282 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



shown in the etiolated and in the usual condition in Fig. 133.* Most etiolated 

 stems fail to develop lateral branches, but the etiolated potato sprout is an 

 exception to this rule. It has much-elongated internodes and rudimentary 

 leaves, but it bears small lateral branches (Fig. 134). 



Many plants that develop only very short stems in light, with leaves in 

 rosettes, like Bellis perennis and Sempervivum (Fig. 123, page 269), form 

 elongated stems in darkness, with spirally arranged leaves. 



Another group of plants that do not produce longer internodes in darkness 

 than in light includes those in which, under normal conditions, the leaves are 

 much retarded in their development and the young internodes quickly become 

 greatly elongated. Such forms, among which belong the hop {Humulus lupulus) 



Fig. 133. — Seedlings of scarlet-runner bean. A, grown in darkness; B, grown in light. 



and Polygonum dumetorum, when grown with the alternating light and darkness 

 of day and night, develop full-grown leaves only on the older internodes, which 

 have ceased to elongate. Thus, the younger, elongating portion of the plant 

 appears very much as if it were etiolated, and no marked difference in the 



' In such twining plants as the scarlet-runner bean the manner of growth of the younger 

 portion of the shoot changes as they become older and the long internodes and small leaves of 

 etiolated plants are produced, even in the presence of light. Thus, if the plant shown in Fig. 

 133, B, continued to grow in light it would soon become terminated by a long, slender shoot 

 such as is shown in A of this figure. This kind of etiolation, generally shown by the younger 

 portions of the stems of twiners, occurs in light, but it is similar to the etiolation of other plants 

 (or of the same plant in its early stages of development) that is brought about by absence of 

 light. As the light-grown shoot becomes older its leaves finally expand, however. This 

 matter receives attention in the text, just below, where Humulus and Polygonum dumetorum 

 serve as examples. See also page 311.— Ed. 



