262 VEGETABLE ORGANOGRAPHY. 



in which the lobes of simple leaves, or -the partial limbs 

 of compound ones^ are united so as to form entire limbs, 

 will readily admit, I think, that these holes are owing to 

 portions of limbs being incompletely united by some 

 defect of development of cellular tissue. They must 

 not be confounded with the round holes which are ob- 

 served in several kinds of Ulva, which result from the 

 destruction of tissue after fructification, and to which we 

 shall hereafter revert. 



Pothos crassinervia also presents a phenomenon which 

 cannot be referred to any class of known facts : when 

 the plant is old, the leaves have kinds of straight callous 

 lines, parallel to the thick sides, and cutting all the veins 

 transversely ; they open below into a kind of slit, closed 

 on the upper side, and bordered by two little limbs. 



The undulations of foliaceous surfaces are produced 

 by a contrary cause to that which, in the ordinary state, 

 produces the lobes ; that is to say, because the cellular 

 tissue is developed between the fibres in greater quantity 

 than can be contained in the plane which separates 

 them : then it forms more parenchyma than the space 

 can hold, and the surface becomes undulated ; we see 

 this, for example, in a variety of Scolopendrium offici- 

 nale, &c. This effect frequently results from a super- 

 abundance of nutriment. 



Section V. 



Of Compound Leaves. 



We have hitherto spoken of leaves as if all their parts 

 were always continuous ; but some are often met with, 

 which, in certain portions of their extent, present arti- 



