172 



BOTANY 



PART I 



and rigid pointed structures that may either be unbranched or 

 branched. They originate by the modification of leaves or parts of 

 leaves (LEAF-THORNS), of shoots (SHOOT-THORNS), or less commonly of 

 roots (ROOT-THORNS). In the Barberry (Berberis vulgaris) the leaves 

 borne on the main shoots are transformed into thorns which are 

 usually tri-radiate, while the lateral branches bearing the foliage 

 leaves stand in the axils of these thorns. In the Cactaceae also 

 (Fig. 197) the thorns arise from leaf-primordia. In Bobinia (Fig. 198), 

 and in the succulent species of Euphorbia, the two stipules of each leaf 



FIG. 194. Transverse sections of the leaf of Stipa capillata. The leaf above in the closed state, 

 the half leaf below expanded. U, lower surface, without stomata ; 0, upper surface, with 

 stomata (S) ; C, chlorophyllous mesophyll. (x 30. After KEENER VON MABILAUN.) 



form thorns. Shoot-thorns are found in Prunus spinosa, Crataegus 

 oxycantha, and Gleditschia (Fig. 199). In Colktia cruciata all the shoots 

 are flattened and spiny, so that, in addition to serving as protective 

 structures, they perform the duties of the leaves which are soon lost. 

 The plant is an American shrub belonging to the Ehamnaceae and 

 grows in dry sunny situations. Root-thorns occur on the stems of 

 some Palms (e.g. Acanthorrhiza). 



Xerophytes may have swollen or succulent leaves or stems. The 

 green, columnar, prismatic, cylindrical, or globular Euphorbiaceae and 

 Cactaceae are examples. Many xerophytes not merely utilise water 

 economically, but, when it is obtainable, store water in special tissues 

 against periods of need. When typically developed such water-storage 



