370 Transactions. — Botany. 



is also aided by the occurrence of the stornata in slight de- 

 pressions, and by the rigidity of the shoot-axes, which are 

 therefore less easily shaken by the wind, and at the same 

 time are possessed of sufficient elasticity to be able to avoid 

 danger of breaking in a high wind. The conditions to which 

 they are exposed on the open plains of the south and south- 

 east of New Zealand are considerable heat, a considerable 

 degree of dryness (especially in summer, when the dry, hot 

 nor' -westers are blowing), and strong winds. 



The conditions of climate prevailing at the present day do 

 not seem to be sufficient to account for their extreme xero- 

 phytic structure. Clematis afoliata and Carmichaelia are 

 derivatives of the forest flora. Probably they acquired their 

 characteristic structure in the days of " Great New Zea- 

 land," when the Southern Alps were higher, and therefore 

 the eastern plains were drier and subject to more exces- 

 sive heat. " The forest flora which could not adapt itself 

 to the climate perished or retreated to more rainy parts" 

 (Diels). 



In C. afoliata and the Carmichaelias the necessary 

 rigidity is attained by means of development of stereom 

 tissue in the cortex ; in Discaria toumatou by means of 

 the abundant development of the woody cells of the xylem, 

 and also to a less extent by the bast fibres. In all there is 

 a double epidermis layer ; perhaps the inner layer may be of 

 use to a slight extent as water-storing tissue. Stornata are 

 only developed above the chloreuchyma, where they are of 

 use. The thin-walled cells round the stereom groups are 

 probably also of use in this respect — i.e., for water-storage. 



The development of mechanical tissue must not interfere 

 with the assimilatory chlorenchyma. In the words of Diels 

 (I.e., p. 248) : " Everywhere is typical palisade tissue effec- 

 tive for assimilation, which is indeed very comprehensible 

 from the physical environment, and the great demands which 

 the total absence of foliage makes from the tissue which 

 represents it. In its centrifugal tendency the chlorenchyma 

 tissue comes into conflict with the not less important sup- 

 porting tissue. The various solutions of this problem de- 

 termine the histological structure of the stem." Palisade 

 chlorenchyma occurs in all the species which have been 

 considered. In Discaria toumatou it extends in an uninter- 

 rupted ring round the stem, so that it is able easily to per- 

 form the work of assimilation. In the other plants it extends 

 inwards as far as the vascular bundles, and hence the pro- 

 ducts of assimilation can be transferred to the sieve-tubes, 

 and the ascending sap is also able to reach the chlorophyll 

 tissue. 



