EXOGENS. 141 



with leaves, if a thread is passed from one to the other, 

 turning always in the same direction, a spiral is de- 

 scribed, and a certain number of leaves and of complete 

 turns occur before reaching the leaf directly above that 

 which began the series. This may be expressed by a 

 fraction, the numerator of which indicates the number 

 of turns, and the denominator the number of leaves in 

 the spiral cycle. In the Peach and Plum-tree the cycle 

 embraces five leaves, and the spiral goes twice around 

 the branch. This is expressed by the formula f . In the 

 Alder three leaves constitute the cycle, and the spiral 

 has only a single turn on the stem ; the disposition of 

 its leaves is represented by the fraction . 



In Exogenous plants the leaves are reticulated, and 

 usually articulated to the stem. The flowers are formed 

 upon a quinary or quaternary type ; that is, their parts 

 are in sets of fives or fours, instead of sets of threes, as 

 Endogens. The embryo has two opposite cotyledons, 

 or seed-lobes, which gives the term Dicotyledonous to 

 the type. 



2. According to the natural system of De Candolle, 

 w-hich is usually followed, Exogens are subdivided as 

 follows : 



i.) THALAMIFLOR^:, (Thalamus, receptacle, and flos, 

 flower.) Calyx and corolla present ; petals distinct, in- 

 serted into the receptacle ; stamens hypogynous, or 

 growing from below the ovary, as Ranunculus, Magno- 

 lia, Poppy, Violet, Geranium, etc. 



2.) CALYCIFLORA. A calyx and corolla present, the 

 petals distinct, but the stamens are perigynous, or at- 

 tached to the calyx ; as Rhamnus, the Leguminose fam- 



