202 AIZOACJ Al 



and colouring so completely, that one could not recognise the wild plant from 

 the description of the cultivated one. 



M. densum, M. stcllatum and other species of the section Harhata hear at 

 the apex of each leaf a ring of peculiarly constructed hairs. Each hair has a 

 much inflated base, which is set in a strong protective cup; the bottom lit this 

 cup, which adjoins the water- tissue, is thm-walled and permeable to water as 

 long as the leaf is young and growing, hence water absorbed from rain or dew 

 by the hairs can pass into the tissue of the leaf, while in dry weather the 

 exposed portion of the hair shrivels up and prevents loss of water from 

 within (big. 91 b on page 200). On old leaves the hairs are dead, for the 

 tissue immediately adjoining their base has become suberi/ed, the plug of 

 cork ((■£/) thus formed preventing communication between the inner tissue and 

 the hair. 



When correcting the proof ot this sheet we received a copy of a paper by 

 Dr O. Oberstein, kindly sent by the author, on "Ban der Blattspitze der 

 Mesembrianthema-Barbata." The author looks upon the hairs as organs of 

 protection for the apex of the leaf against too excessive light, and on older 

 leaves, where a Layer or cork separates the basis ot the hair from the water- 

 tissue beneath, as an ornament or a protection against herbivorous animals. 



The formation of the cork (marked co in Fig. 91 b and indicated, in its 

 initial stage, by the thicker lines ot the walls ot the cells) rather supports 

 our view, for in summer, when the power to absorb dew would be useless, as 

 no dew occurs at that time in the Karoo, it is essential that the leaf should 

 not lose any water through these apical hairs. 



As branches and leaves of the plants of this group are spread out more or 

 less horizontally (see our figure ot M. densum and those ot M. stelligerutn and 

 M. bulbosum in the author's paper cited) the apical hairs cannot afford any 

 protection against light, especially not in M. barbatum, where the leaf pairs arc 

 more than an inch apart, even on the wild plants. Protection against insolation 

 by means ot hairs is, ot course, of common occurrence, but they are always of 

 ordinary shape and not of such an elaborated structure as in this case. 



The theory of ornaments we need not discuss, and protection against 

 grazing animals by such harmless hairs is out ot the question. 



