330 BOOKS AND CURRENT LITERATURE 



even chance that further frosts will occur. The dates are also given 

 when the risks are 33|%, 20%, 10% and 5%. A third table gives the 

 frostless season, and with this as a guide and his knowledge of the 

 number of days it takes to mature a particular crop, the farmer may 

 determine the chance of success. 



Mention is also made of the influence of the Great Lakes. Along 

 the shore of Lake Superior, frosts occur no later in spring than in most 

 portions of the Lower Peninsula and autumn frosts are delayed by the 

 great mass of water that has been heated during the summer season 

 and has been slow in cooling. Consequently the length of the growing 

 season along Lake Superior is about the same as it is in the central 

 counties of the Lower Peninsula. Along the shores of Lake Michigan 

 the autumn frosts do not occur until about October 20, while 50 to 60 

 miles inland they come a month earlier. Elevation shortens the grow- 

 ing season. The highlands of the Upper Peninsula and the northern 

 counties of the Lower Peninsula have a later spring frost and earlier 

 autumn frost than the lake counties on the southern tier of counties 

 in the Upper Peninsula. — R. P. Hibbard. 



The Embryology of Pinus. — Buchholz^ has studied the develop- 

 ment of the suspensor and early embryo in cones of Pinus hanksiana, 

 P. laricio, P. sylvestris and P. echinata, collected weekly during the 

 summer months, and his results change materially the general con- 

 ception of the embryology of the genus. The primitive character of 

 polyembryony due to the cleavage of one egg (as contrasted with 

 polyembryony due to the occurrence of several archegonia in one 

 ovule) which is retained by several genera of Gymnosperms, is much 

 more extensive in Pinus than has been indicated in published de- 

 scriptions. A fertilized egg in the species enumerated above usually 

 produces eight embryos, of which four are the primary embryos com- 

 monly figured in texts, arising from the lowest tier of cells of the pro- 

 embryo, and four are developed from the rosette cells just above the 

 suspensor layer, which thus are shown to be really embryo initials. 

 Rosette embryos are somewhat delayed in development and they 

 probably never contribute the persistent embryo of the seed. Both 

 primary and rosette embryos develop by the division of a fern type of 

 apical cell which is evident in primary embryos up to the 500-700 



' Buchholz, John Theodore. Suspensor and early embryo of Pinus. Bot. 

 Gaz. 66: 185-222. pis. 6-10. 1918. 



