NATURE OF PLANTS 75 



consequently an additional supply of food stimulates them to 

 growth. Buds may also arise upon any part of the stem, root 

 or leaf. These are the so-called adventitious buds and their 

 formation upon roots often accounts for the colonial habit of 

 many trees, plants and shrubs, as in the poplars, Ailanthus, 

 sumacs, etc. They appear to rise naturally in some cases, but 

 in other instances their formation is due to the stimulus of a 

 wound or some other cause and like the dormant bud they serve 

 to prolong the life of the plant. Richards has shown when 

 living cells are exposed to the oxygen of the air, owing to a 

 wound, that these cells are stimulated as a result to renewed 

 activity. This doubtless explains the formation of callus and 

 the healing of wounds, as well as the formation of adventitious 

 buds in many cases. Common examples of buds due to wounds 

 are seen in the vigorous shoots that spring up from the stumps 

 of hard wood trees, pollarded willows, etc. Some buds become 

 fleshy owing to the storage of the food and dropping from the 

 plants serve to propagate new individuals. Examples of this 

 are seen in the fleshy buds on the tips of the branches or in the 

 axils of the leaves of the stone-crop and some lilies and in the 

 flower clusters of some onions. Many aquatic plants have the 

 habit of forming similar buds on the approach of winter. These 

 being compact and heavy with food sink to the bottom of the 

 ponds in the fall and renew their growth in the spring. 



The rapid unfolding of the bud in the spring is a constant 

 source of surprise but when we recall that it contains usually 

 an abundant supply of food and practically all the organs that 

 will appear on the stem during the season we can understand 

 how the bud opens and elongates into a shoot bearing leaves 

 and flowers during the first few weeks of spring. The part of 

 the stem bearing the scale leaves of the bud does not elongate 

 materially since these leaves arc of service onh- during the winter 

 or dry season and soon fall ofl" after the o|X'ning of the bud. 

 Consequently the scars formed by the fall of these protective 

 scales form a ring (Fig. 39, r) each year about the stem which 

 marks the position of each successive bud. These scars are 

 therefore known as annual rings and by counting the number of 



