376 PLANT PHYSIOLOGY 



together at the cut surface and ultimately causes the joined sur- 

 faces to knit into one connected whole; and it is well to get as 

 much cambium together as possible. From the results of grafting 

 some very interesting physiological data have been obtained. 

 Thus Vochting (1894) and Daniel (1921) have shown that when 

 the sunflower (Helianthus annuus), which produces starch as the 

 chief reserve carbohydrate, is grafted to a stock of the artichoke 

 (H. tuberosus), which produces inulin, each plant continues the 

 production of its own characteristic reserve regardless of the kind 

 of material furnished it from the leaves above. The sunflower at 

 the top may form starch and send down sugar to the roots, but 

 in the artichoke at the base this is stored up as inulin. The same 

 individuality is shown when the graft is reversed. 



Sexual Reproduction. — In sexual reproduction, the union of 

 two sex cells called gametes is required. This type of reproduction, 

 which requires a sexual union of two cells, is also called digenetic. 

 The development of sexual reproduction has not occurred suddenly 

 in the evolution of plants but has evolved by gradual steps. In 

 the blue-green algae all reproduction is asexual. Sexual repro- 

 duction first appears with the green algae in forms like Spirogyra, 

 where the gametes are not special cells but are transformed veg- 

 etative cells that unite in pairs to form the zygospores. The 

 gametes look alike (although York thinks the male filaments are 

 less vigorous and contain less starch than the female) but behave 

 slightly differently. One (the female) remains passive in its cell 

 while the other (the male) moves through the conjugation tube 



into it. 



In the black mold (Rhizopus nigricans) the gametes look alike 

 and behave alike, meeting in the middle of the conjugation tube; 

 but Blakeslee has shown that the plants from which they come 

 are either male or female strains, and before conjugation can re- 

 sult, it is necessary to bring together plants of opposite strains 

 or sexes. 



In Spirogyra the gametes are transformed vegetative cells and 

 in Rhizopus they are single cells cut off from the vegetative hy- 

 phae. However, in Vaucheria, another green alga, sex organs 

 which contain the gametes, make their appearance. In this genus, 

 the sex organs are on the same plant, as is true in most of the 

 Thallophytes, but in many Bryophytes like Marchantia the sex 

 organs are on different plants, one of which bears a male sex organ 



