SECT. I 



MORPHOLOGY 



27 



Ciiscuta Trifolii, one of the most frequent of these parasites, is often 

 the cause of the large yellow areas frequently seen in the midst of 

 clover fields. In certain tropical parasites belonging to the family 

 Rafflesiaceae, the jjrocess of reduction has advanced so far that the 

 flowers alone are left to represent the whole plant. Bafflesia Arnoldi, 

 a plant growing in Sumatra, is a remarkable example of this ; its 

 flowers, although they are a metre wide, the largest flowers in 

 existence, are situated directly on tlie roots of the species of Cissus 

 upon which the plant is parasitic. 



Fig. 27. — Twig of Euscus aculeatus. f, 

 leaf ; cl, cladode ; U, flower. (Nat. size.) 



Fig. 2S. — Opuntia monacantha, Haw., showing 

 flower and fruit. (After Schumann, I nat, 

 size.) 



Tendrillar Shoots. — A peculiar form of metamorphosis is exhibited 

 by seme climbing plants through the transformation of certain of 

 their shoots into tendrils. Such tendrils assist the parent plant in 

 climbing, either by twining about a support or otherwise holding fast 

 to it. The twining bifurcated tendrils of the Grape-vine, for example, 

 are modified shoots. In some sub-species of the wild vine (Quinaria 

 [Ampelojms] quinquefolia) and in other species of the genus, e.g. 

 Quinaria Veikhii (Fig. 29), the tendrils are able to form adhesive discs 

 at the tips of the branches, and thus to cling to flat supports. 



Stem-thorns. — Shoots may vmdergo a great reduction by their 

 modification into thorns, as a defence against the depredations of 

 animals. Of shoots modified in this manner, the Black Thorn 

 {Primus spinosa), the White Thorn (Crataegus), and the Honey Locust 



