152 COMPENDIUM OF GENERAL BOTANY. 



ciple is very evident from the fact that the cell-rows are arranged 

 along the lines of greatest tension (Fig. 92). 



AVhen the seed of Anethum Sova (Fig. 92, C) swells during ger- 

 mination, it increases considerably (28 per cent) in the diameter ah; 

 in this diameter there is also a maximnm pressure of the soil ; the 

 diameter in the horizontal direction increases only 11 per cent. The 

 taiiirential-loneitudhial section of Iris-seed shows the mechanical 

 curves. In cross-section we see these lines radiate from the embryo, 

 where they are evidently of physiological, not mechanical, signifi- 

 cance. Haberlandt has made a special study of storage-tissues and 

 the mechanical and physiological arrangements just referred to.' 

 In mytreatment of this subject I have adhered to Haberlandt's 

 interpretations. 



XI. SECRETION. 



The products of plant-metabolism Avhich cannot he further uti- 

 lized i7i thejjlant-economy, and which do not form a part of the 

 cell (as, for example, the cell-wall), are, in general, designated as 

 secretions. In this collective noun I include secretions in the nar- 

 rower sense as well as excretions.^ 



We may designate all those products formed from special or- 

 gans — the organs of secretion, or glands — as secretions, in the nar- 

 rower sense. ' ' Excretion ' ' is not the product of a specific organ ; 

 the waste material collects in certain cells not united to form a dis- 

 tinct structure, while true secretion is invariably associated with an 

 apparatus marked by specific anatomical peculiarities (Haber- 

 landt). 



Our imperfect cliemical knowledge of the subject does not per- 

 mit us to give any detailed description of the phenomena under 

 consideration. We shall l)riefly consider secretion in general. 



The saccharine solution in the nectaries of flowers, the resin of 

 conifers, the etherial oils, many of the formations of calcium oxa- 

 late, the tannin in many cells, and the water of transpiration are all 

 products not required in further metabolic processes. 



We can see the utility of many of these sul^stances and their 

 great importance in plant-life ; therefore secretion does not imply 

 a useless product. It is also evident that a substance, as sugar, 



1 Haberlandt, Physiologische Ptlanzea-Anatomie. 

 ' Ibid. p. 320. 



