670 A TEXT-BOOK OF BOTANY. 



in sub-tropical countries. The starch is a commercial product 

 and is known as sweet-potato starch or Brazilian arrow-root. 

 The grains are more or less bell-shaped and 2- or 3-compound, 

 about the size of wheat-starch grains, and in other ways resemble 

 those of tapioca. 



To this family also belongs rather an interesting group of 

 parasitic plants, namely, dodder (Cuscuta). They contain the 

 principle cuscutin, and quite a number have been used in medicine. 



h. POLEMONIACE^ OR POLEMONIUM FAMILY.— 

 A family mostly of herbs and chiefly of horticultural interest. It 

 contains the genus Phlox, which is indigenous exclusively to North 

 America. A number of the species are cultivated and are included 

 among the most valuable hardy, herbaceous perennials. The 

 flowers are among the most beautiful and persistent of our garden 

 plants. Another interesting genus belonging to this family is 

 Polemonium, a number of species of which have been long under 

 cultivation as border plants. Polemonium reptans is rather com- 

 mon in the woods of the Northern United States (Fig. 367). 



c. HYDROPHYLLACE^ OR WATERLEAF FAMILY.— 

 The plants are herbs or shrubs which are indigenous to Western 

 North America. \^ery few of the plants of this family are of 

 use medicinally, although quite a number are ornamental plants. 



Eriodictyon califoDiiciim (E. glutinosiim) or Yerba Santa 

 is a shrub growing in Northern Mexico and California. The 

 leaves are official (Fig. 368). The flowers are funnel-form, white 

 or purple, occurring in cymes. The fruit is a dehiscent capsule 

 and the seeds are small and few. 



d. BORAGINACE^ OR BORAGE FAMILY.— The 

 plants are mostly herbs with regular blue flowers, occurring in 

 scorpioid inflorescence. The best examples of the group are 

 the forget-me-not (Myosotis), the roots of several species of 

 which have been used in medicine ; and the garden heliotrope 

 (Heliotropiim peruvianum), the fragrance of the flowers being 

 due to a volatile oil. This plant, as well as other species of 

 Heliotropum, contains a poisonous volatile alkaloid. 



At one time there was considerable interest inALKANET^the root 

 of Alkanna tinctoria of Southern Europe and Asia, on account 

 of the red coloring principle alkannin, which is soluble in alcohol, 



