Al'ICAL MERISTEMS 25 



tunica and corpus zones is not always clear. Furthermore, the 

 relation of rib meristem and provascular areas to the young- 

 nodes and internodes is clearly shown in this genus (cf. Kudiger, 

 1939, and Ball, 1941). For comparative purposes, a study should 

 also be made of the shoot apices of various gymnosperms. Here 

 the choice of material is often very limited and hence specific 

 recommendations may be of little value. But the apices of vigor- 

 ous growing shoots of Picca, Abies, or Cedrus are readily sec- 

 tioned and all agree in the absence of the tunica-corpus type of 

 zunation characteristic of angiosperms. Instead, a small group 

 of initials is situated at the summit, from which arise two major 

 tissue-areas or zones, viz.: (1) an outer peripheral zone, which 

 produces the leaves, epidermis, cortex, and provascular tissue, 

 and (2) an inner or central tissue zone which produces exclu- 

 sively the pith. The possible phylogenetic significance of this 

 type of apex is discussed in several recent papers (Foster, 1939b, 

 1941a; Cross, 1939, 1941). Apices of Ginl-cjo (Foster, 1938) and 

 of some type of cycad (Zamia, Johnson, 1939; Cycas revoluta, 

 Foster, 1939a, 1940; Bioon, Foster, 1941b) are also worthy of the 

 student's time, particularly because of the interesting phylo- 

 genetic as well as morphogenetic problems which are raised by 

 their unique growth and structure. 



2. The Boot Apex. The apex of the root differs funda- 

 mentally from that of the shoot in the presence of a root cap. 

 The latter is a thimble-shaped or conical structure which occupies 

 the true physical apex of the root and which acts as a "buffer" 

 for the delicate meristematic tissue which is thus suhtcrminal 

 ill position. Great variation exists with respect to the histo- 

 genetic relationships between the root cap and the subtermiiial 

 meristem. Indeed, the differences are sufficiently evident to make 

 necessary the designation of a number of "types" of root apices 

 which are distinguished (1) by the mode of origin of the cap, 

 and (2) tlie relation of the various so-called "histogens" to the 

 origin of the primary tissue regions in the root proper. (Haber- 

 landt, 1914, pp. 89-94, and Hay ward. 1938. pp. 44-48.) It is 

 an interesting fact that while the highly deterministic scheme of 

 Hanstein (1868) has been largely abandoned for the shoot apex, 

 the structure and growth of the root apex is still generally inter- 



