280 VEGETABLE ORGANOGRAPHY. 



sixteenth, and sometimes the fifteenth or twentieth, 

 which covers tlie first. Tims, in combining these dis- 

 positions either with the distance of the systems and 

 their parts, or with tlie size of the leaves, which go on 

 diminishing from the base upwards, we come to under- 

 stand how all leaves enjoy the action of the solar light. 



All these dispositions can, as we have seen, be reduced 

 to two classes; viz. — 1st, Verticillate leaves, which, 

 when the verticil is reduced to its minimum, become 

 Opposite ; — '2d, Spirally arranged leaves, which, when 

 the spire is reduced to its minimum, become Alternate. 



These two fundamental dispositions can be trans- 

 formed from one into the other. In fact, when the 

 leaves of a verticil are not joined together at their base, 

 and as the stem which bears them gradually elongates, 

 but following a spiral direction, which is very frequent, 

 each leaf is found placed a little above the preceding 

 one, and the leaves, instead of being verticillate or 

 opposite, are spiral or alternate. When the first develop- 

 ment of Dicotyledons is observed, we clearly see that 

 this takes place : the first, or seminal leaves, are always 

 regularly opposite or verticillate, and the following 

 sometimes strictly preserve their original position, as in 

 the Caryophyilege, Labiate, Rubiacese, &;c. : sometimes 

 they gradually separate ; thus, several plants, the adult 

 leaves of which are spirally arranged, have the primor- 

 dial ones opposite,* as is seen in several Leguminosa?, 

 Compositas, &c. 



The contrary takes place, although more seldom, in 

 Monocotyledons ; their seminal or primordial leaves are 

 always alternate or spiral. But it sometimes happens 



* Cassini says that he has observed that those Dicotyledons which have 

 the leaves spirally arranged, have their cotyledons clearly approaching 

 each other on one side of the stem, whilst when they are exactly opposite, 

 the leaves are so likewise. 



