134 BOTANY 
leaves have entirely disappeared, and the work of 
photosynthesis is carried on by flattened twigs called 
cladodes (Gk. klados a branch). In some plants, such 
as the acacia, the leaf blade ultimately disappears and 
the petiole and midrib flatten out to form a phyllode 
(Gk. phyllon a leaf). This, like the cladode, is able 
to carry on carbon assimilation without transpiring so 
rapidly as an ordinary leaf. Seedling acacias have 
normal compound leaves, and the transition from these 
to phyllodes is strikingly shown. 
In the gorse and hawthorn, branches are moditied 
to form spines by which means the leaf surface is 
reduced and transpiration diminished. In gorse, more- 
over, the leaves themselves form spines, and thus the 
transpiring surface is further reduced, a matter of 
importance on the dry hill sides and open heaths. In 
the barberry, only some of the leaves form spines. In 
the cactus the leaves are again reduced to spines. Not 
only is transpiration thereby checked, but the spines 
at the same time protect against animals the fleshy 
stems which have accumulated reserves of food and 
water to tide the plant over the long periods of drought 
that prevail in its desert home. The ice plant produces 
fleshy leaves in which also water is stored. The Wild 
Irishman (Discaria) (Fig. 95) adapts itself to the 
seasons. In spring, the leaves are fairly abundant, 
but, as the season advances, they become oradually 
reduced in number, till, by the autumn, they have 
entirely disappeared, the work of photosynthesis 
being carried on by large green spines. This places 
on transpiration the check that is so needed on the 
sand dunes that form the habitat of Discaria. Dr. 
Cockayne’s historic experiment with this plant has 
earned world-wide fame. Grown by him from the 
leafy seedling form in a warm moist atmosphere it was - 
found that the leaves were retained and spines were 
not developed. This indicates that the Wild Irishman 
