692 ECOLOGY 



of rings coincides with the number of times the leaves are shed; for example, a 

 Theobroma tree known to be only seven-and-a-half years old had twenty-two rings. 

 In Dioon and perhaps in other cycads, rings are not formed annually, but once in 

 every ten or twenty years. 



Concluding remarks on vascular variation. — Differences in the flow 

 of materials through conductive tissues appear to be the chief cause for 

 their structural diflferentiation, both in primary and in secondary wood. 

 Nearly all cases of increased vascular development, whether involving 

 an increase in the length, caliber, or number of the elements or in the 

 thickness of the walls, can be referred to increased conduction; this 

 may result either from high transpiration, as in desert regions and in 

 other situations that are exposed to desiccation, or from an increased 

 flow of structural materials, as in secondary wood and in plants at- 

 tacked by parasites. In the weak development of wood in alpine and 

 polar regions, unfavorable conditions for absorption seem to outweigh 

 the otherwise favorable influence of strong transpiration. The conduc- 

 tive tissues appear to furnish the best evidence found in the plant king- 

 dom in favor of the idea that organs increase through use ; however, it 

 is a more tenable assumption that an increased flow provides more 

 adequately the materials requisite for enlargement and perhaps also the 

 physical stimulus needed for continued growth. 



The role of vascular tissues. — The hadrome. — The hadrome forms 

 the pathway of ascending water, as is evident from the quick wilting of 

 the leaves when a complete section of wood is removed, and from their 

 continued turgescence when a cylinder of bark is removed. The ascent 

 in tracheids and tracheae of water colored with eosin has been micro- 

 scopically observed, and the cessation of such movement, when these 

 tissues are infiltrated with cocoa butter, gelatin, or paraffin, has been 

 demonstrated. In the hadrome also there ascend inorganic salts in 

 solution in the water. While it is generally believed that the move- 

 ment of water is through the lumina, some observers have held that the 

 lignified walls are the chief paths of conduction. There is no adequate 

 disproof of wall conduction ; the stoppage of movement by paraffin in- 

 filtration is often cited as such, but it is probable that the walls as well 

 as the lumina are infiltrated. If, as is now generally believed, the 

 ascending water forms a continuous column involving the entire had- 

 rome, the walls would appear to play an important part in conduction, 

 though doubtless subordinate to the lumina. 



Structural advantages of tracheae and tracheids. — The great elon- 



