GEOGRAPHICAL BOTANY AS THE RESULT OF ADAPTATION. 413 



may be well seen in our own Bryony. In all cases the stems or tendrils 

 grow and form the above structures in response to the mechanical 

 strains experienced by the organs in the course of their development. 



Mangroves. — This name is given to trees of several distinct families, 

 growing in or by the sea in tropical countries. They are especially 

 .abundant in the muddy estuaries of rivers, sometimes growing even below 

 high water. Like herbs of salt-marshes, the structure of Mangroves 

 j-eveals their xerophytic character, for although they live more or less in 

 water, this being salt makes the trees physiologically xerophytic. Their 

 -common peculiarity is to be supported on aerial "stilt-roots," like tent- 

 ropes, and, in addition, they may send down arching roots from the 

 higher branches. 



In the typical Mangrove (Bhizophora) the germination is peculiar ; 

 for the embryo, as soon as it is formed, begins to grow while the fruit is 

 still on the tree, forming a long spindle-shaped radicle which protrudes 

 from the end of the pear-shaped fruit. This is developed at the expense 

 of the cotyledons. When these are exhausted the embryo falls directly 

 into, and then stands erect in, the mud. 



Another peculiarity of several Mangroves is to be provided with 

 pneumatophores " or "air-carriers," for the exchange of gases. These 

 consist of modified roots, sometimes hollow, which rise vertically in the 

 -air, or curve above the water like bent knees. They are provided with 

 orifices by which the oxygen of the air can be absorbed and carbonic acid 

 gas exhaled. It was found in one case that the latter amounted to 

 45 c.c. in one hour, showing that the respiration proceeded from the 

 whole root-system of the tree. 



As the amount of oxygen available in water for the respiration of the 

 plant is less than in air, to meet this aquatic plants have large cavities, or 

 lacuna, in their stems in which air can accumulate ; while the corky 

 -surface of exposed parts of the roots of Mangroves have fissures and the 

 so-called lenticels, through which air can pass. 



The Deciduous Cypress (Taxodium distichum) is an extra-tropical 

 tree ; but it is similarly provided with pneumatophores, showing that they 

 are self-acquired adaptations, independently, by each species in response to 

 similar environmental conditions. 



Tropical Thorn-wood. 



In those tropical, sub-tropical, and warmer temperate regions in 

 which there is great heat, but at the same time a great deficiency of water, 

 as rain only occurs for a few weeks in the year, features of a totally 

 different character prevail. The first and most prominent is spinescence. 

 This is the result of response, but can scarcely be called an adaptation, 

 •because the fact that the shoots are arrested in their growth through 

 the want of water, and therefore necessarily end in points, is of no special 

 advantage to the plant.* 



A more important result is the production of water-storage tissues. 

 These are an obvious advantage during the long dry season. They may 



* The not uncommon teleological view that spines are for the purpose of keeping 

 browsing animals away is inapplicable, since no such creatures occur in the sandy 

 deserts where spinescent plants abound. Evolution excludes teleology. 



