72 BIOLOGY FOR BEGINNERS 



Other methods of climbing are found in plants like the poison 

 ivy, which produces adventitious roots to attach itself, and in the 

 nasturtium, which climbs by hooking its leaf stalks around the 

 supports. 



In any case, the climbing habit is very successful, especially in 

 crowded tropical forests where the shade renders necessary some 

 means for a slender plant to reach up into the light to display its 

 leaves. This the climbers do with least possible outlay of wood tissue. 



Fleshy Stems. Another modification of stems which frequently 

 occurs is developed for the storage of food. The stem assumes a 

 fleshy form, allowing a large storage volume with little exposure 

 of surface. Such fleshy stems are usually developed underground 

 in order to protect their stored food from animals and cold. Like 

 the fleshy root, these underground stems enable the plants to get 

 an early start in spring and also often propagate the plant very 

 successfully. The simplest underground stem is the root stock 

 found in sweet flag and Solomon's seal. Other common forms are 

 the tuber of the potato, and the bulbs such as the onion, lily, tulip, 

 etc. It may seem hard to think of these as stems, yet if we turn 

 to the first paragraph of this chapter, we will find that they have 

 the characteristics mentioned there and are merely modified to 

 adapt them to special functions. 



Bud Structure. A bud is really an undeveloped stem, with the 

 spaces between its leaves greatly shortened, and the leaves them- 

 selves very small and closely packed. The chief function of a bud 

 is to keep the growing point of the stem protected from harm 

 and yet ready for rapid growth at the right time. To carry out 

 this purpose, buds have several interesting adaptations. 



In the first place, they are usually covered with small leaf- 

 like organs called bud-scales, which overlap as shingles do, and 

 protect the tender shoot from loss of water, mechanical injury, 

 rain, and insect attacks. Often the scales are covered with a 

 sticky gum, which aids it, especially as regards the control of water. 



Within the bud, the tiny leaves are frequently packed in a 

 woolly down, which helps protect from injury, especially when 

 the bud is first opening, and may also prevent ill effects from 



