MORPHOLOGY 29 



in differences of form between the successive shoots. These differ 

 in appearance according to the special function performed by them, 

 whether nutrition, storage, or reproduction. In addition to the 

 essential members in the succession of shoots developed in a deter- 

 mined order, there are non-essential members which repeat forms of 

 shoot already present. These may appear simultaneously with the 

 essential shoots, and serve to increase the size of the plant as in 

 many annuals ; in many perennial plants they arise as yearly innova- 

 tions on the stock. Adventitious shoots, as a rule, repeat members 

 which have already made their appearance. Exceptionally, they 

 form necessary links in the succession of shoots ; this is the case in 

 the Podostemaceae (an aquatic order of Dicotyledons found in the 

 tropics, the plants belonging to which resemble Lichens or Liverworts 

 in external form), where they bear the flowers. 



The Habit or General Aspect of Plants is dependent upon the 

 origin, number, mode of growth, and duration of their branches, and 

 on the presence or absence of non-essential shoots. Cormophytes 

 which develop herbaceous aerial shoots, and persist only so long as 

 is requisite for the development and ripening of their fruit, be it one 

 or several vegetative periods, are called HERBS. Herbaceous plants, 

 however, which, although annually dying down to the ground, renew 

 their existence each year by means of new shoots produced from 

 underground shoots, rhizomes, or roots, are further distinguished as 

 PERENNIALS or perennial herbs. SHRUBS or TREES, on the other 

 hand, have woody, persistent shoots, which bear fruit repeatedly. 

 In these the reproductive shoots are shed annually, and, in some, 

 vegetative branches are also cast off, the remaining ones persisting 

 and increasing in thickness. In the Lime the ends of the leafy twigs, 

 in the Scotch Fir the short shoots, and in the Oak, Elm, Willow, and 

 Poplar weak lateral branches are thus lost. The leaves of evergreen 

 trees remain alive for several years, while those of deciduous species 

 only persist for a single vegetative period. 



Shrubs retain their lateral shoots, so that their branches are 

 formed near the ground ; trees, on the contrary, soon lose their lower 

 lateral branches, and have a main stem or trunk, which bears a crown 

 of branches and twigs. In many trees, shrubs, and herbs the main 

 shoot is vertical, while the lateral branches assume a horizontal 

 position, or are directed obliquely upwards or downwards. In other 

 cases the main axis is sympodial, a lateral branch continuing the 

 direction of growth of the primary shoot. Sometimes a main axis 

 is indistinguishable among the group of similarly directed branches. 

 The general appearance of the plant is determined by the direction 

 and thickness of its branches and leafy twigs. If these are all 

 directed upwards the shape is pyramidal, while broadly pyramidal, 

 oval, and rounded forms arise when the branches diverge more 

 strongly. The " weeping varieties " of several familiar trees are due 



