To illustrate Dr. Satish post on  Apocynaceae family, I am posting this tree. All pictures taken in Kolkata- of flowers in October'2007 & others in Feb.'07.
Alstonia scholaris (Apocynaceae, commonly called Blackboard tree, Indian devil tree, Ditabark, Milkwood pine, White cheesewood and Pulai; syn. 
Echites scholaris L. Mant., 
Pala scholaris L. Roberty) is an 
evergreen, tropical tree native to the 
Indian subcontinent and 
Southeast AsiaAlstonia scholaris is a small tree that grows up to 40 m tall and is 
glabrous. The bark is greyish; 
branchlets are copiously 
lenticellate. The upperside of the leaves are glossy, while the underside is greyish.
[1] Leaves occur in 
whorls of 3-10; 
petioles are 1-3 cm; the leathery leaves are narrowly 
obovate to very narrowly 
spatulate, base cuneate, apex usually rounded; lateral veins occur in 25-50 pairs, at 80-90° to midvein. 
Cymes are dense and pubescent; 
peduncle is 4-7 cm long. 
Pedicels are usually as long as or shorter than 
calyx. The 
corolla is white and tube-like, 6-10 mm; lobes are broadly 
ovate or broadly obovate, 2-4.5 mm, overlapping to the left. The ovaries are distinct and pubescent. The 
follicles are distinct and linear. Seeds of 
A. scholaris are oblong, with ciliated margins, and ends with tufts of hairs 1.5-2 cm.
[2] The bark is almost odourless and very bitter, with abundant bitter and milky 
sap.
 
The bark contains the 
alkaloids ditamine, 
echitenine and 
echitamine and used to serve as an alternative to 
quinine. At one time, a 
decoction of the bark was used to treat 
diarrhoea and 
malaria, as a tonic, 
febrifuge, 
emmenagogue, 
anticholeric and 
vulnerary. A decoction of the leaves were used for 
beriberi.
[1] The bark is used in 
homoeopathy for its tonic bitter and astringent properties; it is particularly useful for chronic diarrhoea and dysentry. 
Ayurveda recommends 
A. scholaris for bowel complaints. In Sri Lanka its light wood is used for 
coffins. In 
Borneo the wood close to the root is very light and of white colour, and is used for net floats, household 
utensils, 
trenchers, corks, etc.
[4]