THE MATERIAL OUTGO OF PLANTS 337 



The pressures, recorded in raillimeters 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) . . . 1390 



Betula lutea (yellow birch) . . ... . 1815 



Betula lenta (black birch) ... . . . . 2040 



Vitis vinifera (European grape) . . .... 860 



Much Study has been given to variaiions 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 w^ater 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 Hfe of the plant; others are of no use so far as r^^—^Q 

 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 elands which secrete water, „ F1G.631.-- 



. , . Young gland- 



with no distinctive solutes, hke that which escapes in ular hair of 



guttation and bleeding ; and because there are no dis- Pelargomum : 



1 11 1 1 1 /-ii 1 "^t cuticle ; 0, 



tinctive solutes these are called water glands. Glands volatile oil. — 



are named usually according to the most abundant or l^™in Past 



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 



