THE STAMEN 143 



the microspores are freed from the mother-cell wall — by the intrusion 

 of "tongues" of cytoplasm from the individual tapetal cells among the 

 spores. The nuclei of the tapetal cells migrate into the tongues later, as 

 the processes develop among the pollen grains. Ultimately, the tongues 

 merge to form the periplasmodium, in which the embedded pollen 

 grains enlarge greatly. Vacuoles appear in the periplasmodium, and 

 the nuclei and remnants of protoplasm disappear as the exine de- 

 velops on the grains. The Butomus type of plasmodium is similar to 

 the Sagittaria type but appears earlier, while the spores are still in 

 tetrads. The enlarging tapetal-cell protoplasts press among the spores 

 and unite, embedding them. In the Sparganium type, the tapetal cells 

 — 4-nucleate and vacuolate — enlarge and free their protoplasts at the 

 time of the first reduction division of the spore mother cell. Tetrad 

 formation occurs in the nucleus-rich plasmodium. In the Triglochin 

 type, the tapetal cells disintegrate where they lie, and the cytoplasm 

 and later, during the reduction divisions, the nuclei push between the 

 pollen mother cells. 



The secretory type of tapetum, like the amoeboid type, varies in be- 

 havior; the cell walls disappear or collapse as their cell contents merge 

 in a plasmodium on the periphery of the fertile tissue. Forms of 

 taxonomic significance can perhaps be distinguished, but too little de- 

 tailed information is yet available. This type occurs in the higher mono- 

 cotyledons and in many of the dicotyledons. It has been called the 

 primitive type, but the amoeboid type, which characterizes the lower 

 monocotyledons generally and is found in some of the lower dicotyle- 

 dons, is probably primitive. The resemblance of this amoeboid peri- 

 plasmodium to that of some of the lower vascular plants supports this 

 view. The formation of a peripheral layer in situ — the Triglochin type 

 — is the simpler and apparently the most advanced method. 



Placentoids. Projections of sterile tissue into the sporogenous tissue 

 have been called placentoids. These projections may be large lobes of 

 connective parenchyma on the inner side of the sporangium — much re- 

 sembling the placentae of carpels, with which they were once con- 

 sidered homologous — or plates of laminar tissue extending into or 

 through the sporangia, dividing it more or less completely. In the 

 Gentianaceae and Menyanthaceae, sterile cells break up the sporogenous 

 tissue; in Limnanthemum and Menyanthes, they surround clusters of 

 fertile cells and, in Gentiana, individual fertile cells. In the Onagraceae, 

 two to six spore clusters are set apart in a sporangium; in some 

 Orchidaceae (Phajus), Rhizophoraceae, Loranthaceae, and Mimoseae, 

 there are several to many well-separated clusters. Transverse partitions 

 give the appearance of multisporangiate anthers — Butomus. The parti- 

 tions may become tapetal wholly or in part, and the groups of 



