340 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



conical with the apex resting in the earth. The herb or tree (e.g.Pandanus 

 and Palms) is then supported by stilt- or prop-roots. They are usually 

 cylindrical ; but in some tropical trees, having a very long trunk 

 and a heavy mass of foliage at the top, as has the mahogany, or in 

 short trees similarly with great weight, the roots form flat buttresses. 

 Such are imitated in a small way by engineers, who put " gussets " 

 at the base of iron pillars, to strengthen them. Many are like " flying 

 buttresses." 



All the cases here mentioned, whatever be their uses, are the 

 results of adaptation to the conditions of life. They are, moreover, 

 characteristic of the species, genus, and, not infrequently, of the 

 order to which they belong. 



Stems. — The external forms of stems vary very much, but they 

 are always the consequence of responding to the conditions of life, 

 as anyone may observe for himself. Thus short, thick trunks with 

 a heavy mass of foliage are characteristic of, say, the horse-chestnut 

 and oaks ; while tall, straight stems are seen in pines. The former 

 type indicates a greater freedom of growth, while the latter, by growing 

 thickly, are " drawn." These characteristics have become hereditary, 

 but special varietal forms, as the " fastigiate " and " weeping," may 

 be only feebly hereditary. Thus seedlings of the weeping ash show 

 a tendency at first to " weep," but become erect afterwards, as ex- 

 periments have shown. The weeping willow retains this habit by 

 water, but several planted in the dry St. John's Wood Cemetery, 

 London, exhibited no such tendency. 



The " weeping " habit appears thus to be attributable to moisture 

 in the soil or air ; for the Deodar Cedar, when growing in its native 

 home on the Himalaya Mountains, resembles the Cedar of Lebanon, 

 but in England the habit has become drooping, though the Cedar 

 of Lebanon has retained the stiff character of the branches. 



Some trees of warm climates have enlarged bottle-shaped trunks. 

 The wood-fibres have delicate cell-walls, hence called " cork-wood " 

 (Ochroma lagopus), for their function is to store water. 



Experiments have shown that ordinary woody stems grown in 

 very dry air, in very wet air, and under normal conditions differ very 

 appreciably in the comparative amounts of wood, pith, and cortex, 

 the wood being greatest in the driest conditions and the cellular 

 tissues greatest in the moist ones. 



The comparative uselessness of the wood of poplars and willows 

 is due to their responsive growth in a watery soil. 



The general necessity of wood in trees is of course to support their 

 own weight : in other words, to resist the everlasting pull of gravity 

 downwards. This has become hereditary, because seedlings of trees 

 and shrubs develop a strong cylinder of wood at once when only 

 two or three inches in height — a quite unnecessary proceeding, as may 

 be seen by comparing it with any herbaceous plant, say the flower-stalk 

 of a cowslip, in which only a discontinuous circle of vascular bundles 

 is formed, being quite sufficient for purposes of support. 



