ECOLOGY OF WOODLAND PLANTS. 391 
Vaceinium Myrtillus was one of the species they examined, and 
its sap had a density represented by *23. Their assumption is a 
reasonable one, that a relatively high concentration of the cell-sap 
will favour the rapid absorption of water, and so be of service to 
plants growing under xerophytic conditions. 
(b) SECONDARY AND SUBORDINATE SPECIES. 
Other species have been examined whose distribution is more 
restricted; three of these, Heracleum Sphondylium, Lamium 
Galeobdolon, and Mercurialis perennis, are often locally abundant 
in the moist Oak woods of the Coal-Measures, and they show 
well-marked structural differences iu different habitats. 
HERACLEUM SPHONDYLIUM, Linn. Hogweed: Cow-Parsnip. 
b D H 
This species is of common occurrence in the moist Oak and 
Sycamore woods, especially in parts where the ground has been 
disturbed. It is also frequently met with in open sunny places, 
as in fields and banks. In the more exposed situations it is 
often less than a foot in height. Its leaves are much reduced 
in area, and are thick and hairy. Fig. 61 (p. 392) is a transverse 
section of such a leaf from a plant growing on the sandy soil of 
the Gritstone Plateau ; the epidermis is seen to be strongly cuti- 
eularized and distinetly eorrugated. In surface view the cells 
are oval or rounded and slightly wavy in outline (fig. 62). The 
cells of the palisade are very long and narrow, and consist of a 
single layer only, oceupying more than half of the mesophyll. 
Below this is a layer of more rounded cells followed by two rows 
which are much elongated, and united in such a way as to form a 
characteristic network when seen in surface view. 
Plants growing in the shade are not only taller, but the area 
of the leaf is greatly increased and is less hairy and very thin. 
The epidermal cells are larger and much more wavy in outline 
than in the sun-form (fig. 63), and they are very thin-walled. 
The cells of the palisade (fig. 64) are greatly reduced and are 
pear-shaped, exposing their broad upper surfaces to the light, 
and the air-spaces between them are large. Below this is a 
network formed by two layers of spongy tissue, and the cells of 
the lower epidermis are thin-walled and the stomata raised. 
It has been shown by Noll (73) and Schimper (83) that plants 
growing in deep shade exhibit lens mechanisms in their epidermal 
