124 PLANT PHYSIOLOGY 



food as well as most water must be carried to the active 

 parts. Spring-wood conducts better than autumn wood, 

 although according to Strasburger, * single rows of cells 

 formed last in the autumn-wood possess higher conducting 

 powers than those formed earlier. This he regards as con- 

 tributing to a better connection between the succeeding 

 rings, and this is especially necessary because the new bundles 

 for the forming and growing parts must be adequately sup- 

 plied with liquid while the young spring-wood, with which 

 they connect, is attaining effective dimensions. (For the 

 cause of "annual ring" formation, see pages 191-4.) 



Perhaps in all plants, certainly in many plants growing in 

 desert regions and in places where there is a distinct dry 

 season, tissues are developed in which water may be stored 

 and kept for a long time, in spite of the dryness of the sur- 

 rounding air. It is quite possible that the wood-fibres, 

 present always in the xylem of the vascular bundle and 

 sometimes numerous there, serve as temporary holders of 

 water at the same time that they contribute to the me- 

 chanical strength of the plant. After coUenchyma has 

 served its first purpose in strengthening the rapidly growing 

 parts in which it differentiates so early, its thickened and 

 chemically modified walls, as well as the living cells which 

 formed them, retain water with considerable power, t The 

 most striking examples of water-storing tissues are to be 

 found among desert plants {f^- g-' in the Cactacese and 

 Euphorbiacese),! and in the leaves of Sphagnaeetp, which 

 live under exactly' opposite conditions. The possession by 

 swamp-plants, especially those living in undrained swamps 

 and in bogs, of characters found otherwise only in desert- 

 plants has been remarked by a number of authors. This 

 may be due to the plants trying, by reducing evaporation 

 and therefore the need of absorption, to avoid absorbing in 

 excess any of the poisonous matters (humic acid, etc.) in 



* 1. c. p. 592. 



f Miiller, C. Beitrag zur Kenntniss der Formen des Collenchyms. 

 Berlchte der Deutsch. Bot. Gesellschaft, 1890. 



i See Goebel's Pflanzenbiologische Schilderungen and Volkens's Flora der 

 iigyptisch-arabischen Wiiete, 1887. 



