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CHAPTER XXXIII 



OTHER STRUCTURAL CHARACTERS AND GROWTH-FORMS OF 

 LAND-PLANTS, AND ESPECIALLY OF XEROPHYTES 



IT has already been pointed out that some of the structural features 

 of the growth-forms of land-plants are of such a nature that, while no one 

 can doubt their connexion with a dry environment, their utility to the 

 plant is not obvious. As a case in point we may cite the histology of 

 the sun-leaf (heliophyll), 1 where, as we know, there is a greater depth and 

 number of layers of the palisade cells in the sunlight than in shade, and 

 in dry air than in moist air, with the correlative greater thickness and 

 smaller intercellular spaces of the spongy parenchyma, less-sinuous walls 

 of the epidermal cells, and other characters. Among features of this 

 problematic nature may be mentioned lignification, which is so common 

 among land-plants and so restricted in aquatic plants. 



LIGNIFICATION 



Lignification is a mechanical utility because it increases the plant's 

 power of resisting mechanical force. In many plants, including trees, 

 it is of service in connexion with the storage of water. The birch and 

 the Common spruce are splint-wood trees with shallow horizontal roots ; 

 it therefore suggests itself that the supply of water is controlled by the 

 splint-wood. 



It is worthy of note that lignification stands in direct relation to 

 environment, for it becomes more extensive the drier the habitat (except 

 in succulent plants). Families such as the Umbelliferae, Caryophyllaceae, 

 Linaceae, Labiatae, Rubiaceae, and Dipsaceae, as well as genera which in 

 temperate countries are rich in herbaceous species, become far richer in 

 woody plants in tropical, warm-temperate, or even in Mediterranean 

 countries. 



Lignification is particularly extensive in xerophytes that contain but 

 little sap. In these the wood is dense and hard, but at the same time 

 often brittle. It resembles the so-called ' autumn-wood ', because the 

 lumina of the vessels and cells are narrow, and the reason for resemblance 

 is presumably that the conditions of development are identical ; the 

 narrowness of the constituents is correlated with weakness of transpira- 

 tion, which is due to great reduction in the leaves and unfavourable 

 conditions of growth. 2 According to Cannon 3 * the branches of irrigated 

 plants in the desert about Tucson are poorer in conductive tissue than 

 branches of the same diameter of non-irrigated plants '. The explanation 

 of this must be sought in the difference ' in the length and character of 

 the growing season of the two classes of plants '. What benefit desert- 

 plants derive from this structural character of the wood is not clear. 

 It must, however, be remembered that lignified parts withstand extreme 

 temperatures better than watery thin-walled parts can, and that trees 

 endure great fluctuations in humidity better than herbs do. 



Mechanical tissue is developed in the form of bundles of bast above and 



1 See p. 21. * Henslow enumerates the peculiarities of desert-plants, 1894. 

 3 Cannon, 1905. 



