144 A TEXTBOOK OF BOTANY [Ch. IV, 6 



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

 axillarj^ to leaves, the brandling of phmts should correspond 

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

 as shown by young twigs ; Isut 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 familj- ; and thus is 

 explained the wonderful phjdlotactic S3rmmetry 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 primar}' 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 Bryophj^tes, 

 composed altogether of parenchyma cells without anj-, or 

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

 are passed directly frpm 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. 



