CONSTANCE; SYSTEMATICS OF THE ANGIOSPERMS 433 



time of spore discharge, and whether the grains are borne singly, or cohere in 

 tetrads or pollinia (Engler; Wodehouse, 1928, 1935; Tischler, 1929; Schnarf, 

 1931, 1933, 1937b; Maheshwari 1945, 1949, 1950; Erdtman, 1953). The modes of 

 origin of the tapetum were utilized by P. Clausen (1927) to confirm relationship 

 between Alismatales and Ranales. In general, dicotyledons have a simultaneous 

 division of microspore motlier cells resulting in tetrahcdrally arranged tetrads 

 of spores, whereas monocotyledons show successive divisions producing a non- 

 tetrahedral conformation (Erdtman, 1943), but deviations in this rule appear 

 to show affinity between Alismatales and Eanales ( Suessenguth ) . Whether the 

 pollen grain contains two or three nuclei when shed has been used to recom- 

 mend the exclusion of Heliotropioideae from Boraginaceae (Schnarf, 1937a) and 

 taxonomically within Labiatae (Leitner, 1942). 



Assumptions of affinity based on such individual characters should be judged 

 in the light of facts that a single pollen feature or type may be widespread in 

 related or even unrelated groups, that even closely allied groups may show 

 great discrepancies in microspore characteristics, and that convergences and 

 parallelism are as frequent and confusing here as in other structures (Wode- 

 house, 1928, 1935; Pope, 1925; Erdtman, 1953). Consequently, indications of 

 affinity are doubtless more reliable if based on a spectrum of pollen characters, 

 such as embodied in the palynogram of Erdtman (1953), or correlated with other 

 aspects of plant structure (Heimsch, 1940; Hedberg, 1946; Dahl, 1952). Pollen 

 features have been used to relate KhoipteUa to Juglandaceae (Withner), to re- 

 align members of Liliaceae and Amaryllidaceae (AVunderlich, 1936), to assign 

 genera of uncertain affinity within Sterculiaceae (C. V. Rao, 1950), to underline 

 a connection between Myristicaceae and Annonaceae ( Joshi, 1946), and to clarify 

 relationships in Ericales (Copeland, Doyel and Goss, Kavaljian). The degenera- 

 tion of three microspores of the tetrad in Cyperaceae has been suggested as add- 

 ing to the distinctness of that family, while at the same time relating it to Junca- 

 ceae and separating it from Gramineae (Engler; Wulff, 1939; Wahl, 1940; Ma- 

 heshwari, 1949). Considerable attention has been paid to size of pollen within 

 a restricted group as an indicator of polyploid level (Erlanson, 1931, 1934), but 

 it is also clear that this approach must be employed with caution (Bell, 1954). 

 The occurrence of dimorphic pollen in the same taxon of heterostylic plants 

 (Baker, 1948) has long been known, and Johnston (1952) finds two basically 

 different types of grain in LitJiospermum, a feature which he turns to advantage 

 taxonomically. 



Perhaps the most important data of phylogenetic significance emerge from 

 the distribution of the single-furrowed type of grain, which is characteristic of 

 Seed Ferns, Bennettitales, Cycadales, Ginkgoales, and most monocotyledons. 

 Hallier regarded this type and the occurrence of permanent tetrads as primitive 

 in angiosperms. Although the dicots normally have tricolpate pollen (or modifi- 

 cations thereof), there are now known some twenty groups of dicots with the 

 basic monocolpate pollen (or such derived forms of it as are found in Trimenia 

 and some Chloranthaceae, Nymphaeaceae, Cabombaceae) — all of them in the 

 Ranales (Hallier; Pohl, 1928; Wodehouse, 1936a; Bailey and Nast, 1943a, 1945a, 

 1948; Bailey, 1949; Money, Bailey and Swamy; Canright). The Ranales, then, 

 are the sole group of angiosperms known to have both monocolpate and tricol- 

 pate pollen, the retention of the former presumably being an archaic character. 



