72 SUMMARY OF CURRENT RESEARCHES RELATING TO 



This may possibly be the result of a tendency for the development of 

 stomata to take place chiefly on those parts which are most protected 

 from external influences. 



With reference to the distribution of the stomata, the author 

 divides plants into three groups, as follows : — (1) The foliage- and 

 scale-leaves have stomata only on the under or outer side respectively. 

 (2) The scale-leaves have them on both sides, the foliage-leaves on 

 the under side only. (3) Both kinds possess stomata on both sides; 

 but the foliage-leaves more on the under, the scale-leaves more on the 

 inner side. 



In form the underground stomata are almost always of equal 

 length and breadth. They occur only on those plants whose habitat 

 is sufficiently dry for the tissue to be porous enough for gases to per- 

 meate. None were found on the underground organs of Senecio 

 sarracenicus, Campanula glomerata, Monotropa Hijpopitys, Menyanthes 

 trifoliata, Lamiuvi album, Ballota nigra, Asariim europceum, Typha lati- 

 folia, Caltha palustris, Iris pallida and p>seudacorus, or Pteris aquilina. 

 On the underground axes the number rarely exceeded 10 per sq. mm. ; 

 while on underground leaves it was occasionally much greater : in 

 Saponaria officinalis, 272 ; in Centaurea Jacea, 108 ; in Trientalis 

 europcea, 150 per sq. mm. 



Sieve-tubes of Dicotyledonous Plants.* — The following results 

 were obtained by K. Wilhelm mainly from the examination of the 

 sieve-tube structures in Vitis vinifera, Cucurbita Pepo, and Lagenaria 

 vulgaris, which may be taken as representatives of two different types, 

 the first belonging to a more complicated, the two others to a simpler 

 type. 



Those young bast-cells which are destined to the formation of a 

 sieve-tube undergo as a rule a longitudinal division into unequal 

 cells ; the largest of these becomes an element in the sieve-tube ; the 

 one or more remaining ones, which are much smaller, but themselves 

 of nearly equal size, form the " companion cells " of the sieve-tube, 

 which may undergo still further division. These companion cells are 

 always much smaller than the cambiform cells ; they are marked by 

 containing abundant granular protoplasm with a large nucleus ; and the 

 walls which separate them from the sieve-tube always contain a 

 number of pits, which is not the case with those that separate the 

 sieve-tubes from the cambiform cells. 



The callous nature of the sieve-plates is not, as previously sup- 

 posed, the result of a secondary change ; on the contrary, the trans- 

 formation of a pit into a sieve-plate begins with the local change of 

 cellulose into callus. The sieve-pores are formed where the pits in 

 the callus are separated by a reticulation of unchanged cellulose. The 

 callus is, however, not limited to the neighbourhood of the sieve-pores, 

 but may spread itself over the whole plate ; and this callus-skeleton, 

 which may be isolated, forms an essential part of every completely 



* Wilhelm, K., ' Beitr'age zur Kenntniss des Siebiohrenapparatus dicotyler 

 Pflanzen,' Leipzig, 1880. See Bot. Centralbl., i. (1880) p. 908 ; also ' Nature,' 

 xxii. (1880) p. 602. 



