174 



ORGANOGR API! Y. 



Fig. 350. Fig. 351. 



can see no sufficient grounds for insisting upon this character 

 in the separate portions of a leaf as evidence of its compound 

 nature. The distinctive chai-actcrs of simple and compound 

 leaves as adopted by me, have been already treated of under 

 the head of composition of leaves. 



The presence of an articulation is to a certain extent a cha- 

 racter of distinction between the three great classes of plants. 

 Thus the leaves of Dicotyledonous Plants are in the majority of 

 instances articulated ; while those of Monocotyledonous, and 

 Acotyledonous Plants are non- articulated. Hence the leaves 

 of the latter when they have performed their functions, instead 

 of falling away and leaving a cicatrix as the former, decay 

 gradually upon their respective plants, to which they give 

 a ragged appearance. There are many instances, however, 

 in which the leaves of Dicotyledonous Plants are not arti- 

 culated, as in the Oak, &c. In such cases, the leaves al- 

 though dead, remain attached to their respective plants fre- 

 quently for months, Avhich thus form a striking contrast in 

 their appearance to the surround- 

 ing trees, which have lost their 

 leaves in consequence of being ar- 

 ticulated. 



On the lower surface of the pe- 

 tiole at its base, the parenchyma 

 frequently forms a sliglit swelling 

 {figs. 349, c, and 350), to which 

 the name of pulvinus has been 

 given. A somewhat similar swell- 

 ing may be also seen in many 

 compound leaves at the base of 

 each partial petiole {fig. 350), 

 which is termed the struma. The 

 compound pinnate leaves of the 

 Sensitive Plant afford a good il- 

 lustration of the presence of both 

 pulvinus and struma. 



The form of the petiole varies ; 

 it is usually rounded below, and 

 flattened, or more or less grooved 

 above {fig. 311). In otlicr cases 

 it is cylindrical, especially in tlie 

 leaves of Monocotyledonous PUints, 

 while in other ])lants of tlie same 

 class, especially in Grasses, it be- 

 comes widened at its base, and 

 surrounds tlie stem in tlie form of 

 a sheath or vagina {figs. 259 and 

 351, g). This sheath in all true 



Fig. 3,50. A portion of n branch and 

 leaf of the Sensitive Plant (:l/i- 

 vioKd pudirii), gliowing piilviuiH 

 ut tlie biiHC of the pitiole, niul 

 struma at the base of the parliul 



))etioles. /•'*>/. ;j.")l. Stem of a 



Grass with a leaf attaclied. 1. 

 Ulade. g. Sheathing petiole. U<j. 

 Liijule. 



