OEGANOGEAPHY. 



rate portions of a leaf as evidence of its compound nature. The 



distinctive characters of simple and compound leaves as adopted 



Fia 347. Fia 348 ^^ ^®' ^^.ve been already treated 



'of under the head of composition 



of leaves. 



The presence of an articulation 

 is to some extent a character of dis- 

 tinction between the three great 

 classes of plants. Thus the leaves 

 of Dicotyledonous Plants are in 

 the majority of instances articu- 

 lated; while those of Monocoty- 

 ledonous and Acotyledonous Plants 

 are non-articulated. Hence the 

 leaves of the two latter classes, 

 when they have performed their 

 functions, instead of falling away 

 and leaving a cicatrix as the former, 

 decay gradually upon their respec- 

 tive plants, to which they give a 

 ragged appearance. There are 

 many instances, however, in which 

 the leaves of Dicotyledonous Plants 

 are not articulated, as in the Oak. 

 In such cases, the leaves, although 

 dead, remain attached to their 

 respective plants frequently for 

 months, which thus form a striking 

 contrast in their appearance to the 

 surrounding trees, which have lost 

 their leaves in consequence of these 

 being articulated. 

 On the lower surface of the petiole at its base, the parenchyma 

 frequently forms a slight swelling ( iig. 346, c), to which the 

 name of j5?f/t;i«2fs has been given. A somewhat similar swelling 

 may be also seen in many compoimd leaves at the base of each 

 partial petiole {fig. 347), which is termed the struma. The 

 compound pinnate leaves of the Sensitive Plant afford a good 

 illustration of the presence of both pulvinus and struma. 



The form of the petiole A-aries ; it is usually rounded below, 

 and flattened, or more or less grooved above. In other 

 cases it is cylindrical, especially in the leaves of Monocotyle- 

 donous Plants, while in other plants of the same class, especi- 

 ally in Grasses, it becomes widened at its base, and surrounds 

 the stem in the form of a sheath or vagina {fig. 348, g). This 

 sheath in all true G-rasses terminates above in a membranous 

 appendage {fig. 348, tig), which is either entire, or incised in 

 various ways, or divided into two symmetrical portions ; to this 



Fig. ."47. A portion of a liranch and 

 leaf of the Sensitive Plant (M- 

 inosa pHclica), sliowing pulvinus 

 at the base of the petiole, and 

 struma at the base of the partial 



petioles. Fig. 348. Stem of a 



Grass with a leaf attached. 1. 

 Blade, g. Sheathing petiole. Iig. 

 Ligule. 



