292 MORPHOLOGY. 



nected with the structure of buds were discussed. There is little peculiar 

 to the bulb. The epidermis of the bulb-scales in Allium Moly covers a 

 cellular layer, flat cells of which exhibit the strangest and most irregular 

 forms, and seem to be interspersed between each other, somewhat in the 

 same way as the parts of the well-known child's toy, where a picture is 

 glued upon thin board and sawn up into irregular, variously-shaped 

 pieces, which fit into each other. These cells are thick-walled, and very 

 porous. In Gagea lutea and arvensis a layer of spiral fibrous cells is 

 found in the same part. In Allium ursinum and Colchicum autumnale 

 I do not recollect to have observed this ; nor have I seen it in any bulbs 

 with many leaves. 



b. Bulbels (bulbilli). To plants not perennial by means of a bulb 

 (only Dicotyledonous?), the axillary buds are sometimes developed 

 into bulb-like forms, in which the leaves are only developed as 

 thickened, sheathing parts, and the buds separate from the parent 

 plant by the dying away of the supporting stem or stalk, and become 

 independent plants, as with Dentaria bulbifera. 



From the want of personal investigation, and accurate ones from other 

 quarters, I am unable to say much upon these structures. I cannot 

 decide whether the bulbels of some species of Oxalis belong here. Bul- 

 billi ought to be definitely distinguished from true bulbs, in the manner 

 above stated. 



c. Tubers (tubera). On underground stems the axillary buds 

 (of attenuated scaly leaves) are sometimes developed in such 

 fashion that the entire axis of the bud becomes thickened, fleshy, 

 and of a knobby form ; the leaves are quite in rudimentary con- 

 dition, or scarcely to be recognised, whilst the axillary and 

 terminal buds remain capable of development, and after the dying 

 away of the stems of the parent plant form new stems, as in 

 Solanum tuberosum. 



The growth of the potato from an axillary 

 bud of an underground stem is easy to trace 

 (fig. 177.) ; and if we grow the potato in such 

 a manner that a part of the bottom of the stem 

 must remain above the surface of the earth (a 

 circumstance which occurs continually with po- 

 tatoes badly earthed up), we shall see all the 

 stages and degrees which exist between a per- 

 fectly normal axillary bud and a normal potato. 

 From want of the entire history of the develop- 

 ment, I cannot here decide whether the tubers 

 of the Helianthus tuberosus should be reckoned \ I) |I\I\T 



here or not. Tubers of Cyclamen and others W|| *b> 



certainly do not belong to this class : they are 

 stems. 



d. Tuber buds, tubercles (tubercula). Many plants form small 

 tubers above the earth ; seldom (if ever ?), indeed, as axillary buds, 

 but frequently as adventitious buds, and especially on foliaceous 



177 Solanum tuberosum. Bark of a filiform subterranean stem (a), cut into at b down 

 to the bottom of the axillary bud. c, The young potato ; d, scale-like leaf, which 

 bears the potato as an axillary bud ; x, outline, of the natural size. 



