'^- 



-iM. 



Figure 2. Trimorphic flowers of purple loosestrife 

 only one kind of flower on an individual plant, each with one 

 pistil, and two whorls of stamens, being either long, medium, 

 or short, seed-set is higher when the pistil receives pollen from 

 stamens of similar length, as shown by arrows (after Heywood, 

 V. H. ed., 1993, Flowering Plants of the World). 



... All were guaranteed to 

 be sterile ... I closed my 

 home in October while 

 they were still in bloom. 

 Imagine my surprise the 

 next spring when, upon re- 

 turning to the house, I 

 found hundreds of volun- 

 teer seedlings in every bar- 

 rel, planter, and surround- 

 ing garden." It would ap- 

 pear that loosestrife follows 

 no rules. 



However, there is an ex- 

 planation to these seem- 

 ingly contradictory observa- 

 tions, and it is found in the 

 plant's specialized method 

 of reproduction. 



Purple loosestrife (Figure 2) has three types of flowers, 

 with only one type per individual plant. These trimorphic 

 flowers each differ by having the pistil varying in length 

 (heterostyly), being either short (3-5 mm), medium (7-8 

 mm), or long (9-12 mm). Each type of flower also has two 

 different whorls of stamens that also vary in length, but are 

 never the same length as the pistil or each other. These 

 can be observed easily without magnification. 



Seed-set is highly favored when a pistil receives pollen 

 from stamens of similar length (these kinds of crosses are 

 called "legitimate"!; if pollination occurs in a different man- 

 ner ("illegitimate crosses"), seed-set is extremely sup- 

 pressed. Therefore, the pollination strategy of L. salicaria 

 strongly favors outcrossing among individual plants having 

 different flowers, and this reproductive barrier is called self- 

 incompatibility, or SI. 



This interesting facet of nature was first recognized by 

 the Dutch botanist, Clusius, in the 1500s and later became 

 the fascination of a certain English biologist named Darwin. 

 The natural selective advantage of having three flower mor- 

 phs, as opposed to only two, is that all individuals will 

 have two-thirds of the population available for successful 



outcrossing, instead of only 

 half, thus increasing the odds 

 of each flower bearing a maxi- 

 mum seed-set. Perhaps the 

 difference between two-thirds 

 and half of the population 

 may not seem like a lot, until 

 you consider that an indi- 

 vidual plant can have as 

 many as 3,000 flowers. 



It is now clear how some 

 people may have had differ- 

 ent experiences with "sterile" 

 varieties of Lytkrum. However, 

 you still would be justified in 

 asking if it isn't possible that 

 some of the available culti- 

 vars of Lythrum might actually 

 be safe and sterile. To ad- 

 dress this question, Drs. Neil O. Anderson and Peter D. 

 Ascher, Department of Horticultural Science, University of 

 Minnesota, have conducted thorough investigations on the 

 fertility of purple loosestrife and several of the cultivars of 

 Lytkrum. I contacted Dr. Anderson and he was very glad to 

 share his results, as well as explain the misuse of the term 

 "sterile:" 



The confusion regarding whether or not these culti- 

 vars are "sterile" arises from a misunderstanding of 

 terminology. All Lylhrum species and cultivars possess 

 self-incompatibility (Sl| An old term for SI is "self-ste- 

 rility" Unfortunately, when this latter term is used, it 

 is easily confused with true genetic sterility We now 

 have enough evidence to say that no purple loos- 

 estrife cultivar is sterile Most cultivars are SI, but 

 they produce large quantities of seed when used as 

 either male or female parents in crosses. In several 

 cases, the cultivars are as fertile as the L salicaria 

 populations 



Some growers still might protest that it is absurd to con- 

 sider a few garden specimens of purple loosestrife a threat 

 to the environment. However, they should consider that ev- 

 ery plant has the potential of producing 1.5 million seeds 



Distributor of: 



Tree 



o 



Saver 



supplying Nursery stock for 

 Landscape contractors & Garden centers 

 from many excellent growers 



Call us to Discuss Your Needs 



P.O. Box 64 Mdndoe Falls, VT 05050 



Tree Staking System 

 Finally, a staking system that solves all 



your staking problems. 



Safe Reliable Safe for Tree 



Quick to Install Cost-effective 



Tel. 800-639-1722 



FAX 802-633-2349 



AUGUST ♦ SEPTEMBER 1996 



23 



