104 MORPHOLOGY OF LEAVES. 



except at the base, which is simply pinnate or with one or two 



pairs of simple leaflets. 



Tripinnate or Thrice Pinnate leaves of a regular sort are rare ; 



but, with some irregularity, they occur in many species, as in 



Aralia, &c. This extent of division, and even much greater, is 



common in Ferns. 

 Digitate-Pinnate is where the primary division of the petiole is 



on the palmate or digitate plan ; the secondary, on the pinnate. 



This seems to be the case in the Sensitive Plant, Mimosa pudica, 



Fig. 209. But the leaf is here truly bipinnate with the primary 



divisions very crowded at the apex of petiole. 



Conjugate- Pinnate is the same arrangement, with the primary 



divisions a single pair, at the apex of the petiole, and the leaflets 



pinnately arranged on these. 



Digitately or Palmately Decompound in a nearly regular way 

 is not an uncommon case. 

 Usually, the petiole is succes- 

 sively three-forked, as in Fig. 

 210, when the leaf is said to 

 be biternate (twice ternate), 

 triternate (thrice ternate), or 

 quadriternate (four times ter- 

 nate), &c., according to the 

 number of times it divides, or 

 2 3 4^-times ternately compound. 

 no ^ The ultimate divisions in such 



cases of threes are commonly of the pinnately trifoliolate type. 



201. Pinnae is a convenient name for the partial petioles of a 

 bipinnate leaf, taken together with the leaflets that belong to 

 them. Thus, the Sensitive Plant, Fig. 209, has four pinnae, or 

 two pairs ; the Honey Locust, Fig. 208, a greater number. 

 When such leaves are still further compounded, the pinnae of 

 higher order, or the ultimate ones, take the diminutive term of 

 PINNULE or PINNULES. The blades these bear are the Leaflets. 



202. The Petiole or Leafstalk is a comparatively unessential 

 part of the leaf. It is often wanting (then the blade is sessile) ; 

 it may be absent even in compound leaves of the palmate type, 

 the leaflets rising side by side from the stem. When present, it 

 is usually either round, or half-cylindrical and channelled on the 

 upper side. In the Aspen, it is flattened at right angles with 

 the blade, so that the slightest breath of air puts the leaves in 

 motion. Sometimes it is much dilated and membranaceous at 



FIG. 210. Quadri-ternately compound or ternately decompound leaf of Thalictrum 

 Cornuti. 



