NON-PROTOPLASMIC CELL CONTENTS 107 
germ oil. It also occurs in lettuce, water cress, whole cereals, 
nuts, in cocoanut, corn and cottonseed oils, meat, etc. 
VITAMIN K or Anti-hemorrhagic Vitamin has been found to 
represent the substance necessary in the diet of chickens (and 
possibly other animals) for the prevention of multiple hemor- 
rhages. It has been found in the fat of hog liver, in tomatoes, 
kale and hemp seed. 
CELL WALLS 
The cell walls of plants make up the plant skeleton. They 
are all. formed by the living contents of the cells (protoplasts) 
during cell-divisions. In most plants the cell wall when first 
formed (primary cell wall) consists of pectose or a related car- 
bohydrate. Later the daughter protoplasts secrete two layers of 
a mixture of cellulose, (CeH10O5)n, and protopectin, one on either 
side of a middle lamella, the latter composed of calcium pectate. 
Each of these layers on either side of the middle lamella represents 
a secondary cell wall. A tertiary wall may be laid down by the 
protoplast beneath the secondary wall. (See Fig. 14.) 
The cell walls may remain of such composition or become 
modified to meet certain functions required of them. ‘Thus, in the 
case of outer covering cells as epidermis and cork, whose function 
is that of protecting the underlying plant units, the walls become 
infiltrated with cutin (in the case of epidermal cells), suberin 
(in the case of cork cells), waxy-like substances, which make them 
impermeable to water and gases, as well as protect them against 
easy crushing. Again, in the case of stone cells and sclerenchyma 
fibers whose function is that of giving strength and support to the 
regions wherein found, the walls become infiltrated with dignin 
which increases their strength, hardness, and in the case of 
sclerenchyma fibers, their elasticity also. Moreover, in the case 
of the cells comprising the testa or outer seed coat of the pumpkin, 
squash, mustard, flax, psyllium, etc., whose function is that of 
imbibing quantities of water, the walls undergo a mucilaginous 
modification. These, upon the imbibition of water, swell up and 
form layers of mucilage within the cell cavities. 
The sub-epidermal cells beneath the upper epidermis of 
Buchu leaves exhibit a striking example of mucilaginous modi- 
