THE SHAPES OF TREES 



this " nkonge," as the Zanzibari call it, are placed opposite one 

 another in a plane which, by a peculiarly malicious arrange- 

 ment, is twisted into a spiral. Hence from whatever side these 

 " chevaux de frise " be charged, the unlucky traveller only 

 plunges on to the pricks ; and this happens with aggravating 

 frequency when chasing wounded game through the scrub 

 in the dim light of early dawn. 



Another type of defence extensively adopted consists of 

 circles of stout hairs pointing downwards, which prevent the 

 more simple-minded of the ants from reaching and robbing the 

 flowers. Nearly all the grasses are provided with these spiked 

 collars, for their farinaceous seeds are especially attractive to 

 ants. In many of the flowering plants the same purpose is 

 effected by having the leaves placed in opposite pairs with 

 their bases " connate," or growing together to form a cup, as 

 in the English teazel ; in the rainy season, when alone the 

 plant is in flower, this cup is filled with water, and this natural 

 moat prevents crawling insects from reaching the coveted stores 

 of honey and pollen. 



The method of reducing the waste of seeds in a plant by 

 their germination on the parent is not unknown among our 

 English plants, and cases occur more frequently in the upland 

 meadows (or Alps) of Switzerland. In Africa this "viviparity " 

 is very extensively adopted. The case of the mangrove has 

 already been quoted, but the method is used in the plants of 

 the " Nyika " in an opposite manner ; for in these the leaves 

 act as a kind of parachute, and scatter the young plants 

 broadcast before the wind, instead of restricting them to a 

 narrow belt. 



The action of the wind in moulding the forms of trees is 

 well shown in the open plains of East Africa. The few 

 timber trees that occur there are, like the baobab, fixed by 

 trunks of enormous bulk, so that the wind acting on the leafless 

 branches has but little power over them. In other cases the 

 leaves are narrow and spiny, and the branches expand in flat 

 horizontal sheets, which present only their narrow edges to the 

 wind. In none of these cases does the upper portion of the 

 tree give the wind much leverage by which to overthrow it. 

 The smaller trees are also shaped by the same agency. The 

 acacias which form most of the scrub are low and flat-topped. 



