TREES OF MARLBOROUGH COLLEGE AND ENVIRONS 17 
4. Willows and Poplars 
The silky pubescence on the under-surface of the 
leaves in the dense concentrations of large White 
Willows along the River Kennet cause striking 
scenic effects on sunny, breezy days, as the colour 
switches from light green to silver, either in swathes 
or en masse. The Crack Willows by contrast are 
distinguishable from a distance by willow scab 
disease browning and curling leaves and stunting 
shoot ends. Some large Crack Willows still survive; 
but compared with fifty years ago they are giving 
way to White Willows in height and quantity (see 
‘Diseases’ sub-heading). In some places, Osiers 
(Salix viminalis) predominate. 
Like the Goat Willows (see previous sub- 
heading), the College Poplars (Populus species and 
hybrids) are to be found scattered both north and 
south of the A4, on dry or wet ground. Unlike the 
Goat Willows, most or all were planted rather than 
naturally occurring. Of the eleven types listed (nos 
64-74), six are hybrids and five of these are complex 
hybrids between N. American Black or Balsam 
Poplars and the European Black Poplar (Populus 
nigra). Many are large, but the greatest of all is just 
outside the College boundaries, in George Lane (see 
‘Special Trees’ sub-heading). 
5. Limes and Horse Chestnuts 
Three Limes (Linden) species and two hybrids are 
to be found in the College grounds, but only one is 
both large and common. This is the native 
Common (Hybrid) Lime (Tilia x europea), whose 
parents are the Small-leaved and Broad-leaved 
Limes (T. cordata and T. platyphyllos). Tilia x europea 
is the world’s tallest Lime, and Europe’s tallest 
broad- leaf tree. The larger specimens, at 40m or so 
high, would seem to make it the tallest type of the 
many difference species, hybrids, and varieties of 
tree within the college grounds. All the large 
specimens have densely sprouting bases and masses 
of trunk burrs and sprouts. Some also have suckers 
from underground stems. 
Although no masses of seedlings have been 
noted beneath the College Hybrid Limes (as can be 
found in Savernake Forest), these trees are 
vegetatively invasive in the vicinity of their massive 
bases and can be unpopular on account of the 
honeydew which can sometimes cause the lower 
leaves to become coated with black grimy mould in 
late summer. Also the honeydew can cause pitting 
of the shiny surfaces of parked cars. However this 
honeydew drips on to the soil to provide nutrition 
for nitrogen-fixing cyanobacteria, which in turn 
create usable nitrogen compounds for the tree. 
There is in fact a complex four-way symbiosis, for 
the aphids that create the honeydew also have 
intracellular bacteria which help them use the Lime 
sap more effectively to create proteins. This 
complex symbiosis matches anything to be found in 
the Amazonian rainforests. 
Of the three types of Horse Chestnuts to be 
found in the grounds, only one is large, common 
and seen to produce occasional seedlings and 
saplings by natural spread. This is the Common 
Horse Chestnut or Conker tree (Aesculus 
hippocastanum), now semi-naturalized in Britain 
but originally from Greece and Albania. Large 
conspicuous Horse Chestnuts flank each side of the 
A4, but big specimens are also to be found 
elsewhere in the grounds. Red Horse Chestnuts are 
less common, and are discussed in the ‘Diseases’ 
section to follow. Sweet or Spanish Chestnuts 
(Castanea sativa) are unrelated to Horse Chestnuts. 
Most of the College grounds are either too chalky or 
too waterlogged for these to thrive, but two medium- 
large specimens grow on the Hyde Lane boundary. 
ROSACEAE 
At least 56 tree types, more than a quarter of the 
total, come from this one family alone, out of the 28 
tree families represented. Most Rosaceae trees are 
small, but with clear single trunks to above Sft. The 
paradox is that some large multi-stemmed shrubs 
are far taller and more massive than the neat little 
single-trunk Japanese Flowering Cherry trees 
(Prunus nos 130 & 131). Examples of big shrubs 
include Cherry Laurel (Prunus laurocerasus) and 
Portugal Laurel (Prunus lusitanica) both of which 
can reach 12m high. These are common in many 
parts of the grounds as vigorously expanding 
layering shrubberies, but occasionally produce 
vertical trunks of about 1m in girth at 5ft above 
ground level, meriting inclusion as trees in the 
totals. Even Sloe (Blackthorn, Prunus spinosa), 
usually a 1.5-2m high thorny thicket (as in the far 
south-west of the Nature Trail), can sometimes 
form a proper trunk of a tree 4.5m high. However 
its hybrid with Plum (Prunus x fruticans) always 
forms a more substantial, taller, thorny tree. The 
Prunus genus alone supplies 25 tree types, with 
Wild Cherry (Gean, Prunus avium, discussed 
earlier) as the largest tree of the Rosaceae within the 
College grounds, as well as one of the commonest. 
