534 
PL ANT A GO 
growing in sandy places. Many S. Am. sp. show very marked xero- 
philous characters — dense tufting, small hairy leaves, often grooved on 
the lower surface (cf. Ericaceae), &c. P. maritima L. (the sea-side 
plantain) has linear fleshy leaves (p. 169): it is frequently found at 
high levels in the Scottish Mts. (p. 187), though rarely in the inter- 
mediate regions. 
The seeds of many sp. swell up when wetted and become muci- 
laginous (cf. Linum). Those of P. Psyllium L. (Medit.) are used in 
various ways in silk and cotton manufactures on account of this 
property ; they have also been used in medicine. 
If the young growing mfl. of a P. be vigorously shaken, the spike 
when left to itself droops and only becomes erect again after a con- 
siderable time. This is due to the fact that the strain stretches the 
young cell walls beyond their limit of elasticity (see Sachs’ Lectures 
on the Physiology of PI ants , p. 220). 
Platanaceae. Dicotyledons (Archichl. Rosales). Only genus Plata- 
nus (q.v.). The relationships of P. are obscure; it is placed in 
Unisexuales by Benth. -Hooker, in Saxifraginae by Warming, and 
apparently comes nearest to Rosaceae and Hamamelidaceae. 
Platanthera Rich. = Habenaria Willd. 
Platanus (Toum.) Linn. Platanaceae. 4 sp. N. temp. The plane- 
tree, P. orientalis L., is well known. The bark scales off every year, 
leaving a smooth surface. The axillary bud is developed under the 
base of the petiole, which fits over it like an extinguisher. The stipules 
are united round the stem. Firs, monoecious in pendulous heads, wind- 
fertilised. K 4 or 3. Sta. 4 or 3, opposite sepals. Cpls. 4 or 3, apo- 
carpous. Ovules orthotropous, pendulous. Multiple fruit, each cpl. 
giving a caryopsis, often winged. Seed albuminous. The wood of 
the plane is useful, and also that of the buttonwood of N. Am. P. 
occidentalis L. 
Platycerium Desv. Polypodiaceae. 5 sp. Afr., Malaya, Austr. They 
are epiphytic, but sometimes grow on steep rock surfaces, and exhibit 
a very interesting structure. The rhizome is short and bears alter- 
nately leaves of two kinds. The young leaves are protected by hairs. 
Of the two kinds of leaves, the one stands more or less erect (the 
‘mantle’ leaf) the other is pendulous, usually much branched and 
bears the sporangia in irregular areas on its lower surface. Assimi- 
lation is chiefly carried on by the pendulous leaves. Two types of 
mantle leaf occur, represented in P. grande J. Sm. and P. alcicorne 
Desv., the two common sp. In the former the base of the leaf, 
which is unbranched, clings closely to the supporting trunk, whilst 
the upper part spreads out and makes a niche in which humus collects; 
in this the roots ramify and absorb food. This plant grows to a great 
size. In P. alcicorne on the other hand the whole of the mantle leaf 
clings to the support, and the only humus-supply the roots have is that 
furnished by the decay of old mantle leaves and perhaps of the tree 
