THE' PRINCIPLE OF COHESION. 49 



of flowers, respectively. Cohesion by contact is the cause of 

 the anthers being sjngenesious in the Compositce. It applies, 

 sometimes at least, to the two margins of each carpel when in 

 contact np the axis of an ovary, as of that of a Lily. The 

 stigmas of Asclepias are at first free, but later in their deve- 

 lopment they become coherent by contact. 



Congenital cohesion takes place almost from the very 

 commencement of growth and development of the parts, so 

 that when full-grown there may be no trace of the line of 

 cohesion. Fibro-vascular cords, indeed, often occur in the 

 very position of it, not unfrequently branching off in various 

 ways, as, e.g., at the fork to nourish the adjacent free portions 

 of the limb. This occurs in the calyx of Stachys and the 

 corolla of Frimula, etc. In Gavipanula rotundifolia the fibro- 

 vascular system of the corolla becomes completely altered, and 

 instead of representing that of distinct leaves in contact by 

 their edges, the veins ramify and anastomose all over the 

 general space between the two adjacent dorsal ribs, com- 

 pletely obliterating all trace of the line of union between 

 them. In the case of the Primrose, however, the calyx has 

 the exact appearance of five pinnately nerved leaves being 

 united by their thin and impoverished edges, where there is 

 nothing but translucent tissue without any cords at all. 



It is important to observe this more or less complete 

 modification of the fibro-vascular system under congenital 

 cohesion, as it shows how much more highly differentiated 

 a condition has been acquired than when the parts are free. 

 In the latter case they represent more closely the forms and 

 venation of distinct foliar organs. 



As a curious instance of cohesion of both kinds in the 



same organ, may be mentioned the corolla of Phyteuma ; the 



basal portion of which consists of five petals congenitally 



united ; but the five portions of the limb cohere by contact 



7 



