of about 90 F., for a few days. If the experiment has 

 been properly conducted adventitious buds will make their appearance in the 

 axils of the veins. 



Study the supernumerary buds on twigs of the Apple, Lilac and Butternut 

 or Hickory, and note the difference of arrangement. 



3. Compare the shapes of the stems of the Bulrush, the Wheat. Pepper- 

 mint, Yellow Dock, Wild Parsnip, Prickly pear Cactus and other familiar plants. 

 Observe the twining stems of the Hop and Morning-glory, and note how they 

 differ in their modes of twining. Observe how the Blackberry, the Wild Clem- 

 atis, the Poison Rhus, the Virginia Creeper, the Pumpkin, the Pea and the 

 Green-briar differ in their climbing organs and in their modes of climbing. 



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