164 MINUTE STRUCTURE OF THE LEAP. 



surface of the stem ; but it may occur a little higher up, so that 

 some of the petiole remains attached to the stem 1 (Rubus, 

 Oxalis, etc.). 



461. Evergreen leaves are those which remain upon the stem 

 without much apparent change during at least one period of 

 suspension of vegetation. The leaves of some evergreens per- 

 sist through only one year, falling off as soon as those of the 

 succeeding year have fully expanded. It is not unusual in warm 

 temperate climates to have trees and shrubs which are normally 

 deciduous in colder regions retain their leaves until new ones 

 are produced. 



Pines and spruces lose some of their oldest leaves every j'ear, 

 but new ones are as regularly formed. Their branches are never 

 completely defoliated, but may bear at one time the leaves which 

 have been formed during several years. 



462. The colors assumed by leaves before they fall can be 

 better examined after the subject of the pigment of chlorophyll- 

 granules has been treated in Part II. 



463. The fronds of ferns and the leaves of their allies present 

 few peculiarities, and do not need to be here examined. The 

 formation in ferns of the sori, or spore-dots, the sporangia, or 

 spore-cases, and the spores themselves falls properly within the 

 province of Volume III. 



464. The leaves of mosses are characterized by great sim- 

 plicity of structure. For their study any of the species of Poly- 

 trichum, or Hair-cap Moss, will answer. In these there is no 

 true fibro-vascular bundle ; a series of somewhat elongated and 

 rather firm cells, known as the conducting thread, takes its place. 

 Upon this conducting thread the parenchyma cells are distributed 

 more or less regularly, on one side forming slender elevations 

 four or five cells in height. The cells contain chlorophyll, and 

 generally much starch. 2 



465. In the thallophytes there is no clear distinction of leaf 

 and axis ; the tissue consists throughout of parenchyma more or 

 less modified. In some algae there is often a lateral parting of 

 the frond into segments resembling leaves ; but as they are not 

 leaves morphologically, they need no further consideration here. 



1 For full and interesting accounts of the changes which cause the fall of 

 the leaf, see Mohl's paper in Botan. Zeitung, 1860, p. 1, and also Van Tieghem 

 and Guignard in Bull. Soc. hot. de France, 1882. 



2 In Strasburger's Botanische Practicura, p. 304, the student will find a 

 full and interesting account of the structure of the leaves of Polytrichum and 

 Mniuiu. 



