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A TEXTBOOK OF THEORETICAL BOTANY 



sometimes only a single terminal segment, the apical bud of which aborts, 

 but which bears one or two fully developed foliage leaves. A new shoot 

 arises from the axil of a scale at the base of the pseudo-bulb, and this in 

 its turn ends in another pseudo-bulb, so that the appearance is presented 

 of a row of green tubers connected together at their bases by a slender stem 

 bearing scale leaves. 



There are numerous instances of the axillary buds on aerial shoots becom- 

 ing transformed into small bulbs or bulbils. Good examples are shown by 

 Dentaria bidbifera (Fig. 933) and Liliiim hiilhiferiim, in which nearly all the 



Fig. 934. — Agave atner/cana. Part of an inflorescence 

 showing the replacement of flowers by detachable 

 bulbils. 



axillary buds are so transformed. These bulbils are readily detached and form 

 an effective means of vegetative propagation. In the genera Allium and Agave 

 (Fig. 934) and in some alpine plants, e.g., Polygonum viviparum and Poa alpina, 

 the flowers are often transformed into bulbils, but the latter cases are 

 undoubtedly related to the very short growth season available to the plants 

 and perhaps to the scarcity of pollinating insects at high levels. Finally 

 there is the case, described by Goebel, of Cryptocoryne (Araceae) in which 

 the embryo itself becomes a bulbil and is shed naked, detached from its 

 cotyledon, which remains in the seed. These latter cases of modification 

 of floral structures, though not morphologically equivalent, are included 

 under the biological term " vivipary," of which we shall say more when 

 dealing with Mangrove trees in Volume IV. 





