498 JOURNAL OP THE ROYAL HORTICULTURAL SOCIETY. 



■within the seed-coat. This occurs, however, only at germina- 

 tion, and as a rule the amount of stretching is inconsiderable. 

 The co-operation of a leaf in sinking the plant at a later stage of 

 development has only been observed in Oxalis rubella and in 

 perhaps one or two other species of this genus. The phenomenon, 

 as described by Hildebrand,* is of so remarkable a nature that 

 we include a brief description of it here. 



The seed of Oxalis rubella germinates normally enough. 

 Two cotyledons are raised up on a long hypocotyl, whilst the 

 tap-root at its junction with the hypocotyl undergoes local 

 enlargement into a spindle-shaped body. By further growth at 

 the base of the cotyledons a sheath common to them both is 

 formed. From the epicotyledonary bud a single leaf arises, the 

 base of whose petiole swells up, so that it fits tightly the coty- 

 ledonary sheath. That portion of the petiole immediately in 

 contact with the small epicotyledonary bud and below the 

 swelling now begins to elongate, and, as the petiole is tightly 

 jammed in the cotyledonary sheath above, the result is that the 

 proximal end of the petiole, together with the epicotyledonary 

 bud, are forced into the substance of the hypocotyl. The interior 

 of the hypocotyl being soft is readily compressed, and the 

 epicotyledonary bud is pushed down through the whole length of 

 the hypocotyl till it reaches the spindle-like enlargement of the 

 root. The petiole then ceases further elongation, and the bud, 

 conveniently placed within the enlargement of the root, grows 

 into a bulb-like structure, which subsequently expands. The 

 means employed in the descent in this plant are remarkable and 

 unique. 



In the foregoing pages I have attempted to bring together 

 the main facts which have been ascertained — largely by Rimbach, 

 Stroever, and de Vries — in connection with the various means 

 employed by plants in reaching their normal level in the soil. 

 Owing to the difficulties attending continuous observations upon 

 the subterranean parts of plants our knowledge is still much at 

 fault, whilst the conditions under which many of the phenomena 

 are exhibited are unknown. It is because the practical horti- 

 culturist has such unrivalled opportunities for studying these 

 little known phenomena in numberless instances that I am glad 

 that the facts set forth above should appear in the Royal 

 Horticultural Society's Journal. 



* Hildebrand, Bot. Zcitung, 1888, p. 193. 



