58 MORPHOLOGY OF THE ANGIOSPERMS 



of "soft cells," to one that has also sclerenchyma cells of one or more 

 types variously arranged, and to a secondarily simple type, consisting 

 largely or wholly of sieve tubes and companion cells. 



Internal (or intraxijlanj) phloem, as well as that external to the pri- 

 mary xylem, is present in a considerable number of the more highly 

 specialized angiosperm families — the Myrtaceae, Solanaceae, Gentiana- 

 ceae, Asclepiadaceae, Onagraceae, Convolvulaceae, Campanulaceae, 

 Apocynaceae, Cucurbitaceae. This phloem, although always scanty, dif- 

 fers considerably in amount, and it lies within the primary xylem. 

 Although shown to function effectively in girdled tomato plants, it has 

 been called vestigial, perhaps because the sieve tubes are in small 

 isolated clusters. 



Phylogenetically, the presence of internal phloem is puzzling. In 

 ferns, amphiphloic structure is often prominent, and the internal tissue 

 is as well developed as the external. The absence of internal phloem 

 from the more primitive angiosperm families — some has been reported 

 for the Rosaceae — seems to make unlikely a derivation from amphi- 

 phloic ancestors. Its strong development in herbaceous genera in fami- 

 lies with many woody genera can perhaps be related to the presence of 

 the small amount of secondary phloem. Its absence in the most primi- 

 tive woody families seems unlikely to be the result of loss. Its wide- 

 spread presence must be evidence that it existed in ancestral taxa, and 

 the suggestion has been made that this is evidence that the angiosperms 

 are polyphyletic and one of the ancestral taxa was amphiphloic. But the 

 families with internal phloem do not seem to be related, even distantly. 

 The presence of internal phloem — a major anatomical character — must 

 play a prominent part in the search for the ancestors of the angiosperms. 



Strands of secondary phloem embedded in secondary xylem are 

 termed interxijlanj . The nature of this phloem is chiefly of ontogenetic 

 and histological interest, but its presence within the xylem has been 

 critically studied in only a few taxa. There are two methods by which 

 phloem becomes interxylary. In some genera, like Entada and Com- 

 bretiim, some strips of the cambium cylinder form phloem cells to the 

 inside for a brief period, then return to normal activity; the strips of 

 phloem are thus embedded in xylem, which was formed normally. By 

 the other method, sti-ands of phloem are normally formed by the 

 cambium, but the initials that formed them cease to function, and new 

 arcs of cambium develop external to the phloem strands and form 

 xylem, which encloses the phloem strands, as in Sfnjchnos. 



Phloem of peculiar origin is present in the complex tissues formed 

 inwardly by secondary or accessory cambia, as in the Chenopodiaceae. 

 In this family, the cambium forms both xylem and phloem to the in- 

 side, mostly in cell aggregates that suggest vascular bundles. 



