THE DEPTH IN THE SOIL AT WHICH PLANTS OCCUR. 497 



previous season. As is well known, the growing apex of the 

 corm is situated laterally at its base (cf. fig. 116), and it is there 

 that the new conn is produced. In the case of a Colchicum 

 plant growing at its normal depth in the soil, the new corm 

 arises as an outgrowth at the base on the same level as the old 

 corm (fig. 11G, b). But when the corm has not yet reached the 

 normal level, as after germination, or if a full-grown corm be 

 raised and planted at an insufficient depth, the portion of stem 

 from which the new corm develops does not project horizontally, 

 but bends downwards (fig. 116, a), so that the new corm is formed 

 about half a centimetre below the level of the old one. This 

 process being continued from year to year, the corm is ultimately 

 formed at a certain average depth, which is maintained. So far 

 as the roots are concerned, it does not appear that they have any 

 influence in this sinking of the plant. 



Other cases in which the stem initiates the change in level 

 are Paris quadrifolia, Dentaria btdbifcra, Adoxa, Circcea, &c. 

 The exact nature of this response on the part of the stem is an 

 obscure matter demanding further investigation. 



Amongst these curving stem structures we may also include 

 the " droppers " of Tulips. These are particularly well marked, 

 and of very constant appearance in Tulvpa sylvestris. A ' ' dropper ' ' 

 is a stolon-like structure arising within the old bulb, which 

 pushes through the bulb-scales, and contains at its free extremity 

 a little bulb which sprouts in the usual way at a varying distance 

 from the parent bulb. Not infrequently this stolon grows down- 

 wards from the old bulb, and the new bulb is thus produced 

 some little distance below. So far as my own observations 

 extend, these droppers appear to behave as would be expected ; 

 i.e. when the bulbs are planted near the surface the droppers 

 descend ; when very deep and below the normal level of Tulipa 

 sylvestris in the soil they curve upwards. For convenience I 

 have termed these droppers " stolon-like." In point of fact, 

 they have a peculiar morphology, and are not simply axial 

 structures. 



III. — Elongation of a Foliar Member. 



The sinking of the plant is aided by the stretching of the 

 Cotyledons in numerous instances. As has been indicated above, 

 this is usually the case when the cotyledon or cotyledons remain 



