144 A TEXTBOOK OF BOTANY [Cn. IV, 6 



Since new buds, which give origin to new branches, are 

 axillary to leaves, the branching of plants should correspond 

 with their phyllotaxy. This, indeed, is true in principle, 

 as shown by young twigs; but as plants grow older the 

 regularity of their branching becomes greatly disturbed by 

 irregular shading and diverse natural accidents. Flowers 

 always originate from axillary buds, and hence clusters of 

 flowers also exhibit the plans of phyllotaxy. This becomes 

 especially striking when flowers are condensed closely 

 together in heads, as in the Composite family ; and thus is 

 explained the wonderful phyllotactic symmetry of Dahlia 

 flowers, and of the head of a Sunflower in "seed" (Fig. 100). 

 Other structures which show such symmetry strikingly well 

 are cones of various trees, plants of compact growth, like 

 the Mamillaria of the Cactus family, and various rosette 

 plants. In all of these cases the primary spiral is difficult 

 to trace because of its condensation ; but incidentally there 

 arise a number of secondary and tertiary spirals, and these 

 it is which become so strikingly evident. 



6. THE TRANSFER OF WATER AND FOOD THROUGH PLANTS 



A secondary function of stems is the conduction of water 

 from the roots to the leaves, and of food from the leaves to 

 the roots. 'We now consider the method of these important 

 processes. 



In the lower plants, the Algae, Fungi, and Bryophytes, 

 composed altogether of parenchyma cells without any, or 

 with only a rudimentary, system of veins, both food and water 

 are passed directly from one cell to another. The process is 

 a slow one, and in land plants prevents any great develop- 

 ment of size, as the very low growth of all Bryophytes, or 

 Moss plants, exemplifies. The higher plants, however, both 

 Flowering plants and Ferns, have developed veins, or vas- 

 cular bundles, which permit the comparatively rapid trans- 

 fer of both water and food through long spaces of stem, thus 

 rendering possible the growth of those plants to tall trees. 



