Less serious, because not hurtful to the general health of the tree, is the "witch's broom" which forms upon the silver fir, sometimes greatly disfiguring it. This is caused, or at all events accompanied by, a fungus (_Æcidium elatinum_), which passes one phase of its existence upon certain humble herbs of the Pink family, such as the mouse-ear chickweed and sandwort. For purposes of timber probably the common (_A. pectinata_) and the Caucasian silver fir (_A. nordmanniana_) are the pick of the genus, but there are also many other species of singular beauty. Their beauty, indeed, especially in a young state, has proved somewhat of a snare, inducing people to plant them in gardens and pleasure-grounds where they soon outgrow their environment, and, being isolated from their kind, are apt to send up many leaders and so forfeit their true character. Several years ago I was staying in a country house in the south of England, where a royal personage was paying a visit. It was arranged that the said personage should plant a memorial tree; a site was selected on a close-shaven lawn, and I could not but deplore the tree chosen for the honour. It was perhaps the least majestic of all the silver tribe, namely, the Spanish fir (_A. pinsapo_), a species which seldom responds freely to the conditions provided in this country, and, when it does so, is of gloomy appearance. The largest silver fir in the world is the North American (_A. grandis_), which in a young state might easily be mistaken for _A. pectinata_, but soon exhibits its true nationality by the extraordinary rapidity of its growth. It races upwards at the rate of three feet a year, and, distancing all surrounding growth, suffers the penalty inevitable in our stormy climate, unless it should be provided with shelter from a sufficient company of its peers. At the Avondale School of Forestry _A. grandis_ is reported to be less liable to injury from spring frost than the common silver fir. No doubt there are spaces in the wilder parts of this island where this grand tree might be grown into splendid forest, but as an isolated specimen it can never develop its true dimensions, which are out of all proportion to our native woodland. The timber is neither strong nor durable; indeed, of the nine species of North American silver firs, Professor Sargent reports favourably in this respect upon one only, _Abies nobilis_ to wit, a tree of which, personally, I have formed a very high opinion for the climate of the northern and western parts of the United Kingdom. It has suffered in reputation with many experienced planters, owing to a liability to lose its leader when it outgrows its surroundings, as it very speedily does; but, as in the case of the common silver fir, that is the consequence of bad forestry; if _A. nobilis_ were planted in masses, the trees would protect each other. No forester can look unmoved at the group at Murthly in Perthshire, several of which are well over 100 feet high.