1908] THIESSEN—DIOON EDULE 369 



age has developed about nine or ten leaf primordia, but only about 

 one-third have developed into leaves. 



Internal structure. — During germination, in connection with 

 the development of the primary root, not all of the tissue of the tip of 

 the plantlet resumes growth, but only the plerome and a limited 

 portion of the periblem or cortex surrounding it becomes meristem- 

 atic and pushes through the hard and caplike tissue at the tip of 

 the embryo. The cortex which does not resume growth, as well as 

 the caplike tissue, frays off (fig. 9, //'). Also, as growth progresses, 

 the outer layers of the newly formed root, some distance back of the 

 root tip, keeps on fraying off; and underneath a phellogen soon 

 appears and a layer of cork several cells thick is formed. Along each 

 of the four protoxylem strands of the root, continuous from the 

 vascular plate, lateral roots have their origin at definite intervals, 

 being arranged in four rows. Certain cells among and near the 

 protoxylem elements become meristematic, and form the tip of the 

 lateral root, which pushes through the cortex. 



The vascular cylinder increases in dimensions uniformly with the 

 growth of the seedling as a whole. New bundles are inserted as new 

 leaf primordia appear, and gradually fill in more and more the 

 vacant spaces between the original strands, so that immediately above 

 the original plate the vascular cylinder is quite compact, while beyond 

 this it continues to be represented by separated strands in the potential 

 vascular tissue. 



The vascular strands of the cotyledons. — Not much need be added 

 to the statements in reference to the cotyledonary strands. The 

 transition from endarch to exarch xylem is very much more gradual 

 and the mesarch stage is located relatively much farther up. This 

 is due to the fact that most of the growth of the cotyledon in length 

 occurs at its lower extremity. No secondary wood is developed in 

 cotyledons. The phloem also has increased in bulk, but mature sieve 

 tubes are never developed. 



The leaves and scales. — As said above, not all the primordia de- 

 velop into leaves, most of them remaining abortive and forming scales. 

 Although about one-third of the primordia develop leaves, it does not 

 follow that every third primordium becomes a leaf in regular succes- 

 sion. It sometimes happens that two leaves are developed from 



