1596 A TEXTBOOK OF THEORETICAL BOTANY 



for the embryo at germination is still quite undifferentiated. It merely 

 increases in size into a minute green tuber, which gets no further unless 

 it is invaded by the right mycorrhizal fungus, when it develops a growing 

 point and produces the first small leaves (Fig. 1456). 



Several cases are known of seedlings in which the plumule is abortive. 

 Some genera of the Papilionaceae have no vestige of a plumule, even its 

 vascular traces having disappeared. The cotyledons in these plants 

 {Scorpiiirus, Securigera, Tetragonolobiis) have serial buds in their axils and 

 the whole epicotyledonary structure arises from these buds. 



A very striking and well-known case is that of Streptocarpus (Gesner- 

 aceae). The features are not quite the same in all species but the general 

 outline is as follows. The freshly germinated seedling has two equal, or 

 sometimes unequally sized cotyledons, with a minute plumule between 

 them. One cotyledon then begins to grow rapidly, while the other coty- 

 ledon and the plumule remain unchanged. The growing cotyledon acquires 

 the size and appearance of a large foliage leaf and is, in fact, the only leaf 

 usually produced by the plant. Its thickened petiole forms a prolongation 

 of the hypccotyl, pushing aside the other cotyledon and the plumule, 

 which abort and disappear. In some species even the primary root also 

 disappears and the hypocotyl takes its place and forms adventitious branch 

 roots. At the base of the cotyledonary lamina there now arises an exo- 

 genous bud, which grows into the main stalk of a cymose inflorescence. 

 In the axil of this stalk there arise, serially, secondary and tertiary inflor- 

 escence buds. It is argued that their presence proves that the original 

 inflorescence bud is not adventitious but is an axillary bud of the cotyledon, 

 carried upwards by the vigorous growth of its petiole. 



We have remarked above upon peculiarities in the seedlings of parasitic 

 plants and the same remark applies to some insectivores. In some species 

 of Pingiiicula there is only one cotyledon and the primary root is abortive, 

 but it is in Utricularia that modification is most profound. No compre- 

 hensive account of the genus is here possible as nearly every species has its 

 own peculiarities. Plasticity of organs reaches an extreme in this genus 

 where leaves, shoots and, in some related genera, roots alter their character 

 and substitute for one another in bewildering fashion. 



The embryo is undifferentiated at first and never produces roots. The 

 minute, green tuber forms two cotyledons in some species. One of these 

 may remain leaf-like and the other develop into the main axis, or both may 

 remain small and the axis be formed from a third leaf, the plumule being 

 abortive in all cases. In Utricularia vulgaris, there seems to be no coty- 

 ledons, their place being taken by a whorl of unequal, green spikes, one or 

 more of which grow out into the long axes which bear the insect-catching 

 bladders. 



The rootlessness here is probably associated rather with the aquatic 

 than the insectivorous habit, since some other floating aquatics, f.^^., Cera- 

 tophyllum, are also rootless. 



The morphological oddities of the Podostemaceae as well as their unique 



