1 1 S PLANT BIOLOGY 



SUGGESTIONS. — Some of the best of all observation lessons are 

 those made on dormant twigs. There are many things to be 

 learned, the eyes are trained, and the specimens are everywhere 

 accessible. 123. At whatever time of year the pupil takes up the 

 study of branches, he should look for three things: the ages of 

 the various parts, the relative positions of the buds and leaves, the 

 different sizes of similar or comparable buds. If it is late in 

 spring or early in summer, he should watch the development of 

 the buds in the axils, and he should determine whether the 

 strength or size of the bud is in any way related to the size and 

 vigor of the subtending (or supporting) leaf. The sizes of buds 

 should also be noted on leafless twigs, and the sizes of the former 

 leaves may be inferred from the size of the leaf-scar below the 

 bud. The pupil should keep in mind the fact of the struggle 

 for food and light, and its effects on the developing buds. 

 124. The bud and the branch. A twig cut from an apple tree 

 in early spring is shown in Fig. 155. The most hasty obser- 

 vation shows that it has various parts, or members. It seems to 

 be divided at the point / into two parts. It is evident that the 

 part from/ to // grew last year, and that the part below/ grew 

 two years ago. The buds on the two parts are very unlike, 

 and these differences challenge investigation. — In order to under- 

 stand this seemingly lifeless twig, it will be necessary to see it as 

 it looked late last summer (and this condition is shown in Fig. 

 156). The part from / to //, — which has just completed its 

 growth, — is seen to have its leaves growing singly. In every axil 

 (or angle which the leaf makes when it joins the shoot) is a bud. 

 The leaf starts first, and as the season advances the bud forms in 

 its axil. When the leaves have fallen, at the approach of winter, 

 the buds remain, as seen in Fig. 155. Every bud on the last 

 year's growth of a winter twig, therefore, marks the position 

 occupied by a leaf when the shoot was growing. — The part below 

 / in Fig. 156, shows a wholly different arrangement. The leaves 

 are two or more together (aaaa), and there are buds without 

 leaves (bbbb). A year ago this part looked like the present shoot 

 from / to h, — that is, the leaves were single, with a bud in the 

 axil of each. It is now seen that some of these bud-like parts 

 are longer than others, and that the longest ones are those which 

 have leaves. It must be because of the leaves that they have 

 increased in length. The body c has lost its leaves through some 

 accident, and its growth has ceased. In other words, the parts 

 at aaaa are like the shoot ///, except that they are shorter, and 

 they are of the same age. One grew from the end or terminal 

 bud of the main branch, and the others from the side or lateral 

 buds. Parts or bodies that bear leaves are, therefore, branches. 

 — The buds at bbbb have no leaves, and they remain the same 



