98 



INTRODUCTION TO CYTOLOGY 



of aleurone grains in maturing endosperm (p. 96), the appearance of 

 stained masses when certain vital dyes are used, and the formation of 

 crystals. Very striking alterations in the form, number, and reactions 

 of vacuoles have been observed in stimulated secretory and conducting 

 cells of insectivorous plants^ and in diseased tissue. ^° ' 



Carbohydrates. — The most conspicuous visible carbohydrate mate- 

 rials in plants are starch and cellulose. In most green plants the excess 

 carbohydrate elaborated by the process of photosynthesis (p. 65) is 

 deposited in the form of starch granules. These range in size up to 

 about 200m and vary considerably in appearance in different plants. 



Each granule is made up of a series of 

 concentric layers successively depos- 

 ited about a center, or "hilum." 

 Meyer showed that in certain cases 

 the stratification is correlated with 

 the alternation of day and night and 

 therefore with a periodic activity of 

 the protoplast. Granules from wheat 

 grown under constant conditions are 

 reported not to show it (Sande- 

 Bakhuyzen, 1926). In case the gran- 

 ule starts to form near the middle of 

 an amyloplast it may develop sym- 

 metrically, but commonly it lies near 

 the periphery and becomes very 

 eccentric in form, owing to the une- 

 qual deposition of new material on its various sides. The amyloplast 

 may become greatly distended as the granule grows, often reaching 

 invisible thinness; in extreme cases it becomes ruptured and remains in 

 contact with the granule only at one side, where all new material is 

 thenceforth deposited. Several granules may start to develop simul- 

 taneously in a single amyloplast and later grow together to form a "com- 

 pound granule" with more than one hilum. In case the parts making 

 up the compound granule are enveloped in one or more common outer 

 layers, the granule is said to be "half-compound." The successively 

 deposited layers making up the granule differ mainly in water content, 

 the innermost layers being richest and the outermost poorest in water. 

 As a result of this non-uniformity the granule often splits radially when 

 dehydrated. Viewed through a polarizing microscope with crossed Nicols 

 the granule appears as a light body traversed by a dark cross (Fig. 50). 

 By this means starch can often be detected in very minute plastids. 



9 Gardiner (1885), Dufrenoy (1927), Mangenot (1929a6), Homes (1929). 

 i« Dufrenoy (19286c, 19296c, 1930c, 193 lac), Dufrenoy et al. (1929de). Kuster 

 (1929) describes pathological changes in protoplasm. 



Fig. 



50. — Potato-starch grains photo- 

 graphed by polarized Hght. 



