HOW THE BARK BREATHES 



I IK 1-3 all other living; things, ])lants 

 must breathe or they will not 

 J continue to live. The more 

 highly specialized among them 

 are therefore provided with elaborate 

 respiratory systems, consisting of pas- 

 sages which conduct air to all parts of 

 the plant, and openings on the surface, 

 through which oxygen can be taken in 

 and carbon dioxide given out, substan- 

 tially as is the case with animals. 



The extsrnal o])enings of this ventil- 

 ating system are of three general types: 

 stomata or valves on the surfaces of 

 leaves and young shoots;^ ventilating 

 pores, which occur in certain aerial 

 roots; and lenticels, pores in the older 

 wood, whose presence can be noted by 

 the unaided eye in almost any plant, 

 and which are ]jhotographed, enlarged, 

 in Figs. 3 and 4. 



The earlier naturalists were quite in 

 the dark as to the function of these 

 pores. Guettard, who described them 

 in 1745, designated them merely as 

 glands; De Candolle (1826) thought 

 they were a kind of bud, from which 

 roots later put forth; linger (1838) 

 believed they had something to do 

 with reproduction; but as early as 1809, 

 DujK'tit-Thouars declared their ]Jurposc 

 was ventilation, and the work of .several 

 students during the next half century 

 demonstrated that this opinion was 

 well founded. 



Although he mi.sundcrstood their 

 imri)ose, Dc Candolle gave them the 

 name which they now bear, because of 

 the resemblance of one of these pores to 

 a minute, bi-convex lens, in general 

 shape. 



They arc usually found on both stem 

 and root of a plant, but may also appear 

 on leaf-stems, and sometimes on fruits — 

 the walnut anrl horse-chestnut, for 

 instance. 



Yet, in spite of their wides])read 

 distribution, they have been sought in 



vain on some plants. They appear 

 not to exist on the European grape 

 {Vitis vinifera), although they can 

 easily be seen on its close relative, the 

 Scuppcrnong grape of the southern 

 United vStates (\'itis rotundi folia); they 

 have been not discovered on the Italian 

 honeysuckle, the trumpet creeper Te- 

 coma radicans, some species of Clematis, 

 the Philadelphus or mock-orange, Deut- 

 zia, Ktibtis odoratus, etc. 



It has been exjjlained that these plants 

 are provided with a regularly repeated 

 annular formation of the bark, and 

 therefore do not stand in such need of 

 lenticels for the purpose of ventilation. 

 But this hypothesis carries little weight, 

 when one finds that other plants with 

 similar bark have lenticels. Why, for 

 instance, should the climbing honey- 

 suckles lack these organs, while these 

 which do not climb possess them ? And 

 why are they present in the bittersweet 

 {Solanum didcamara), the Boston Ivy 

 (Ampeloi)sis), the Wistaria, and other 

 plants which have habits of growth and 

 formation of bark similar to those 

 above referred to, which lack lenticels? 



Another difficulty in the way of 

 believing the idea once held, that they 

 are indispensable to the respiration of the 

 ]3lant, is the fact that during a consider- 

 able part of the year they are ])artly or 

 wholly closed by the formation of a 

 layer of cork underneath them. This is 

 l)articularly the case in winter, the 

 plant's resting ])criod, when little venti- 

 lation is necessary; but oKservers have 

 found that this closing of the ventilators 

 often begins in early summer, so that 

 s])ring is the only .season when the 

 lenticels are functioning. 



Ftu-ther, it has been discovered that 

 tlie lenticels are in some cases per- 

 manently closed : they look normal from 

 the outside, but are in reality of no 

 value whatever to the phiiil for breath- 

 ing. 



' For an excellent photograph of stomata on a wheat leaf, enlarjjed alioiiL .UK) diameters, see the 

 JouRN.M, f)F HKKKDirv, Vol. VI, No. ?>, p. 12,S, March, 1<)1S. 



491) 



