160 



BRITISH PLANTS 



weak stems. Some of the leaves bear tuberous buds 

 (bulbils) in their axUs, but they have the same struc- 

 ture as the imderground tubers. Subterranean leafy 

 bulbils are found in Saxifraga granulata (the meadow- 

 saxifrage), and these form the chief means of propa- 

 gation. 



5. Aerial Bulbils. — These are fleshy brood-buds which 

 arise on aerial stems. They become detached, fall to the 

 ground, root, and grow into new plants. In the tiger-lily 

 they develop in the axils of the lower leaves, and when 

 the plant dies they separate. In some species of onion — 

 e.g., Allium vineale — they replace some or all of the 

 flowers. In very wet places^ such 

 as water-meadows, it is rare to find 

 any flowers at all on the plant. In 

 drier places, where the chances 

 against the survival of the bulbils 

 are increased, flowers are produced 

 instead. 



6. Viviparous Plants. — New plants 

 sometimes appear on the most un- 

 likely parts of plants, and almost 

 any organ may, if the proper stimu- 

 lation is present, become the seat 

 of vegetative budding. Thus the 

 leaves of Begonia become viviparous 

 (Latin vivus, alive ; pario, I produce) 

 if they are pegged to the ground and 

 their veins slit across. Buds are 

 formed adventitiously at the lower 

 ends of the severed veins. From 

 these buds little plants arise, which, 

 if separated, take root easily in the soil and grow. 

 Similar plantlets are formed on the radical leaves of 

 the cuckoo-flower {Cardamine pratensis) when growing 

 in wet, fertile meadows (Fig. 64). In many alpine grasses 

 ■ — e.g., Poa alpina and Festuca ovina — the flowers become 

 transformed into plantlets (Fig. 65). The stimulation in 

 all these cases is produced by the accumulation of excess of 

 nutritive material at certain points. In the Begonia the 

 retreat of the sap down the leaf is stopped at the points 

 where the veins are severed and the buds arise. In the 

 alpine grasses flowers are not produced because of the 



Fio. 64. — Radical Leaf 

 OP Cuckoo - Flower 

 {Cardamine pratejisis), 



SHOWING rORMATION 



OF New Plantlet on 

 Terminal Leaflet. 

 (After Kerner.) 



