1 124 -^ TEXTBOOK OF THEORETICAL BOTANY 



tunica layer (Tg) in exactly the same way as the foliar primordia on a stem.* 

 Intercalary growth in this rim produces the perigynous disc, which is 

 therefore of foliar origin. Alternating with the sepals, the petal primordia 

 also arise in T.,. There is sufficient variation in the position of the primordia 



Fic. 1098. — Riibiis rosnefoliits. On the left is an apex still in the vegetative state showing 

 the last foliage leaf primordium. On the right an apex passing over into the flowering 

 state showing sepal primordia with virtual elimination of the plastochrones. (After 

 Engard.) 



of the perianth members to indicate that plastochrones have not been 

 entirely suppressed, but this initial asymmetry disappears in later develop- 

 ment. The sepal formation is the last act of the vegetative stage and the 

 production of the stamen-like petal rudiments marks the beginning of a 

 new phase, or what has been called a " new route " in development. 



The stamens also originate in the T2 layer and the staminal archesporium 

 develops from the Tj layer of the stamen itself, so that the pollen possesses 

 the characters of the second tunica layer of the apex, which may, as has 

 been shown in chimaeras, be different from those of the first layer. 



The carpels are also T., structures. Each appears first as a simple hump, 

 in which the first procambial strand to appear becomes the abaxial or dorsal 

 trace. The ovarial cavity is produced by wing-like expansion of the adaxial 

 margins, forming a gradually deepening groove. The ovules begin in the 

 T2 layer of these wings and the nucellus and embryo sac belong to the 

 T2, but the ovular integuments belong to Tj. In Riibiis, the vascular system 

 of the floral apex is essentially that of the vegetative apex, but shortened. 



We have not yet got so detailed an account of the above process in many 

 other cases, but the information available shows that in a number of other 

 flower rudiments the distinction of tunica and corpus persists from the 

 vegetative into the floral apex. In Vinca there are three types of apex: 

 I. The juvenile vegetative apex which forms leaves only. 2. The adult 

 apex which forms both leaves and flowers. 3. The floral apex which forms 

 only a flower. All three are morphologically alike, with two tunica layers 



* See Volume I, p. 846, for a discussion of the apical structure of the vegetative stem. 



