THE ANGIOSPERMAE i597 



habitat in tropical waterfalls and their peculiar distribution, have long made 

 them famous among botanists. Some genera have adopted a thalloid 

 habit which makes their vegetative parts scarcely distinguishable from 

 ]ichens or algae. Others closely resemble Jungermanniaceae. Willis first 

 observed the germination stages in Ceylon. What he then observed is 

 probablv common to most Asiatic and African species, though the story 

 of the American species may be different. The young embryo has two 

 cotyledons but no radicle and attaches itself to the rock by the stumpy 

 hypocotyl. The further development of the plant is wholly due to adven- 

 titious roots springing from the hypocotyl, which rapidly elongate and flatten 

 themselves on the rock surface, to which they become attached by exo- 

 genous lateral outgrowths called haptera. These roots are green and 

 assimilatory and often spread into thalloid expansions. From their upper 

 sides endogenous buds arise in acropetal order, from which leafy and 

 flowering shoots arise, the latter during the dry season when the water 

 level is low. We shall have another occasion to deal more fully with the 

 family under Bionomics in Volume IV. 



Lastly we would mention a unique occurrence in some species of 

 Oxalis, especially O. hirta. Here the interior of the stele in the primary 

 radicle separates from the endodermis, which with the cortex forms a tube. 

 By collapse of the central tissues of the stele, the plumule is drawn down 

 through this tube, below the soil level, where, in a protective pocket of root 

 tissues, it develops into a bulb. 



Vivipary is a word that is used in more than one sense. It may be 

 applied to the precocious germination of seeds within the fruit, the young 

 seedlings being dropped on the ground in a vegetative state, or it may be 

 applied to the special case of vegetative reproduction in which the flowers 

 are replaced by detachable bulbils or buds. In the latter case the young 

 plant is merely an offshoot of the parental sporophyte. The first case is 

 the only true vivipary and if we exclude the second there is no need for 

 the special term of biotechnosis which has been coined for precocious 

 germination. Instances of precocious germination are frequently noticed 

 in certain plants growing in damp situations and such events are probably 

 ecologically conditioned, like the precocious germination of spores which 

 may occur under similar circumstances in Bryophyta and Pteridophyta. 

 Many observations of precocious germination have been made on plants 

 in greenhouses and it seems to be only a casual phenomenon in such cir- 

 cumstances. 



The Mangrove trees are in a different category, for some of the pre- 

 dominant trees in these swamp forests, e.g. RhizopJiora, Bruguiera and 

 Ceriops, always produce their seedlings in this way and drop them into the 

 tidal mud on which they grow (Fig. 1457). Rhizophora is particularly 

 striking, as the seedling may reach a length of 50 cm. before dropping and 

 hang on the tree like fruit. The single seed germinates in the capsule and 

 the radicle emerges from the micropyle and then forces open an operculum 

 at the top of the fruit, emerging into the open air, where it continues to 



