DALBEY: ANATOMY OF GRINDELIA. 39 



Toward the base of the bract the outline approaches triangu- 

 lar, being flattened on one side. Vascular bundles consisting 

 of a large amount of phloem (fig. 35, I) occur near the cen- 

 tral part of the bract, and in cross section are arranged in 

 the shape of an arch. (Fig. 35.) The upper part of the bract 

 contains large masses of mesophyll cells separated by water- 

 storage tissue, while the lower part is made up of more 

 scattered masses of mesophyll and a larger amount of water- 

 storage parenchyma tissue, which often contains large aerat- 

 ing spaces. Resin ducts are found in the parenchyma ad- 

 jacent to each bundle, the central one reaching a diameter of 

 .03 mm. The mesophyll cells appear to contain more chloro- 

 plasts than the mesophyll of the leaf. 



The epidermis of the bract is the same in structure as the 

 epidermis of the leaf, and contains stomata and a large num- 

 ber of resin glands similar to those on the leaf. There is a 

 thick cuticle layer covering the epidermis. 



SUMMARY. 



1. 'The stem, leaves and involucre scales are covered with an 

 aromatic sticky exudate. 



2. This exudate comes from multicellular superficial glands 

 that occur as frequently as 1.2 to the sq. mm. on the upper 

 surface and 3.4 to the sq. mm. on the lower surface of the leaf. 



3. The exudate appears to be essentially a gum-resin, some 

 of its substance being soluble in water, some in ether, alcohol, 

 and in xylene, and it stains as mucilage does with methylene 

 blue, and as oils and resins do with alcannin and Sudan III. 



4. Where the glands occur at the tips of the marginal serra- 

 tions clusters of tracheids extend to them from the marginal 

 veins, and where they occur over the surface of the leaf they 

 are subtended by masses of water-storage cells. 



5. These water-storage cells compose about one-third of the 

 mesophyll ; they surround the veins, and often extend from one 

 surface to the other. 



6. It would seem from their position and the nature of their 

 exudate that the glands might serve in the absorption of water 

 from dew or rain. The gum of the exudate could imbibe the 

 water and give it over to the storage tracheids and parenchyma, 

 and the resin could protect against evaporation. 



