136 



ORGANOGEAPHT. 



leaves in such plants is to be opposite or whorled also, and that 

 when they become alternate, this arises from the prolongation or 

 extension of the nodes : while in Monocotyledons on the con- 

 trary, which have but one cotyledon usually, or if more than 

 one, then placed alternately, that the regular position of the 

 leaves is alternate also, and that when they become opposite or 

 whorled, that this arises from the non-development or shortening 

 of the successive internodes. The investigations however of 

 Bonnet nearly a century ago tended to prove that all leaves and 

 their modifications have normally a spiral arrangement on the 

 stem ; and he was led to this belief by observing that if a line 

 be drawn from the bottom to the top of a stem, so as to touch 

 in succession the base of the different leaves upon its surface, it 

 would describe a spiral around it; he found also, that the 

 relation of the leaves to each other was constant, each being 

 separated from the other by an eqvial distance, so that if 

 we started with any particular leaf and waited until another leaf 



Fig. 263. 



was reached which corresponded ver- 

 tically with it, and then proceeded to 

 the leaf beyond this, we should find 

 that that would also correspond ver- 

 tically with the one next above that 

 which we started from, and so on each 

 leaf as it succeeded the other above 

 would be placed vertically over one of 

 the successive leaA'es below, but that 

 in all cases in the same plant the num- 

 ber of leaves between the one started 

 from, and that which corresponded 

 vertically with it was always the 

 same. Thus if we take a branch of 

 the Apple or Cherry-tree {Jig. 263), 

 and commence with any particular 

 Fig. 263. Aportionof ahranrh of leaf which we will mark 1, and then 

 a Cherry-tree, with six leaves, proceed upwards connecting in our 



the sixth of which is placed i.v i <? t i 



A-erticaiiy over tiie first. Tiie course the base of succeeding leaves 

 right-hand figure is the same by a line, or piece of string, we shall 



branch niaprnifled, the leaves n i .i . in ^i i 



iiaving been removed, and nnct that we Shall pass the leaves 

 numbers i.iaced to indicate the marked 2, 3,4, and 5, but that when 

 points of their insertion, i .i i i /. .i ^ ^i • 



we reach the one marked 6, that this 



will correspond vertically with the 1st, and then proceeding 

 further, that the 7th will be directly over the 2nd, the 8th over 

 the 3rd, the 9th over the 4th, the 10th over the 5th, and the 

 llth over the 6th and 1st, so that in all cases when the sixth 

 leaf was reached including the one started from, a straight line 

 might be drawn from below upwards to it, and that consequently 

 there were five leaves thus necessary to complete the arrange- 

 ment. Bonnet also discovered other more complicated arrange- 



