CHAPTER THIRTEEN 



ECOLOGICAL GROUPS OF STEMS 



IN the tenth chapter we discussed the normal structures of 

 the upright stems of land plants. It was pointed out that 

 each of the three great groups of seed plants has a charac- 

 teristic arrangement of its tissue systems. The essential 

 tissues, like the water-conducting, food-conducting, storage, 

 and mechanical tissues, are present in all. 



Stem structures and habitats. Plants growing in different 

 habitats, as ponds, swamps, and deserts, have very different 

 stem structures. The stem of a leafless desert plant must of 

 necessity be different from that of a leafy submerged plant. 

 The tropical climber has a stem quite unlike that of a plant 

 whose main stem is underground. These differences con- 

 sist not so much in the arrangement of the several tissues as 

 in modifications in their amounts and proportions. 



Stems of mesophytes. The native plants of the eastern 

 United States grow under medium conditions of moisture, 

 light, and temperature ; and they are characterized by large 

 leaf area, leaves of soft texture, and much-branched stems. 

 The vegetation culminates in the forests of the rich, well- 

 watered soils of the river valleys. Here may be found oaks, 

 walnuts, elms, and sycamores from 100 to 150 feet in height 

 and with trunks from 4 to 14 feet in diameter. 



In the moist canons of the Sierras of California, the giant 

 sequoia reaches heights of from 250 to 320 feet above the 

 ground, with extreme trunk diameters of 35 feet. This tree 

 is the largest and is perhaps the oldest of all living things. 

 The redwood, its near relative, grows in the fog-abounding 

 ravines of the Coast Ranges. Its trunk does not attain a 

 diameter of more than 28 feet, but it surpasses the giant 



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