34 



THE CELL AND PROTOPLASM 



in certain types of plants may be formed 

 of other substances {e.g., of ehitin, as in 

 Phycomyces, whose structure has been care- 

 fully investigated by E. S. Castle). The 

 niicroeapillaries of the secondary wall may 

 be filled with liquid, or with lignin, cutin, 

 suberin, hemicelluloses,^ or a great variety 

 of other organic compounds, and even at 

 times with crystals, such as silica. 



It is advisable to raise the question, how 

 may one be certain that the structural pat- 

 terns visible under the microscope are not 

 artifacts, produced for example by stresses 

 and strains in an expanding disk-shaped 

 section? There are four lines of evidence 

 which should be emphasized in this connec- 

 tion : 



(1) Specific, complex structural patterns 

 whose coarser details are clearly visible in 

 unswollen material are characteristic of 

 particular types of cells and of specific 

 types of plants. 



(2) Fragments of the secondary wall, 

 whatever their shapes and siz3s, exhibit, 

 when swollen, their own particular part of 

 the general structural pattern from which 

 they were remo^'ed. 



(3) During the earlier stages of forma- 

 tion of the secondary wall, it may be ob- 

 served that the cellulose is deposited in the 

 form of coalesced fibrils (Fig. 8). 



(4) In the case of cotton, the hairs elon- 

 gate for a certain number of days; then the 

 secondary wall is formed during a period of 

 from twenty to forty days. By collecting 

 material each successive day after a known 

 date of flowering, Kerr has shown that two 

 lamellae of varying porosity are formed 

 during each 24 hours, a denser lamella 

 being deposited during the day and a more 

 porous one during the night. The width 

 and the porosity of these lamellae fluctuate 

 more or less with variations in environ- 

 mental conditions. In the cotton fields near 

 Raleigh, North (-ai-olina, characteristic pat- 

 terns of lamellae are frequently formed 

 wliicli ai-e closely correlated with sjiecific 



^ A. G. Normnii lias suf^gestcd tliiit in Mic c.-isc of 

 the "celluloHiUis" the ch.-iin hkiIcciiIcs of hcnii 

 cellulose may lie associated witli cliaiii iiiolcculcs of 

 cellulose witliin the micelles. 



variations in temperature (Figs. 9 and 10). 

 Thus, the hairs of different cotton plants 

 which have grown during the same period 

 of varying environmental influences ex- 

 hibit identical patterns of lamellae, and it is 

 possible to cross-date daily growth rings, 

 much as the annual rings in the stems of 

 western yellow pine may be cross-dated. In 

 addition, Anderson and Kerr have shown 

 that lamellation of the secondary wall of 

 the cotton hair may be eliminated by grow- 

 ing jilants under constant light and tem- 

 perature (Fig. 11). By varying these en- 

 vironmental factors, lamellae may be in- 

 duced to form as desired (Fig. 12). 



Since the concentricities of secondary 

 walls are not artifacts, they provide a con- 

 venient means of determining the approxi- 

 mate diameters of the microfibrils. If the 

 diameter of the unswollen wall is carefully 

 measured and the number of constituent 

 lamellae in a swollen section is counted, a 

 fairly close approximation of the diameter 

 of the lamellae in their unswollen state can 

 be obtained by dividing the diameter of the 

 unswollen wall by the number of constituent 

 lamellae. The finer types of lamellae grade 

 down to 500 A or less in diameter. Since 

 such lamellae are composed of a single layer 

 of coalesced microfibrils, these threads, in 

 their unswollen condition must likewise ap- 

 proach 500 A or less in diameter. 



The evidence presented thus far indicates 

 that in the field of microscopically visible 

 structures, the secondary wall of plant cells 

 is composed of a continuous system of 

 coalesced microfibrils which is perforated 

 by a continuous system of intercommunicat- 

 ing niicroeapillaries. The cellulosic matrix 

 is an extraordinarily porous structure, yet 

 it exhibits a very high tensile strength. It 

 is of interest to compare the visible struc- 

 ture of cellulose with pictures of its struc- 

 tui'e which have been ]iostulated in the tridy 

 submici'oscoi)ic field. For many years fol- 

 lowing the publication of Niigcli's iiiiccllar 

 liy])othesis, the micelles oi' ci-ystallites of 

 cellulose were considered to be disci'ete 

 entities separ-ated on all sides by intermicel- 

 \nv s]iaces. Sul)se(|ueu1ly, evidence obtained 

 hirgely by x-i-ay analyses was interpreted 



