TISSUES AND SIMPLE ORGANS. 153 



may be a secretion in one part of the plant, and in another part it 

 may be a plastic substance. 



Concerning the physiological significance of many secretions, it 

 may be mentioned that the sweet secretions of the nectaries and 

 the etherial oils are of importance in cross-fertilization, due to the 

 attraction they have for the appropriate pollen -bearing insects. 

 The sticky secretion of the stigma serves to retain the pollen as 

 well as to aid in the formation of the pollen-tube. Resinous secre- 

 tions serve to cover and protect injured parts. Certain sticky se- 

 cretions from superficial glands serve to keep off injurious crawling 

 insects (KEENER). According to STAHL,' the acicular bundles of 

 calcium oxalate which occur so frequently in various tissues serve 

 as a protection against animals, particularly snails, that attempt to 

 feed upon the plants ; tannin serves a similar purpose. Sticky resin- 

 ous secretions sometimes unite the bud-scales (in winter) and protect 

 them against moisture and decay. Waxy coatings (example of use- 

 ful excretion) reduce the transpiration and evaporation of moisture. 

 The secretions of insectivorous plants must also be included here. 



The translucent spots on many leaves frequently indicate the 

 location of glandular structures, mostly internal glands as distin- 

 guished from external glands; two examples of the latter are 

 shown in Fig. 93. Besides the external and internal glands, we 

 shall refer more particularly to the duct-like secreting organs. 

 The resin- ducts of conifers (they occur in the wood, bark, and 

 leaves), the oil-ducts of the Umbelliferce, and the resin-ducts of 

 Cycas may be mentioned as the more important examples. 



Of the c ' excretions ' ' there are receptacles containing a mucous 

 substance ; again we find cells more or less filled with resin or oil, 

 receptacles bearing tannin or crystals, also the so-called " cysto- 

 liths ' ' occurring in Ficus. Receptacles for rnucus occur in the 

 Malvaceae. In the Apoidece, Composite, and Convolvulacece we 

 find resin- bearing tubes resembling the laticif erous tubes (DE BARY). 



In agreement with DE BARY 2 I wish to emphasize that not all 

 secreting organs are the result of cell- fusion ; many of them are 

 intercellular ducts and chambers. If they are formed by the 

 crowding apart of cells, they are said to be formed according to the 

 " schizogenous " method. Example: the resin-ducts of conifers. 



1 Pflanzeu und Scbneckeu, 1888. 



2 Comparative Anatomy. 



