802 



THE AMERICAN NATURALIST [Vol. XLI 



knowledge of these forms has been very materially increased during 

 the past decade. 



Among the most important of the more recent papers are those 

 of Chamberlain (The Ovule and Female Gametophyte of Dioon. 

 C. J. Chamberlain. Bot. Gaz., XLII, Nov. 1906, pp. 322-358. Pis. 

 XIII-XV. Preliminary Note on Ceratozamia. Ibid., XLIII, Feb. 

 1907, p. 137) and that of Caldvs^ell on Microcycas calocoma (Microcy- 

 cas Calocoma. O. W. Caldwell. Bot. Gaz., XLIV, Aug. 1907, pp. 

 118-141). 



The three genera treated in these papers are all peculiar to America. 

 Dioon and Ceratozamia being Mexican, while Microcycas is confined 

 to a limited district in the western part of Cuba. 



Professor Chamberlain made careful studies in the field, where he 

 collected a good deal of material, but his studies also included living 

 material sent to Chicago from the region where Dioon grows. The 

 latter is abundant in a region about twenty-five kilometers from 

 Xalapa, the capital of the state of Santa Cruz. Apparently Dioon is 

 confined to this very limited area. The plant much resembles Cycas, 

 but does not attain the dimensions of C. revoluta or C. circinalis as 

 these occur in their native habitats. The largest specimen seen had a 

 height of only three meters; but nevertheless it was estimated that 

 these plants were at least oiu^ thoiisiind years old. The growth is 

 excessively slow, and a ( jn-cful study of tlie rate of growth of plants 

 in cultivation has lev! io tliis ('xtnuirdiiiary estimate of the age of the 

 larger plants. 



The plants are said to fruit freely exery other year. The ovulate 

 cones are very large, sometimes weighing six kilograms or more, and 

 the large size of the sporophyll approximates that of Cycas, although 

 the sporophylls are arranged in a definite cone. The lower leaves of 

 the cone are sterile and there are intermediate forms between these 

 sterile leaves and those that bear the ovules. Each perfect sporophyll 

 bears two very large ovules, which may reach a length of four centi- 

 meters. The ovules do not ordinarily attain their full development 

 unless pollination takes place. 



Full details are given of the metlnxls iis.mI in stu.lyino- xUv develop- 

 ment of the ovule and there is also a ronipl.-te acc-nnt of it. morphol- 

 ogy. The qu(>sti..n of the possit.l.- douMr nat„n- nf tlu" ititcu'innent 



At the thuv of pollination thnv is a rorisidcrahle ainonnt of tissue 

 at the apex of the nncellus, al>()ve the embryo sac; but later this is 

 destroyed, partly by the growth of the pollen tubes and embryo sac. 



