8 ACACIA 



vertically this exposes less surface to radiation. A mere inspection, 

 though it shows the phyllode to he a leaf-structure (it has an axillary 

 bud), does not show that it is not a 1. turned edgewise, though it 

 shows no twist at the base. Occasionally however there are reversions 

 to type (i.e. to the ancestral form) on the plant, some phyllodes 

 occurring with leaf-blades of the ordinary bipinnate type. This is 

 still better seen in germinating seedlings. The first 1. are typical 

 bipinnate 1., followed by others with slightly flattened stalks and less 

 blade, and so on, until finally only phyllodes are produced. In A. 

 alata R.Br. and others, the phyllodes are decurrent on the stem, like 

 the 1. of thistles. In many the slips, are repres. by large thorns, 

 swollen at the base. In A. spkaerocephala Cham, et Schlecht. (Cent. 

 Am.), the thorns are inhabited by colonies of ants, which bore into 

 them and clear out the internal tissue. The ants live on the A. and 

 are fed by it. Extrafl. nectaries occur on the petioles, and yellow 

 sausage-shaped food-bodies on the tips of the leaflets. These consist of 

 parenchymatous cells containing food -st lifts, and are eaten by the ants. 

 If attempt be made to interfere with the tree the ants rush out. (Cf. 

 Cecropia, and see Nature, Aug. 1893, for an account of the leaf- 

 cutting ants: and cf. Schimper, Plant Geogr. p. 140. This was the 

 first case of myrmecophily (symbiosis with ants) discovered; see 

 Belt's Naturalist in Nicaragua.) Other sp. are myrmecophilous also. 

 A few are twiners, others hook-climbers. Most are xerophytes, 

 often forming char, features in vegetation and scenery, e.g. the babul 

 (A. arabica Willd.) with its low, spreading habit, is almost the only 

 tree in many parts of the dry plains of India, and others are common 

 in S. Afr. &c. In Austr. (esp. S. Austr.) the A. take a great part in 

 the formation of the scrub, a concourse of shrubby plants of many 

 genera, which covers the almost waterless country with a waste of veg. 

 about 6 10 feet high, of a general bluish green effect, and with few 

 herbs or grasses beneath. 



The fl. (diagram, see order) has oo long sta. , affording little pro- 

 tection to the pollen. In A. hotnalophylla A. Cunn. (S. E. Austr.; 

 Myall) the seed hangs out on a long red funicle. 



Many valuable products. A. Senegal Willd. (Soudan) yields the 

 best gum-arabic; the gum exudes from the branches principally 

 during the prevalence of the dry desert winds. Other sp. yield 

 inferior qualities. A. catechu Willd. (E. Ind.) yields catechu orcutch 

 (used in tanning), by digestion of the wood in hot water. With this 

 the true khaki cloth is dyed and shrunk. A. decnrrens Willd. (Austr. ; 

 black wattle) yields good tan bark; inferior barks from A. pycnantha 

 Benth. (S. E. Austr.; golden wattle), A. dealbata Link (Austr.; 

 silver wattle) &c. That of A. arabica Willd. is largely used in 

 India. The wood of many is valuable, esp. Australian black-wood, 

 A. mclanoxylon R.Br. Many have sweetly scented fls. ; those of 

 A. Farmsiam Willd. (trop.) are the Cassie flowers of perfumery. 

 A. annata R.Br. (temp. Austr.; kangaroo thorn), A. horrida Willd. 

 (S. Afr.) &C. form good hedges or sandbinders. 



Acacia, false, Robinia Pseud-acacia L. 



Acaena L. Rosaceae (in. 9). 80 =fc, Mex., Calif., Polynes. Fr. 

 hooked. Some sp. bud from junction of leaf and stalk. 



