The Nature and Origin of Stipules. 47 



This anatomical arrangement shows that the so-called addition- 

 al leaves of the whorls in Galium are in reality stipules and that 

 the Stellatse agree with the rest of the Rubiacese in having oppo- 

 site leaves. The tendency of the family however to produce ver- 

 ticillate leaves has been strongly felt in this group but has taken 

 an unusual course, the increased assimilative area having been 

 evolved through the stipules instead of by an increase in the 

 number of true leaves. The explanation is thus made compara- 

 tively simple except in those cases where the number of stipules 

 at a node is more than four. 



As a general rule, in plants with stipulate leaves, each leaf is 

 provided with two stipules. But when the leaves are opposite, 

 the two on the same side of the node often coalesce, forming a 

 single interpetiolar stipule, as in the case of Gephalanthus 

 ( fig. 78 ). That this coalescence is secondary is shown by the 

 fact that the distal portions only of the veins of the two stipules 

 have united. Now in the Stellatse also, this must have been the 

 original condition, but the interpetiolar stipules have been greatly 

 developed to serve assimilative purposes, the veins having mean- 

 while united completely to form a midrib. The increase in size 

 has advanced until in Galium the stipules are of the same size 

 and form as the leaves and morphologically indistinguishable from 

 them, except in G. bifolium where the stipules are smaller. In 

 this condition they remain in the broader-leaved species, as G. 

 pilosum Ait., G. latifolium Michx. and G. lanceolatum Torr. 

 But in the narrower-leaved species, a still greater foliar expan- 

 sion being desirable, separation has been re-accomplished, proceed- 

 ing probably ft'om the tip downward, as is illustrated in Rubia 

 peregrina L. with whorls of four. In this species stipules are 

 occasionally found with two midribs (fig. 79), most widely sepa- 

 rated at the apex or even coalescing toward the base. In Galium 

 Aparine L. and other species in which the number of stipules is 

 abnormal, we may suppose this condition to have arisen from a 

 repetition of the process of division which has produced the six- 

 leaved whorls. This is not improbable, since even in the four- 

 leaved forms the stipules have already entirely lost their original 

 morphological character and have taken on a more generalized 

 nature, making them fit material for development along new lines 

 of evolution. Embryological evidence is not wholly wanting, al- 

 though the family stands so near the head of the plant series. In 



