THE MATERIAL OUTGO OF PLANTS 337 
The pressures, recorded in millimeters of mercury (760 = i atmosphere), vary 
from o to 
Ribes rubrum (red currant) 358 
Acer platanoides (sycamore maple) 347 
Acer saccharum (sugar maple) 1033 
Psedera quinquefolia (Virginia creeper) 615 
Betula alba (white birch) 139 
Betula lutea (yellow birch) 1815 
Betula lenta (black birch) 2040 
Vitis vinifera (European grape) 860 
Much study has been given to variations in the amount and pressure 
of bleeding; seasonal and possibly diurnal fluctuations have been dis- 
covered; but inasmuch as turgor pressure must be influenced by tran- 
spiration, itself of infinite variability, the precise results of these studies 
are not important. 
The limited movement of water through submersed aquatics which 
has been described cannot be due to transpiration, and is probably not 
a case of guttation. The experimental evidence is scanty and the 
movement may be referable to the larger heating effects on the leaves 
as compared with the stems. This should create a slow movement of 
water out of the leaf, to be supplied from below. 
Secretion. Secretion is a much more general and varied phenomenon 
than guttation or bleeding. It is performed by more limited and 
specialized tissues, called glands, and the variety of substances which 
escape is much greater, though the amounts lost are much smaller. 
Many of the secretions are of such a nature that they play an important 
part in the life of the plant; others are of no use so far as 
we know and are therefore called waste products. No dis- 
tinction can be made in plants between useful secretions 
and waste excretions. 
Glands. There are some glands which secrete water, ., lG - 6 3 I -~" 
Young gland- 
with no distinctive solutes, like that which escapes in u lar hair ot 
guttation and bleeding; and because there are no dis- Pdargomum: 
c, cuticle; o, 
tmctive solutes these are called water glands. Glands volatile oil. 
are named usually according to the most abundant or From PART 
characteristic material they secrete. Thus those in whose 
secretion calcium salts become conspicuous by concentration are called 
lime glands; digestive glands secrete water containing enzymes. Most 
common of all are the nectar glands or nectaries, abundant in flowers, 
but found also on other parts as extra-floral nectaries (fig. 1183), whose 
