THE LEAVES OF VASCULAR PLANTS. 277 



in such a manner that those of the second pair cut those 

 of the first at a right angle, those of the third cutting 

 those of the second at a right angle, and springing con- 

 sequently immediately above those of the first ; and so 

 on in the same manner : this disposition is very evident 

 in plants with a square stem, as the Labiatse ; and also 

 in those with a cylindrical one, as the Lilac, for example. 

 I know hut one exception to this law, — it is Globulea 

 ohvallata, the opposite leaves of which are disposed in 

 spiral pairs ; that is to say, the second pair only cuts the 

 first at an acute angle ; the third cuts the second at the 

 same angle ; and it is only the sixth or seventh whicli 

 comes immediately over the first, so that each system is 

 composed of six or seven pairs spirally arranged. Ajuga 

 Genevensis, according to the observation of Roeper, 

 presents something analogous in the lower part of the 

 stem. 



As for verticillate leaves, the number of which is 

 regular, each leaf of a verticil arises in the place which 

 corresponds to the interval between two leaves of the 

 lower verticil ; whence it results that the leaves of the 

 third correspond to those of the first, those of the fourth 

 to those of the second, &c. &c. : this is a corresponding- 

 law to that of opposite leaves ; but it is observed with less 

 accuracy, because the symmetrical disposition is deranged 

 as often as the number of the leaves of the neighbouring 

 verticils is found to vary, which is so much the more 

 frequent the greater the number is. 



The constancy of the opposite or verticillate position 

 of leaves is sometimes very great, sometimes very slight, 

 according to anatomical circumstances ; thus, this po- 

 sition is exactly followed — 1st, when the leaves are 

 united together externally by their base, which happens 

 in those said to be Connate; — 2d, when a kind of 

 plexus of vessels, or a transverse portion, unites them 



