SPINY PLANTS.

Timaru Herald, Volume LXXXIII, Issue 13012, 27 June 1906, Page 3

SPINY PLANTS.

WILD IRISHMAN AND SPANIARD. The form of a leaf depend very much upon the surroundings of the plant, especially as regards its -water supply; The object of the plant is to expose as' large a leaf surface a>: possible to the action- of light, while at the same balance must be-maintained between the loss -of water and its supply.'!■ If more water leaves the; plant than esters it,' : tihe leaves must wither, and this is a danger that the plant.must guard against at all hazards. Plants living in dry situations have adopted most ingenious to reduce traatapirajtion (Ibssrjof water) .-to-- a,'minimnm;i .and. plants wjalci.have-risuch. devices-ore called. .-'There age many ways hv which --a' plant can adjust its leaf to the; conditions of a' desert climate- The leaf may be set obliquely Mto the. sun, as in Eucalyptus; it may provide itself with a water-storage apparatusi as in Mesembryantihemum; the total leaf, surface may be reduced as in many Veronicas; or the leaves may become spines, as in Speargrass v and Wild Irishman. '■"-..,.£,..,; .-. ; ._.,,..', .;■•... :. f ... .:*/ ..■:.•--;• An unusually "large proportion *iof the trees and shrubs in New Zealand are zerophytic in structure. This type of plant structure seems to indicate- that the past climate of New Zealand was muchnmore arid, than at present. ; The vegetation"! of the Canterbury Plains, in particular, is of a desert type, and this is explained üby geological evidence. Captain -Huttoii 'has pointed out that during the Piocene period., •the Southern Alps- were much higher'-t-han. they are now, and he considered that the land surface then extended eastward to the Chathams and southward.to the.-Auck-land Islands. Under. these circumstances the climate of the interior would approach tht continental in character. ; The plains to the , eastward of the Alps would be subjected to a. great range of temperature, and great drought. Dr Cockayne explains the present condition of some of the lowland plants by reference to these past, conditions. When the land again sank, the climate would become moister and more insular. Zerophytic plants, if still plastic, would resume their original form, or if unable to do this, would become extinct or retire -,to the shingle fans, dry rocks, and river beds, where we now find them.; •

Discaria—Wild Irishman—has been -re-d-need to these sitraits to prolong its existence. The leaves of the Wild Irishman are fairly abundant in spring, become fewer in summer, and are altogether wanting in autumn and -whiter. Most of the shoots axe reduced to sharp green spines, sometimes with brown tips. These when old become very hard and dry. The small deciduous leaves and the hard green spines' at once proclaim it a plant of dry localities. Dr Cockayne has studied its development, and has also shown by an experiment that Discaria was originally /a spineless leafy plant, adapted to a moist habitat. The seedling plant is erect, .leafy, and bears no spines. After it ttias .attained an inch or ..two in height, spines begin to develop in the leaf axils and the foliage becomes gradually sparse. If, hoW ever, the plant is now put into a warm moist chamber; no more spines will be developed, the leaves will be retained, the' plant returns to its seedling: form, and this will be retained so long as the plant remains in an atmosphere saturated with moisture. We must therefore assume that Discaria had originally small thin leaves, adapted to a moist climate, but has [.changed its <ftaiacters to meet the needs of a drier atmosphere.

Aciphyllum (Speargrass) belongs to tile Umbellifera, the plant family which 'in*clnde? the carrot and parsley, celery and' anises carraway and, hemlock. There are 13 species of Aciphylla in New Zealand, and most of them are characterised by long swferd-like, spinous leaves,, which form 1 a dense tuft of spikes, a. : foot or two in. lieight.. The long flower stalks are clothed with similar; spiny bracts guarding, the flowers.. No plant .could be : better defended against the attacks, of grazing animals, -and the earlier' botanists concluded that the- purpose of spines was protective. Ore - thing, puzzled them. Why should plants be guarded by suchterrible thorns'in a land where no animals browsed? Wallace suggested that jthey might have gained their spines to prevent their being trodden down by moas, which for countless ages took tie place of mammals in New Zealand. However the trend of recent investigation is to show that the spines are not protective in that sense, but are modifications resultinsr from

the struggle against drought. -That-'the 'leaves'should - 'be reduced to spines in Aeiphylla is especially -remarkable, because the leaves of other members of the family are often large and well developed. The leaves, however, of some species axe quite soft) ,«nd grass-like when young, and sheep eat them greedily. On well-stocked stations therefore, the plants suffer severely, and ere soon exterminated. By a curious irony of fate, the protection, of the spines is wanting when most needed. The young plants are greedily eaten by cattle, and as the old ones die off, no others are allowed to take their place. ' . The "whipcord" Veronicas, in which the leaves have become diminished to mere scales, closely packed upon the stem", ar« explained in the < same way.—(From Laing and Blackwell's "Mants of New Zealand.")