12 ILLINOIS BIOLOGICAL MONOGRAPHS [102 



MORPHOLOGY 



Des Moulins in his tudes was probably the first to use the characters 

 of the seeds to differentiate groups of Cuscutas. He showed that it is possi- 

 ble to separate the genera on the differences in the shapes of their seeds. 

 Different botanists have since studied the structure of the seed. 



The number of seeds produced in each capsule varies from one to four, 

 because of the abortion of one or more of the ovules. Certain species 

 characteristically produce four seeds while others regularly produce but 

 one or two. The shape of the seed is determined, to a certain degree, by 

 the number developed in a capsule. When four are ripened they have 

 two flattened surfaces and an outer convex surface. When but one is 

 ripened, however, it generally possesses a spheroidal shape. The hilum 

 is an oblong, linear or short, rounded, raised or sometimes sunken area 

 situated towards one end or to the side of the seed. It is transverse, i.e. 

 at right angles to the broadest diameter, or oblique. It is situated near 

 the center of a roundish areola, the "umbilical area" of Engelmann and 

 "seed scar" of Hillman (1907), which is usually smoother and of a different 

 shade of color and may be somewhat striated. A cross section shows the 

 seed to possess four layers of cells enclosing the spirally coiled, filamentous, 

 acotyledonous embryo in the albumen. The outer layer of cells having 

 their exposed walls somewhat convex and cuticularized gives a roughened 

 appearance to the seeds. Guttenberg (1909) was able to show differences 

 in the size and shape of the cells making up the testa and keyed out six 

 species (C. suaveolens , C. arvensis, C. epilinum, C. trifolii, C. europaea and 

 C. arabica). , 



The stems do not offer external characters sufficiently constant to aid 

 in more than a general way in specific differentiation. W^ithin certain 

 limits the size is of use, but they show wide variation, even on the same 

 plant. In describing the stems of the different species it has been thought 

 best to divide them with relation to their diameters into three categories 

 using comparative terms to designate each of them. Sections of an 

 average stem were measured for each species, but sections from other parts 

 of the same plant will show wide variation. The size varies from .10 mm. 

 up to 1 or 2 mm. The term "slender" is used in the descriptions to desig- 

 nate those stems that ordinarily possess diameters of not more than .35 or 

 .40 mm., the term "medium" for those of about .40 to .60 mm., and "coarse" 

 for those with a diameter greater than .60 mm. Combinations of two of 

 these terms as "slender to medium" are used in cases where the size of the 

 stem lies about the border line between two of the divisions, as well as 

 qualifying adjectives like "very slender." At the best, the size of the 



