384 



SCIENCE 



[N. S. Vol. XXXVII. No. 949 



each group appearing to occupy more space. At 

 this time no apparent change in the density or size 

 of individuals has taken place, but their axes now 

 lie in many directions. The nuclear membrane 

 appears and the nucleus has the form of an oval 

 with the shorter diameter in the direction of the 

 spindle. It may be that the spreading apart of 

 the chromosomes is for the purpose of facilitating 

 their growth, as they now appear somewhat larger. 

 Probably the chromosomes do not grow, but be- 

 come less dense, since they do not stain so intensely 

 at this time. The nucleoles make their appearance 

 as small oval bodies, from one to four in number. 

 These usually combine later, to form a single 

 nucleole. 



The chromosomes now appear to become angular 

 and to lose their curved form. Threads of linin 

 appear and portions of the chromatin appear to 

 be drawn out along these threads. No continuous 

 spirem is formed, however. The chromosomes now 

 come together in groups, forming various-sized 

 homogeneous, angular masses. This is the condi- 

 tion in which the chromatin is found in the greater 

 number of so-called resting cells in the Gentian. 

 There are also present small particles of chromatin 

 material along the linin threads which connect the 

 larger masses. There is no uniformity in the 

 number of the larger masses, and in this plant 

 there is no indication of prochromosomes, i. e., 

 there is no relation between the number of chro- 

 mosomes and the number of these masses. 



Each mass appears to be composed of smaller 

 fairly distinct bodies, but these smaller bodies do 

 not represent the chromosomes, since they are 

 much more numerous and vary in size. These 

 small bodies later become arranged along the linin 

 thread, forming a spirem. 



The spirem increases rapidly in diameter, takes 

 a homogeneous stain, and occupies a peripheral 

 position in the nucleus. Irregularities on the 

 surface of the spirem suggest the position of the 

 chromatin bodies of which it was made up. Seg- 

 mentation of the spirem follows and the chromo- 

 somes are formed. 



These are long at first, but soon shorten to rod- 

 like bodies, three or four times as long as broad. 

 They are distributed throughout the nucleus and 

 pairing does not appear to take place. 



The main points in the history of the chromatin 

 in the somatic cells of this plant are : 



1. The aggregation of chromosomes in the di- 

 aster, from which, later, the chromosomes separate 

 out. 



2. The absence of a dispirem. 



3. The presence in the resting nucleus of chro- 

 matin masses which vary in size and shape. 



4. The breaking up of these chromatin masses 

 into smaller fragments, more numerous than the 

 chromosomes. 



Physiological and Economic Significance of the 



Structure of the Tracheids of Conifers: I. W. 



Bailey, Harvard University. 



The so-called striated tracheids of conifers are 

 a specialized type of tissue structurally organized 

 to resist compression. Gothan 's hypothesis that 

 ' ' spiralstreif ung ' ' are spiral cracks confined to 

 heartwood, and are produced by chemical changes 

 and mechanical stresses in the transformation of 

 alburnum into duramen, is not substantiated by a 

 study of the origin and distribution of striations 

 in the various coniferous genera. 



Cracking or slitting of tracheid walls in drying 

 occurs sporadically and is confined to the so-called 

 summerwood. Tiemann in his "slit" theory of 

 the penetration of gases and preservatives into 

 seasoned wood has not taken into consideration the 

 important fact that drying-cracks do not rupture 

 the middle lamella and are confined to the sec- 

 ondary and tertiary walls. Injection experiments 

 show that the membranes of bordered pits in 

 freshly cut green sapwood are perforated and 

 permeable to gases, colloids and finely divided 

 solids held in suspension. 

 The Leaf Trace and Pitting of the Araucarinew 



and their Melation with those of the Cordai- 



talean Forms: E. B. Thompson, University of 



Toronto. 



The venation of the Araucarian and Cordai- 

 talean leaves is typically dichotomous, though in 

 some of the modern forms a false trichotomy has 

 been acquired. In both groups the dichotomous 

 condition persists in the secondary wood, double 

 traces extending to the pith in many instances. 

 The bundles of the double trace are far apart in 

 the seedling of Agathis. Ginkgo has been up to 

 the present the classical example of the double 

 trace in the secondary wood for comparison with 

 the Cordaitalean forms. The double trace in the 

 cortex of the AbietineK has been considered a 

 vestige of this condition. 



In pitting, the cone and root of the Araucarineas 

 show a more accentuated Cordaitalean character 

 than that of the stem. The ordinary tracheids of 

 the cone, for example, may have the pits as much 

 as 5-seriate and extending from end to end of the 

 tracheid. The ray-pitting of the tracheids retains 



