103] NORTH AMERICAN AND WEST INDIAN CUSCUTA—YUNCKER 13 



stem is of small taxonomic value and undoubtedly varies with relation 

 to the situation under which the parasite grows. It is a known fact that 

 a plant of a certain species of dodder growing on an unfavorable host, as 

 some grass for example, will not have the same luxuriance of growth and 

 consequent large diameter of stem as another plant of the same species 

 growing on a more favorable host like an Impatiens. The internodal 

 length varies within wide limits and is probably determined to a great 

 extent by environmental factors such as the food supply and the distance 

 to be traversed before being able to fasten onto another stem. 



The internal structure, however, according to Mirande (1900), exhibits 

 characters which are of value. He was able to divide Cuscuta into three 

 subdivisions by using characters of the vascular bundles. His histological 

 division corresponds to that of Engelmann on external morphological 

 characters. He finds that the Monogyna group shows bundle characters 

 that, according to his opinion, are least modified by parasitism. The 

 group Grammica, on the other hand, shows the greatest modification and 

 reduction of parts, though Engelmann placed it intermediate, while the 

 group Succuta, according to Mirande, holds an intermediate position. 

 Further comparative study of the groups Succuta and Monogyna, which 

 are predominatingly Old World species, may clear up the question regard- 

 ing the sequence of forms. 



The leaves of Cuscuta are reduced to scales which in themselves 

 apparently offer no characters of taxonomic value. One usually subtends 

 each branch and pedicel. They serve no apparent purpose, possessing 

 but little if any chlorophyll and rarely functioning as photosynthetic 

 organs. A few stomata are found on the scales of some species. Mirande 

 has shown that the Monogyna group shows the least and the group 

 Grammica the greatest amount of vascular reduction in the leaf. 



It is in the flower that we find the effects of the parasitic mode of life 

 least apparent and obtain the best characters for the separation of the 

 different groups and the differentiation of species. The proportion of 

 one part to another, their relative shape, size, and positions assumed offer 

 fairly constant differential characters. Owing to the small size of the 

 flowers some of the characters are difficult to determine without the aid 

 of a magnifier. The size of the flower ranges from about 1 millimeter in 

 length for C. karperi to 6 or 7 millimeters for the larger flowers, e. g. 

 C. rostrata, C. corymbosa, etc. The size varies slightly for different indi- 

 viduals of a species. Identification of some species as C. glomerata, 

 C. compacta, C. ceratophora, etc. can be made fairly accurately on external 

 characters without dissection, but others like C. californica, C. poly- 

 gonorum, etc. ordinarily require that a dissection be made to determine 

 scale and staminal characters. The texture of the floral parts shows all 



