EXTERNAL CAUSES OF GROWTH AND FORMATION. I 



3<>5 



(Fig. 88, /), we find that the internodes and petioles of the former have become 

 greatly elongated, while the leaf -blades remain small and undeveloped. Micro- 

 scopic investigation shows that the leaves remain in their embryonic condition, 

 the tissues exhibiting little differentiation. Further, the last phase of growth in 

 the stem is incomplete, for the mechanical elements are wanting, and hence 

 the etiolated shoot is quite soft ; apart from this the individual cells are very 

 much more elongated than in the normal shoot and their number is much 

 greater. The majority of Dicotyledons with long internodes behave in the 

 same way, but even in the case of the so-called ' rosette plants ', such as Semper- 

 vivum (WIESNER, 1891 ; BRENNER, 1900), etiolation results in the diminution 

 in size of the leaves, the elongation of the condensed internodes, and in an 

 opening out of the leaf rosette (Fig. 89, ///). These phenomena are not univer- 

 sal, however ; organs which grow normally in the dark, naturally react dif- 

 ferently from those which grow in light. Thus it would not be possible, for 

 example, to cause an extension in subterranean bulbs with condensed internodes. 

 Bulbous plants, e. g. bulbous species of Oxalis, exhibit totally different etiola- 

 tion phenomena. In their case elongation of the stem does not take place, but 

 the petioles of the leaves, on the 

 other hand, elongate very consider- 

 ably, while the leaf-blade remains 

 small (Josx, 1895). The petioles of 

 Oxalis deppei, for example, growing 

 in the dark, but not full grown, were 

 fifty-eight to seventy-eight cm. 

 long, while the controls which were 

 standing in a room in moderate sun- 

 light, had petioles from eighteen to 

 twenty-three cm. long. 



Many Monocotyledons, whose 

 stems are outstripped by their 

 leaves in rapidity of growth, behave 

 in the same way as do species of 

 Oxalis. Such plants, both in light 

 and in darkness, form shoots of 

 about the same length, but in the 

 dark the leaves, owing to a con- 

 tinued capacity for growth in their basal meristems, exhibit a marked increase 

 in length in the dark, but are generally smaller than when grown in light. 



It has been customary to distinguish these two types of etiolation as the 

 monocotyledonous and dicotyledonous respectively. In both of these groups 

 of plants, however, there are numerous exceptional cases, in which etiolation 

 does not take place or where the plants in question behave in the etiolated 

 condition quite differently from their allies. Among plants which do not 

 elongate their axis in the dark are certain climbers such as Humitlus and Dios- 

 corea, but their behaviour becomes intelligible when it is remembered that 

 climbing plants form, to start with, very long internodes in light, with leaves 

 which for a long time remain small in size. Further, plants are known in which 

 the leaf-blades are not actually smaller in the dark than in the light, such as 

 Beta, Taraxacum and Tragopogon. As already noted no elongation of the 

 shoot takes place in bulbous species of Oxalis, while among Monocotyledons 

 Tradescantia behaves quite like a Dicotyledon ; the leaves remain small and 

 the internodes become elongated. Again among the grasses the Paniceae 

 e. g. Zea mais have much elongated hypocotyls, and the leaves of hyacinths 

 are smaller and shorter in the dark than in the light. Finally in the Cac- 

 taceae shoots grown in the dark remain shorter, often considerably so, than 

 when grown in light (VOCHTING, 1894 ; GOEBEL, 1895). 



Fig. 89. Sempervivum assimile. /, grown under nor- 

 mal conditions; //, grown in a moist atmosphere; ///, grown 

 in darkness. After BRENNER (1900). 



