MICROSCOPIC STRUCTURE OF WEED SEEDS 575 



Common Sunflower (Helianthus annuus L.). 



The microscopic structure of the fruit and seed has been studied 

 by Hanausek, Harz, Winton and Moeller. The obovoid achenes 

 are more or less four-sided. The pericarp in some varieties is 

 nearly black, in others it is striped with white and black. The 

 pericarp consists of thin porous walls which are dark colored in 

 the black seeded varieties, although in the varieties with striped 

 seeds only a part of the cells are colored. Some of the cells are 

 elongated, forming duplex hairs, which are attached to what 

 Hanausek called the "foot cell." The underlying hypodermal 

 cells of 4-6 rows of cells are thick-walled and porous, the cells ar- 

 ranged in rows. These contain the blackish pigment, pitchlike in 

 character. The third layer consists of thick-walled sclerotic 

 parenchyma cells. These cells are more or less isodiametric. This 

 layer contains the fibrovascular bundles which occur adjacent to 

 the thin-walled parenchyma cells. Extending into the sclerotic 

 parenchyma are radial rows of thinner- walled parenchyma cells. 

 This layer is followed by large thin-walled parenchyma cells. The 

 testa consists of thin-walled loose parenchyma cells. The outer or 

 epidermal cells are roundish and have obscurely beaded walls; 

 the spongy parenchyma follows and contains the fibrovascular 

 bundles. The spongy parenchyma is followed by the rectangular 

 cells of the inner epidermis. The endosperm consists of one or 

 two rows of aleurone cells. The epidermal layer of the embryo 

 consists of small, rather thin-walled cells of the cotyledons and 

 underneath this on the upper surface are several rows of palisade 

 cells. These cells contain irregular spherical aleurone grains larg- 

 er than those in the epidermal cells. 



Crownbeard, (Verbesina hdianthoides Mx.). 



The microscopic structure of the achene includes a series of small, 

 rather thick-walled epidermal cells, followed by elongated or short, 

 rather thick-walled parenchyma cells, The pigment layer is com- 

 posed of thick-walled cells, whose walls contain a blackish pigment. 

 The testa consists of two layers ; an outer of elongated, thick- walled 

 cells and an inner layer of shorter cells also colorless. The cells 

 of the embryo are much larger and contain protein grains. 



Boot-jack, Spanish Needle (Bidens discoidea (T. & G.) Britton). 



The pericarp consists of an outer epidermal layer underlaid by 

 a similar layer of elongated, thick-walled cells; between which are 



