73 



ECOLOGY 



TItc possible factors. While the exact factors determining elongation in par- 

 tially buried stems are not clearly known, it is significant that only those stems 



exhibit elongation which develop adventi- 

 tious roots in the moist sand (fig. 1050). 

 Some trees (as the red cedar) ordinarily are 

 of low stature, while other trees (as the euca- 

 lyptus) are very tall; the factors determining 

 the potential height of the stem in various 

 species are quite unknown. Increasing height 

 probably is accompanied by accelerated tran- 

 spiration, and an increasing root system makes 

 possible increased absorption. It is likely, 

 however, that the transpiration increase 

 gradually exceeds the increase of available 

 water, owing to the conjunction of increased 

 transpiring surface, increased exposure to 

 transpiration, and increased length of the 

 conductive tract; ultimately the available 

 water may be sufficient merely to make good 

 the loss by transpiration, leaving no surplus 

 for elongation. This condition, ultimately 

 reached in all trees, may be reached much 

 sooner in some species than in others, owing 

 to peculiarities of structure and behavior. 

 If this hypothesis is valid, stem elongation in 

 dune sand may be explained by the fact that 

 the absorptive system increases as fast as the 

 transpiration is accelerated ; the distance 

 traversed by water in reaching the topmost 

 organs remains essentially the same, since 

 the adventitious roots keep pace in their de- 

 .velopment with the increasing elongation of 

 the stem. 



FIG. 1050. The apical portion of 

 a plant of the red-osier dogwood 

 (Cornus stolonifcra) that has been 

 almost buried by dune sand; note the 

 adventitious roots (r) that have issued 

 from the stem; the main shoot (m) 

 has been killed, but one of the lateral 

 shoots (ri) still keeps above the sand; 

 note that the scars (w) left by the fall- 

 ing of the scales of the previous winter 

 bud are at the sand line (s), showing 

 that the plant was almost completely 

 buried. 



Stem dwarfing. Alpine and low- 

 land cultures. Alpine stems com- 

 monly are dwarf, and, if much-branched, 

 they are compact and bushy, often 

 forming dense mats or cushions. In 

 some very careful experiments both al- 

 pine and lowland individuals of many 



species were split into two parts, one 

 of which was grown in an alpine garden, and the other in the lowlands; 

 the portions taken from alpine districts to low altitudes developed slender 

 elongated stems, while the portions taken from the lowlands to the 



