472 



ANNUAL REPORT 



DISSEMINATION OF WEEDS. 



We can conveniently make two modes: 



1. Natural agencies — Wind, animals, water, snow, ex- 

 plosive properties of the fruit. 



2. Where man is chiefly concerned — Impure seeds, railroads 

 and other carriers, planting for ornamental purposes. 



In most cases the fruits and seeds of our first division have 

 certain structures in form of burs, hairs, etc., which enable 

 them to be disseminated. 



In many of the composite, the calyx, the outer set of the 

 floral envelopes, is made up of a collection of fine, thread-like 

 bodies, collectively called the pappus. In Prickly Lettuce {Lac- 

 tiica scariola, L. ) The acheuium (a one-seeded seed like fruit), 



L/f.P del. 

 MM-sc 



3. (Bidens Connata) h. Aclienium 

 with its awns barded downwardly. 

 a. One of the awns more highly mag- 

 nified. 



Trichoiiies of (Desiuodium canes cens) a. 

 Hooked trichoma, b. Ordinary trichome. e. 

 Epidermal cells magnified 122 times. 



is carried out into a long beak bearing the pappus at its end. 



See Fig. 1. In another composite, The Pasture Thistle (Cnicus 

 pumilum, Spreng. ) The pappus is made uj) of numerous fine and 

 long threads which are plumose. See fig. 2. Such fruits are easily 

 carried about by the wind, the akenes being in most cases light. 

 The seeds or fruits disseminated by animals, at least among most 

 of our weeds, are hooked and barbed. In Bidens connata the 

 acheniu m bears three awns which are barbed downwardly. See 

 Fig. 3. In Echinosjyermum lappula the small nutlets (forming 

 burs which are not seeds but quarter portions of seed-like 

 fruits — 6 b), are covered with a double row of grappling organs. 

 See Fig. 5. These burs easily fasten themselves to any passing 



object. 



In Tick Trefoil (^Desmodium canescens, D C), a troublesome 



roadside weed in the South, the pod breaks up into several 



transverse joints, and by means of the small trichomes (plant 



