136 DERMAL SYSTEM 



Birch, or acicular crystals of cerine in bottle-cork (Quercus Suber) ; 

 calcium oxalate crystals also occasionally occur, either as dense masses 

 {Quercus Suber) or in the form of raphides (Testudinaria elephantipes). 



Cork, like the epidermis, always forms a continuous envelope, 

 which is only interrupted by special intercellular passages that serve 

 for ventilation [cf. Chap. IX. b]. Its component cells are usually 

 arranged in radial rows. The total thickness of the corky envelope is 

 very variable ; for while some cork-films " s comprise no more than two 

 or three layers of cells, a cork-crust may be thicker than the branch 

 which it envelops. Cork-films are smooth integuments, and as a rule 

 consist entirely of tabular cells. Cork-crusts, on the contrary, are 

 mainly composed of wide, soft-walled cork cells, narrow strips of 

 tabular cells being intercalated towards the ends of successive 

 vegetative periods. These massive crusts never form evenly distri- 

 buted smooth integuments, but are arranged in the shape of projecting 

 Manges or wings, separated from one another by deep longitudinal 

 furrows. This condition arises partly owing to asymmetrical production 

 of cork, and partly owing to the development of cracks in consequence 

 of the growth in thickness of the whole organ. The majority of woody 

 plants form cork-films ; cork-crusts are comparatively rare, but occur, 

 for instance, in the well-known Cork Oaks (Quercus Suber, Q. occidentalis, 

 Q. Pseud osidier), on the young branches of Acer campestre, IAquidctm- 

 bar styraciflua, Ulmus subcrosa, Euonymus europaea, and of spp. of 

 Banlcsia and Hakea, and 'on the stems of climbing spp. of Aristolochia. 

 It has been demonstrated by Yon Hohnel in the case of Ulmus suberosa, 

 Euonymus europaea, Liquidambar styraciflua, Passiflora limbata, etc., that 

 the layers composing the crusts are to a large extent unsuberised, and 

 consequently cannot be regarded as genuine corky tissues. For such 

 tissues Von Hohnel proposes the name phelloid : their massive develop- 

 ment is supposed to compensate to some extent for their feeble 

 suberisation, the defect in quality being as it were counterbalanced 

 by an increase in quantity. This interpretation cannot be applied 

 universally : Yon Hohnel himself states that in certain cases (Quercus 

 Suber, Acer campestre, Aristolochia cymbifera, A. Pcixotoa, etc.) the cork- 

 wings are entirely made up of genuine cork. 



Some account must next be given of the physical properties of 

 cork. From the physiological point of view the most interesting 

 property of cork is its very slight perviousness to water. This point 

 has been quite clearly demonstrated by a number of experiments 

 carried out in order to test the influence of cork upon the activity 

 of transpiration. Both Nageli and, more recently, Eder 79 have com- 

 pared the transpiration from peeled potatoes with that of the intact 

 tubers. In one of Eder's experiments the readings indicated that while 



