THE STEM 191 
There are two plans of branching in trees. When the trunk, 
or main stem, extends vertically upward to the tip, as it does in 
the pines, junipers, spruces and other conical trees, the type of 
branching is called excurrent; when it divides into several more or 
less equal divisions as in the elm and other spreading trees, it is 
said to be deliquescent. ‘The deliquescent plan is the more com- 
mon one among our deciduous trees. 
Fic. 121.—A, tunicate bulb of Onion (Allium Cepa), in longitudinal section; 
B, Scaly bulb of Canada Lily (Lilium canadense); C, Same as B, but in longitudinal 
section; D, Bulb of Garlic (Allium sativum) with portion of the scales cut away so 
as to enclose the young bulbs within; E, Portion of stem of Tiger Lily (Lilium 
tigrinum), showing bulbils in the axils of the leaves; G, Corm of Crocus showing a 
terminal and one lateral bud. The horizontal lines represent the scars of the few 
scales that have been removed. At the base is the remnant of last year’s corm. 
The various buds will develop into new corms. F, Vertical section of a familiar 
corm. H, flat corm of Cyclamen; I, Bulblets at the apex of a flower stalk of the 
Onion. (After Gager.) 
A Surus is a perennial woody plant of smaller stature than a 
tree whose stem is relatively short and usually branches near the 
ground. Examples: Viburnum, Elder, Buchu, Krameria, etc. 
Fatt or Leaves.—The fall of leaves in autumn from our 
deciduous woody dicotyledons and gymnosperms is caused by the 
formation beneath the leaf stalks of an abscission or cut-off layer 
or band of delicate cells. ‘The cellulose walls of these break down 
into pectic acid and pectin which forms a mucilaginous substance, 
the middle lamellae between the cells disintegrates, the veins 
become ruptured and the leaf falls of its own weight. A protec- 
