PHYSIOLOGY OF THE RED MANGROVE. 655 



The difference in palisade and tannin cells is not so pronounced 

 here as in the preceding set of comparisons, the main dift'erence 

 being in the amount of water storage tissue. On account of the 

 slight quantity of rain water held in the soil, or of the chemical 

 action of the soil producing a lessening in concentration of the salt 

 water as it seeps inshore, the inshore leaves are thinner and show 

 the tendency toward adaptation, as seen in the fresh-water leaves. 

 It might be mentioned in connection with these preparations 

 that the drawings were made by means of a camera lucida and that 

 the actual leaf thicknesses were as follows : Fresh-water leaf, .54 

 mm., salt-water leaf, .65 mm., inshore leaf, .42 mm., offshore leaf, 

 .54 mm. The first pair of comparisons must not be based with the 

 measurements of the second pair, as the material was collected at 

 different times of the year, different regions and were perhaps dif- 

 ferent in leaf size or age. 



ViviPARY AND Dispersal. 



Perhaps the most peculiar of all the adaptations of mangrove- 

 habit plants is that of vivipary, and this seems to be best developed 

 in members of the Rhizophoracese that grow in the deepest water 

 and softest mud. This adaptation has a very vital ecological signifi- 

 cance in connection with dispersal, as remarked at the beginning 

 of this chapter concerning Praeger's experiments, and the more 

 recently published results of Guppy. Vivipary, according to Goe- 

 bel's^^^ view, is only found in plants which grow under very warm, 

 moist conditions and this wet environment which quickly germi- 

 nates seeds has produced the habit, that is the habit arose by the 

 differences in the readiness to germinate in various seeds. 



The first sign of vivipar}' then would be the falling to the 

 ground of an immature seed, with the embryo still undeveloped, a 

 condition somewhat analogous with that of the seeds of certain 

 orchids ; next would be the stage when the seeds germinate as soon 

 as shed on the ground ; third is a type represented by Lagtmcularia 

 in which the seedling just begins to germinate on the tree, then 

 fourth, where germination is completed on the tree, but the seed- 

 ling immediately falls as in Avicennia and the climax is reached in 



^•^3 Goebel, K., loc. cit., p. 123. 



