STEMS 663 



velopment has been discovered to be similar, while subsequent closure 

 through cork development is due to desiccation. In some plants cork 

 and complementary tissue develop alternately, the reason apparently 

 being that lenticel closure checks transpiration and permits water accu- 

 mulation and the consequent development of complementary tissue, 

 which bursts through the cork and opens the lenticel. Then active 

 transpiration again takes place, desiccation ensues, and a cork layer 

 once more develops. 



When a woody stem (as in the willow) is submerged, the tissues usually be- 

 come surcharged with water, and complementary cells develop from the phellogen to 

 such an extent that they burst th ough the bark (particularly at the lenticels, be- 

 cause they are the points of least resistance), forming whitish patches, the so-called 

 ■waler lenticels, which differ from ordinary lenticels only in that the greater water 

 supply causes larger emergences. 



The role of lenticels. — Lenticels are regions of gas exchange, taking 

 the place of stomata in stems after the inception of secondary growth, 

 and making possible the continued activity of the chlorophyll after cork 

 formation has begun. Only a somewhat structureless organ such as a 

 lenticel, consisting of an indefinite patch of loose cells, is fitted for gas 

 exchange in bark, where growth and rupture occur continually. Lenti- 

 cel closure involves a decrease in gas exchange, hence reducing syn- 

 thesis and perhaps retarding respiration; however, as closure occurs 

 chiefly at the inception of dry or cold weather, it is likely that the gain 

 from the reduction of transpiration is greater than the loss resulting 

 from decreased synthesis. Probably the small amount of oxygen used 

 in respiration is obtained in part through the bark, which is not wholly 

 impermeable; the lenticels, even when closed, are more permeable 

 than other regions of the bark. 



After the lenticels disappear, the bark continues to increase in thickness, gas ex- 

 change becoming less and less, until it ceases to be appreciable, except, perhaps, 

 beneath furrows, where the protective layers are thin and frequently ruptured by 

 stem enlargement. It has been suggested, however, that the death of trees partly 

 submerged by water or by dune sand is due to the checking of stem respiration, thus 

 assuming that gas exchange through bark may not be inconsequential, though in 

 such cases death may be due to the cutting off of oxygen from the roots ; in any 

 event, continued vigor characterizes trees like the willows and poplars, in which sub- 

 mergence by water or sand incites the development of adventitious roots on the 

 buried stems, while oaks and pines, having no capacity to develop such adven- 

 titious roots, soon die. 



The distribution of lenticels. — No systematic study has been made of lenticel 

 distribution. Usually lenticels are most abundant just beneath the nodes. In some 



