398 GENERAL PLANT PATHOLOGY 



Protective Tissues. — The protective tissues of galls consist of the 

 epidermal, or covering tissues, and the stone cells which form part of 

 the mechanic tissue. The epidermal tissue will be considered as a pro- 

 tective tissue irrespective of its origin whether from the epidermis of the 

 host, or as a new formation. The outer epidermis of sac and walled 

 galls consists of relatively large, often flat cells which have a cuticle of 

 moderate development. Occasionally this epidermis may consist of 

 more than one layer. A gall found on a Californian oak Quercus Wisli- 

 zeni, has the outer walls of its epidermal cells and the upper part of the 

 side walls thickened so that the cell cavity becomes conic in shape (Fig. 

 162). Cork, as a covering for galls, is extremely rare. Wound-cork 

 is found occasionally in these gaUs, while bark is even rarer in a few 

 apterous galls. 



Hair structm-es, or trichomes, are not unusual in galls. The 

 majority of prosoplasmatic galls are naked or onlj' slightly pubescent 

 and some galls are entirely without any covering tissue. 



Mechanic Tissue. — These consist of stereids (sclerotic, or stone cells) 

 or sclerenchyma fibers almost entirely and they surround the larval 

 chambers so that their occupants are protected from outside pressure, 

 or sudden blows. Lacaze-Duthiers called the stone cell tissues in galls 

 "couche protectrice." The arrangement of the stone cells, their 

 structure and their position in the gall tissues are of the greatest diver- 

 sity. In the majority of cases, the stereids are round, in other galls 

 they are angular, while in others, they are stretched like palisade cells 

 and stand perpendicular to the upper surface of the gall body similar to 

 those in many fruit and seed shells. Sometimes the sclereid cell is thick- 

 ened only on one side, the delicately waUed part being outside as in the 

 galls of Andricus quadrilineatus and sometimes they are inside as in an 

 elliptic gall of the oak, etc. The walls of the sdereids may be pitted, 

 and, therefore, porous, while in other cases the pitting may be very 

 scanty and other peculiarities have been described by pathologists 

 who are intimately acquainted with the structure of galls. 



Nutritive Tissues. — The tissues of galls which are eaten by the animal 

 occupants of the different galls, or the contents of which are beneficial 

 to the larvae have been termed by cecidologists nutritive tissues. The 

 position of these nutritive tissues in the galls and their contents must 

 be considered next. No gaU is entirely without nutritive tissues and 

 these not infrequently form the largest part of the gall and in those 



