XI V] PHYLLOCLADES, CLADODES, ETC. 147 



a cylindrical form, and protection by a thick cuticle, &c, 

 are characteristic of some of them growing in dry climates 

 or on exposed moors, hills, &c. e.g. Hakea, Rushes 

 others, e.g. Subularia, Isoetes, grow more or less completely 

 submerged in water, and some species of Allium are 

 meadow plants. Whether these erect Juncoid leaves are 

 of advantage in such exigencies as the drying up of the 

 shallow water in which they grow may be a question : it 

 appears probable that plants with their roots in cold wet 

 soil e.g. Juncus, Scirpus, Garex, Equisetum, &c. may have 

 considerable difficulty in absorbing water sufficiently fast 

 to supply a copious transpiration-current to the leaves 

 in the air above on a hot sunny day, and this state of 

 affairs is common in the case of such plants. 



The suggestion has also been made that the radial 

 structure facilitates equal illumination on all sides in 

 Polar regions, where many such plants are found. 



Many plants are so modified that their leaf-nature is 

 not obvious at all, as in the case of tendrils and leaf- 

 spines; while in other cases we find flattened branches 

 (pJtylloclades or cladodes) replacing leaves. 



With regard to the latter, there is an adaptation met 

 with in the leaves of many Acacias whereby the whole leaf 

 rs reduced to a flattened petiole which assumes some 

 simple leaf-like form, but has its edges vertical. Such 

 phyllodes can be recognised from their axillary buds, their 

 venation, freedom from scale-leaves, and in many cases 

 may be seen in all stages of transition to ordinary leaves 

 on the young plants. Again they are bilateral in struc- 

 ture, not dorsi- ventral like ordinary leaves. 



The fact that phyllodes occur in plants of brilliantly 

 lighted regions, e.g. Australia, where the lamina of ever- 

 green leaves of trees like Eucalyptus also tends to place 

 itself edge-on to the incident light, has suggested that 



102 



